Copyright © 1997, American Society for Microbiology
The 3
9
Untranslated Region of the B19 Parvovirus Capsid
Protein mRNAs Inhibits Its Own mRNA Translation in
Nonpermissive Cells
CORALIE PALLIER,
1,2ANNA GRECO,
3JANINE LE JUNTER,
1ALI SAIB,
2ISABELLE VASSIAS,
1AND
FREDERIC MORINET
1,2*
Service de Microbiologie, Unite´ de Virologie,
1and CNRS UPR 9051,
2Hoˆpital Saint-Louis, 75475 Paris Cedex 10, and
CNRS UMR 5537, Faculte´ de Me´decine—Lyon RTH Laennec, 69372 Lyon Cedex 08,
3France
Received 8 July 1997/Accepted 22 August 1997
Although parvoviruses are found throughout the animal kingdom, only the human pathogenic B19 virus has
so far been shown to possess a limited host range, with erythroid progenitor cells as the main target cells
supporting B19 propagation. The underlying mechanism of such erythroid tropism is still unexplained.
Synthesis of the NS1 nonstructural protein occurs in permissive and nonpermissive cells, such as
megakaryo-cytes, whereas synthesis of the VP1 and VP2 capsid proteins seems to be restricted to burst-forming units and
CFU of erythroid cells. In nonpermissive cells, the NS1 protein is overexpressed and the NS1 RNAs are the
predominant RNA species. However, the VP1 and VP2 proteins are not detectable, although the corresponding
mRNAs are synthesized. Since all transcripts have part of the 5
*
untranslated region (5
*
UTR) in common but
distinct 3
*
UTRs characterizing the nonstructural- and structural-protein mRNAs, we investigated, in
tran-sient transfection assays, the possible involvement of the 3
*
UTR of the capsid protein mRNAs in VP1 and VP2
protein synthesis in nonpermissive Cos cells. The results showed that (i) the 3
*
UTR of mRNAs coding for the
capsid proteins repressed VP1 and VP2 protein synthesis, (ii) the 3
*
UTR did not affect nuclear export or
mRNA stability, and (iii) mRNAs bearing the 3
*
UTR of the capsid protein mRNAs did not associate with
ribosomes at all. Taken together, these results indicate that in nonpermissive cells, the 3
*
UTR of the capsid
protein mRNAs represses capsid protein synthesis at the translational level by inhibiting ribosome loading.
The Parvoviridae family includes many common agents of
disease in animals. However, the only known human
patho-genic member of this family is the B19 virus, recently
reclas-sified as belonging to a new genus called Erythrovirus. In
indi-viduals with underlying hemolytic disorders, B19 virus
infection is the primary cause of transient aplastic crisis. In
immunocompromised patients, persistent B19 virus infection
may lead to pure red cell aplasia and chronic anemia. B19 virus
infection in utero can result in fetal death, hydrops fetalis, or
congenital anemia. Human erythroid progenitor cells are the
main target cells of the B19 virus (22). However, despite such
a remarkable erythroid tropism, B19 virus infection can also
impair megakaryocytopoiesis (39). The disrupted
erythropoie-sis observed during productive B19 virus infection is initiated
by virus replication in erythroid progenitor cells in the bone
marrow and is followed by their destruction. In contrast, the
etiology of the thrombocytopenia encountered in B19 virus
infection is explained by the expression of the B19 virus
ge-nome without viral replication, and the expression of the B19
virus genome may be toxic to cell populations that are
non-permissive for viral replication. Thus, human hematopoietic
cell culture remains the only in vitro system for B19 virus
propagation (29). The basis of this narrow erythroid specificity
is not quite clear, but it is partially mediated by the tissue
distribution of the B19 virus cellular receptor, the blood group
P antigen (3). The susceptibility of bone marrow cells to B19
virus infection is known to increase during erythroid
differen-tiation: thus, the CFU consisting of late erythroid progenitor
cells (the CFU-E) are more strongly inhibited by B19 virus
infection than the burst-forming units consisting of more
prim-itive erythroid progenitor cells (the BFU-E) (41). Although
cellular factors may be involved in viral replication specificity,
as in the case of the adenoviruses (43), permissivity also
ap-pears to be controlled at the level of viral gene expression and
replication.
Parvoviruses are small, nonenveloped icosahedral viruses
containing a linear single-stranded DNA genome of
approxi-mately 5 kb, whose sense can be positive or negative. The 5
9
end of the positive-sense strand of the B19 virus genome
con-tains the sequence coding for the NS1 nonstructural protein,
and the 3
9
end harbors the sequences coding for the two
struc-tural proteins, VP1 and its amino-terminal-truncated version,
VP2. The VP2 protein constitutes the major capsid
polypep-tide, accounting for about 95% of the viral proteins in the
virion. The B19 virus transcriptional pattern sets it apart from
most of the other parvoviruses (Fig. 1A). All transcripts are
initiated at the single strong left-side promoter called P6. The
sequence encoding the NS1 polyadenylation site is located in
the middle of the genome, whereas the sequence encoding the
VP1 and VP2 polyadenylation site is on the far right side (2, 18,
26). The existence of a single promoter indicates that viral gene
expression is not a function of promoter choice. Moreover, the
P6 promoter exhibits strong activity in many cell lines (16). At
least nine polyadenylated RNAs have been identified (4, 26,
35), and the proteins encoded by five of them have been
de-scribed (18, 26, 40). The only unspliced mRNA encodes the
NS1 nonstructural protein, a 77-kDa pleiotropic regulating
protein which causes cellular toxicity (19, 27), activates
tran-scription from the P6 promoter (7), and probably plays a role
in viral DNA replication, as documented for the
adeno-asso-ciated viruses (AAVs) and the minute virus of mice (24, 37).
* Corresponding author. Mailing address: Service de Microbiologie,
Unite´ de Virologie, Hoˆpital Saint-Louis, 1 avenue Claude Vellefaux,
75475 Paris Cedex 10, France. Phone: 33 1 42 49 94 93. Fax: 33 1 42 49
92 00. E-mail: fr.morinet@chu-stlouis.fr
9482
on November 9, 2019 by guest
http://jvi.asm.org/
Two mRNAs code for each of the two capsid proteins, VP1
and VP2, of 84 and 58 kDa, respectively (30). The relative
amounts of the minor VP1 protein and the major VP2 protein
in the viral capsid are regulated in part by the presence of
multiple AUG codons upstream of the commonly used
trans-lation initiation codon of the VP1 protein (28). These spurious
triplets, which have been shown to be responsible for the
re-duced translation efficiency of the VP1 mRNAs, are removed
by splicing events from the VP2 mRNAs. Consequently,
syn-thesis of the VP2 protein is greatly improved. In addition to
these transcripts, small, abundant RNAs of two size classes,
whose functions in the virus life cycle are not yet understood,
are produced (18, 40). Synthesis of the NS1 protein occurs in
both permissive and nonpermissive cells, whereas synthesis of
the VP1 and VP2 proteins seems to be restricted to BFU-E
and CFU-E (17, 26). Permissive cells, which support B19 virus
propagation, indeed allow full-length transcription of both the
nonstructural- and structural-protein genes, although the
cap-sid protein mRNAs are the predominant RNA species. On the
other hand, in nonpermissive cells, the steady-state level of the
nonstructural-protein mRNAs is higher than that of the
struc-tural-protein mRNAs. The small amount of the capsid protein
mRNAs can be explained by the premature arrest of
transcrip-tion in the middle of the genome, as demonstrated by Liu et al.
in transient transfection assays (17). Thus, in nonpermissive
cells, transcription of the nonstructural-protein gene is
pre-dominant and leads to overexpression of the corresponding
NS1 protein, inducing cell death (20). Although the level of the
capsid protein mRNAs is low, the corresponding VP1 and VP2
proteins are not detectable, indicating that capsid protein
syn-thesis is also regulated at the posttranscriptional level.
In this work, we further investigated the regulation of capsid
protein synthesis in nonpermissive cells. We checked for
cis-acting regulatory elements that might control capsid protein
synthesis. Since all the B19 virus transcripts share a common
nucleotide sequence at the distal part of the 5
9
untranslated
region (5
9
UTR) but have completely distinct 3
9
UTRs
char-acterizing the nonstructural and capsid protein mRNAs (26),
we focused on the 3
9
UTR of mRNAs which code for both the
VP1 and VP2 proteins. We showed by transient transfection
assays that the 3
9
UTR of mRNAs coding for these capsid
proteins significantly represses VP1 and VP2 protein synthesis
in Cos cells. Our results indicate that this is not the
conse-quence of nuclear retention or of increased instability of the
corresponding mRNAs but that the 3
9
UTR of the capsid
protein mRNAs regulates capsid protein synthesis at the
trans-lational level by inhibiting ribosome loading.
MATERIALS AND METHODS
Construction of recombinant plasmids.The pBS-B19 plasmid is a derivative of the pEMBL-B19 plasmid (31). It contains the 5,110-bp EcoRI fragment of the B19 virus genome, subcloned at the EcoRI restriction site of the pBluescript II SK (1) plasmid (Stratagene) corresponding to the full-length B19 virus genome, except for the terminal repeat DNA sequences.
To construct the pVP1-39VP and pVP2-39VP plasmids, referred to below as pVP1(2)-39VP, the DNA regions corresponding either to the VP1 open reading frame (ORF) and the DNA sequence coding for the homologous 39 UTR (nucleotides [nt] 2438 to 5105 in the B19 virus genome) or to the VP2 ORF and the DNA sequence coding for the VP 39UTR (nt 3119 to 5105 in the B19 virus FIG. 1. (A) Transcription map of the B19 virus. The NS1 nonstructural protein is encoded by the only unspliced mRNA species from the left of the genome; the VP1 and VP2 capsid proteins are encoded by overlapping mRNAs in the same reading frame. (B) Schematic representation of the VP1, VP2, and NS1 expression
plasmids used in this study. The positions of some of the restriction enzyme sites used for the cloning are indicated.
on November 9, 2019 by guest
http://jvi.asm.org/
[image:2.612.75.535.71.384.2]genome) were amplified by PCR from the pBS-B19 plasmid as a template, with the following primers: VP1 upstream, 59-GGAATTCCCAAGCTTGGGTAGA
TTATGAGTAAAAAAAG-39; VP2 upstream, 59-GGAATTCCCAAGCTTGGG
CCAAGCATGACTTCAGTT-39; and VP112 downstream, 59-GCTCTAGAGC
CATCTTGTACCGGAAG-39(characters in italics represent part of the B19 virus sequence). The EcoRI and XbaI restriction sites (underlined) were introduced for further cloning. The PCR products were then digested and inserted between the EcoRI and XbaI restriction sites of the pSG5 plasmid (Stratagene). To construct the pVP1-39SV40 and pVP2-39SV40 plasmids, referred to below as pVP1(2)-39SV40, the DNA regions corresponding to the VP1 and VP2 ORFs (nt 2438 to 4792 and 3119 to 4792 in the B19 virus genome, respectively) were amplified by PCR from the pBS-B19 plasmid as a template, with the upstream primers listed above and the downstream primer 59-GGAATTCCCAAGCTTG GGTGTTTACAATGGGTGCACACG-39. The EcoRI restriction site (under-lined) was introduced for further cloning. The PCR products were then digested and inserted into the EcoRI restriction site of the pSG5 plasmid. The pNS1-39SV40 plasmid contains the sequence coding for the NS1 ORF (nt 430 to 2454 in the B19 virus genome), which was amplified by PCR from the pBS-B19 plasmid as a template, with the upstream primer 59-GGAATTCCCAAGCTTG GGCCTAACATGGAGCTATTTAGAGG-39and the downstream primer 59-GG AATTCCCAAGCTTGGGTTTTTACTCATAATCTACAAAGCTTTGC-39, and was inserted into the EcoRI restriction site of the pSG5 plasmid.
The sequences of all the cloned DNA fragments were verified by sequencing with a T7 sequencing kit (Pharmacia Biotech) (35).
The firefly luciferase (pCMV-Luc) and Escherichia coli b-D-galactosidase (pCMV-LacZ) expression plasmids were kindly provided by Uriel Hazan and Marc Alizon (ICGM, Paris, France), respectively.
Cell culture and transient transfection assays.Cos6 cells were grown as monolayers in Dulbecco modified Eagle medium (DMEM; Gibco-BRL) supple-mented with 10% heat-inactivated fetal calf serum (Eurobio).
Eight hours before transfection, 1.23106cells were plated onto
100-mm-diameter petri dishes. Transfections were carried out with 12mg of total DNA and 20mg of Lipofectin reagent in 4 ml of Opti-MEM, according to the manu-facturer’s instructions (Gibco-BRL). All DNA plasmids used for transfection were purified on anion-exchange resin columns (Qiagen). We used 10mg of the relevant VP1 or VP2 expression plasmid together with 2mg of the pCMV-LacZ or pCMV-Luc plasmid per plate. Sixteen hours after transfection, the medium was replaced with fresh DMEM. Cells were harvested at various times after transfection, and RNAs and proteins were extracted. These were analyzed 40 and 64 h posttransfection, respectively, the optimal times for RNA and protein analysis. The pCMV-LacZ and pCMV-Luc plasmids were used as internal trans-fection efficiency controls. Galactosidase and luciferase activities were quantified by luminometry and colorimetry, respectively, and were standardized as a func-tion of the protein concentrafunc-tion in extracts from cells transfected with the VP1, VP2, or NS1 expression plasmid, as previously described (38). Each transfection was performed in duplicate, and all experiments were repeated at least three times.
Antibodies.To produce the VP1 and VP2 antibodies, the pET-VP1 and pET-VP2 plasmids were constructed. They contain the 2,370- and 1,689-bp
HindIII fragments of the pVP1-39SV40 and pVP2-39SV40 plasmids, respectively, cloned at the XhoI restriction site of the pET-14b plasmid (Novagen), after filling by Klenow treatment. The VP1 and VP2 proteins were produced in E. coli BL21 DE3 and were purified on nickel-exchange resin columns according to the man-ufacturer’s instructions. New Zealand White rabbits were then immunized as previously described (21), and the specific anti-VP1 and anti-VP2 polyclonal antibodies obtained were called Ab-R1 and Ab-R2, respectively.
Protein analysis.The VP1 and VP2 proteins were synthesized in vitro from 1 mg of the pVP1-39VP, pVP2-39VP, pVP1-39SV40, or pVP2-39SV40 plasmids incubated in the TNT T7 coupled reticulocyte lysate system (Promega) contain-ing 40mCi of [35S]methionine (ICN) per assay. The VP1 and VP235S-labeled
proteins, which were of the expected size (30), were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as described below and were revealed by autoradiography of the dried gel.
Indirect immunofluorescence assays were performed with 73104cells per
slide. Transfected cells were cytospun, fixed in 3.7% paraformaldehyde for 10 min at 4°C, and then permeabilized with methanol for 5 min at 4°C. Next, cells were incubated with human anti-B19 virus serum, diluted 1:100, from a patient convalescing from B19 virus infection (kindly provided by B. Cohen, Colindale, London, United Kingdom) and were stained with a fluorescein isothiocyanate (FITC)-conjugated goat anti-human antibody (Sanofi Diagnostic Pasteur), di-luted 1:100. This serum recognized the VP1 and VP2 proteins but not the NS1 protein, as evidenced by the fact that the indirect immunofluorescence assay was negative on HeLa-NS1-21 cells, in which NS1 protein synthesis was induced by dexamethasone (15). Concurrently, the rabbit polyclonal antibodies Ab-R1 and Ab-R2 were also used for specific revelation of the VP1 and VP2 proteins. These antibodies were used at a 100-fold dilution, and cells were stained with FITC-conjugated goat anti-rabbit antibody (Sanofi Diagnostic Pasteur), diluted 1:100. The NS1 protein was detected with a rabbit polyclonal antibody (kindly provided by C. Astell, University of British Columbia, Vancouver, Canada) used at a 250-fold dilution and was revealed as described above.
For Western blot analysis, cells were heat disrupted at 100°C for 5 min in Laemmli sample buffer (14). About 50mg of proteins was resolved by SDS–12%
PAGE and transferred by electroblotting onto a polyvinylidene difluoride mem-brane (Immobilon-P; Millipore). The memmem-branes were incubated first with an anti-VP1 or anti-VP1 and -VP2 polyclonal antibody (kindly provided by S. Kajigaya, National Institutes of Health, Bethesda, Md.), diluted 1:50, and then with an alkaline phosphatase-conjugated goat anti-rabbit antibody (BioSys), di-luted 1:800. The complexes were stained with NBT (nitroblue tetrazolium)-BCIP (5-bromo-4-chloro-3-indolylphosphate toluidinium) as the substrate (Sigma) (31).
Radioimmunoprecipitation of proteins was performed as follows: cells were transfected for 56 h and metabolically labeled for 8 h at 37°C with 200mCi of [35S]methionine per ml in methionine-free DMEM (ICN) supplemented with
10% fetal calf serum. Cells were then lysed for 15 min in ice-cold buffer (50 mM Tris-HCl [pH 7.4], 100 mM NaCl, 5 mM MgCl2, 1% Nonidet P-40, 0.5% sodium
deoxycholate, 0.1% SDS, 1mg of aprotinin/ml, and 100mg of phenylmethylsul-fonyl fluoride/ml). The VP1 and VP2 proteins were immunoprecipitated from cell lysates with human anti-B19 virus serum, diluted 1:10, from the patient convalescing from B19 virus infection for 18 h at 4°C, as previously described (10). Antigen-antibody complexes were collected on protein A-Sepharose (Sig-ma) for 2 h at 4°C. Immunoprecipitated proteins were resolved by SDS-PAGE as described above, and the dried gel was submitted to autoradiography.
mRNA analysis.For in situ RNA hybridization assays, transfected cells were cytospun, fixed in 3.7% paraformaldehyde for 20 min at 4°C, and permeabilized with 100 mM ethanolamine for 5 min at room temperature. Cells were then incubated either for 2 h at 37°C with 200 U of DNase I/ml in DNase buffer (40 mM Tris-HCl [pH 7.4], 6 mM MgCl2, 10 mM NaCl, 10 mM CaCl2, and 2 mM
dithiothreitol) or for 1 h at 37°C with 100mg of RNase A/ml in RNase buffer (40 mM Tris-HCl [pH 7.5] and 23SSC [13SSC is 0.15 M NaCl plus 0.015 M sodium citrate]). The VP1, VP2, and NS1 probes corresponding to the ORF sequences were prepared by PCR using the pVP1-39SV40, pVP2-39SV40, and pNS1-39SV40 plasmids, respectively, as templates and the primers used for cloning. The am-plified DNA fragments were purified by using the Wizard PCR Preps DNA purification system (Promega) and were labeled by random priming, by using the DIG DNA labeling kit (Boehringer-Mannheim) in the presence of 0.035 mM [digoxigenin]dUTP. Hybridization and detection of RNA were performed under nondenaturing conditions, as previously described (44).
mRNAs were quantified by Northern blot analysis. Total RNAs were extracted with RNA-B (Bioprobe Systems). Cytoplasmic poly(A)1RNAs were purified
with the Oligotex direct mRNA kit (Qiagen) after lysis of the transfected cells with either Nonidet P-40 or Triton X-100 at a final concentration of 0.5%. Twenty micrograms of total RNAs or 2mg of cytoplasmic poly(A)1RNAs was
loaded onto a 2.2 M formaldehyde–1.2% agarose gel, blotted onto a nitrocellu-lose filter (Optitran BA-S 85, NC reinforced; Schleicher & Schuell), and hybrid-ized according to standard procedures (8). The probes used to detect the VP1, VP2, NS1, and human glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNAs were obtained by labeling the 2,354-bp EcoRI fragment of the pVP1-39SV40 plasmid, the 1,673-bp EcoRI fragment of the pVP2-39SV40 plasmid, the 2,024-bp EcoRI fragment of the pNS1-39SV40 plasmid, and the 318-bp
SacI-BamHI fragment of the pTRI-hGAPDH plasmid (Ambion), respectively, by
using a nick translation kit (Boehringer-Mannheim) and [a-32P]dCTP
(Amer-sham). Membranes were hybridized sequentially, first with the VP1, VP2, or NS1 probe and then with the GAPDH probe. After autoradiography, the correspond-ing mRNAs were quantified by scanncorrespond-ing densitometry of the membranes with the GS-525 molecular imager system (Bio-Rad). The results obtained for the VP1, VP2, and NS1 mRNAs were normalized by the correcting factor obtained with the GAPDH probe.
The stability and half-lives of the VP1 and VP2 mRNAs were determined from cells transfected for 40 h with the relevant plasmids and incubated with either 20 mg of cycloheximide/ml or 10mg of actinomycin D/ml (46). Total RNAs were extracted 6 h after addition of cycloheximide, or at hourly intervals until 12 h after addition of actinomycin D, and the VP1 and VP2 mRNAs were quantified by Northern blot analysis as described above.
Distribution of specific mRNAs among polyribosomal fractions. Polyribo-somes were prepared from four 100-mm-diameter plates of cells transfected with the relevant plasmids. Sucrose gradient fractionation of polyribosomes was car-ried out from postmitochondrial supernatants as previously described (6, 11). Briefly, transfected cells were washed in cold phosphate-buffered saline in the presence of 100mg of cycloheximide/ml and were scraped off from the monolayer in phosphate-buffered saline. Cells were then centrifuged, resuspended in ice-cold buffer (250 mM sucrose, 50 mM Tris-HCl [pH 7.4], 25 mM KCl, 5 mM MgCl2, and 100mg of cycloheximide/ml), and lysed by addition of either Nonidet
P-40 or Triton X-100 at a final concentration of 0.5%. Nuclei and mitochondria were pelleted by two successive centrifugations, one at 7503g and one at
12,0003g. The postmitochondrial supernatant was layered onto a 10 to 40%
linear sucrose gradient in the same buffer containing 2 mM dithiothreitol. After separation by ultracentrifugation in a Beckman SW41 Ti rotor at 40,000 rpm for 75 min at 4°C, 20 fractions of identical volume were collected from the top of the gradient. Each fraction was incubated with 100mg of proteinase K/ml and 1% SDS, and the RNA was extracted with phenol-chloroform and precipitated with ethanol. The RNA from each fraction was resuspended in 10 mM Tris-HCl (pH 7.4)–1 mM EDTA–50 U of RNasin (Promega)/ml. The distribution and quan-tification of the various mRNAs in the different fractions were then determined on Northern blots, as described above.
on November 9, 2019 by guest
http://jvi.asm.org/
RESULTS
Inhibition of capsid protein gene expression.
To determine
to what extent the sequence coding for the 3
9
UTR of capsid
protein mRNAs is involved in regulating the expression of the
homologous gene, we constructed the pVP1-3
9
VP,
pVP2-3
9
VP, pVP1-3
9
SV40, and pVP2-3
9
SV40 plasmids (Fig. 1B). In
the pVP1-3
9
VP and pVP2-3
9
VP plasmids, the DNA sequence
coding for the VP1 and the VP2 ORFs and the downstream
homologous 3
9
UTR of the capsid protein mRNAs were
cloned in the pSG5 plasmid controlled by the simian virus 40
(SV40) early promoter. We decided not to preserve the
ho-mologous DNA sequence coding for the 5
9
UTR in our
plas-mids, as it includes multiple AUG codons upstream of the
commonly used translation initiation codon of the VP1 protein
(28). In the pVP1-3
9
SV40 and pVP2- the DNA sequence
cod-ing for the 3
9
UTR of the capsid protein mRNAs was replaced
by that of the SV40 early mRNAs. The pNS1-3
9
SV40 plasmid,
which we constructed as a control, contained the NS1 ORF
instead of the VP1(2) ORF.
Cos cells were transfected with the pVP1-3
9
VP, pVP2-3
9
VP,
pVP1-3
9
SV40, or pVP2-3
9
SV40 plasmid. The pNS1-3
9
SV40
plasmid was used as a positive control for viral gene expression.
The galactosidase (pCMV-LacZ) or luciferase (pCMV-Luc)
expression plasmid was included in each transfection as an
internal control for transfection efficiency. Cells were
har-vested 64 h posttransfection, and the presence of the VP1 and
VP2 proteins was revealed by immunofluorescence assay using
human anti-B19 virus serum from a patient convalescing from
B19 virus infection. No detectable fluorescence was seen in
cells transfected with the pVP2-3
9
VP plasmid, whereas about
5% of the cells transfected with the pVP2-3
9
SV40 plasmid
exhibited a fluorescent pattern (Fig. 2A). Identical data (not
shown) were obtained for cells transfected with the VP1
plas-mids. Similar results were obtained with the specific Ab-R1
and Ab-R2 polyclonal antibodies, directed against VP1 and
VP2, respectively (data not shown). Furthermore, the NS1
protein was detected in about 30% of the cells which were
transfected with the pNS1-3
9
SV40 plasmid and incubated with
a specific NS1 polyclonal antibody; this percentage reflects the
transfected yield, because galactosidase activity was detected in
situ in about 30% of the transfected cells in all experiments.
The ratios of galactosidase and luciferase activities to total
protein concentration were 0.21
6
0.03 and 1,103,676
6
71,395, respectively. No fluorescence was detected either in
cells transfected with the pSG5 plasmid or in the control
mock-transfected cells incubated with the antibodies.
These results were confirmed by Western blot analysis with
anti-VP1 and anti-VP1 and -VP2 polyclonal antibodies to
re-veal the VP1 and VP2 proteins, respectively. As shown in Fig.
2B, no protein was detectable in cells transfected with the
pVP1-3
9
VP or pVP2-3
9
VP plasmid, whereas in cells
trans-fected with the pVP1-3
9
SV40 or pVP2-3
9
SV40 plasmid, the
VP1 and VP2 proteins of 84 and 58 kDa, respectively, were
detected. No protein was detected in the cells transfected with
the pSG5 plasmid or in the control mock-transfected cells, but
the Western blot pattern of the nonspecific signals was the
same as that of cells transfected with the pVP1-3
9
VP or
pVP2-3
9
VP plasmid. These results were also confirmed by
radioim-munoprecipitation assays (data not shown).
Taken together, these results show that the VP1 and VP2
proteins are synthesized only in Cos cells transfected with the
pVP1-3
9
SV40 or pVP2-3
9
SV40 plasmid, not in Cos cells
trans-fected with the pVP1-3
9
VP or pVP2-3
9
VP plasmid. This
sug-gests that the sequence coding for the 3
9
UTR of the capsid
protein mRNAs is indeed involved in repressing capsid protein
gene expression.
No inhibition of transcription or nucleocytoplasmic
trans-port, and no destabilization of transcripts.
To test the
possi-bility that the repression of capsid protein synthesis in
trans-fected cells was due to the inhibition of transcription of the
corresponding genes, we explored Cos cells transfected with
the pVP2-3
9
VP or pVP2-3
9
SV40 plasmid for the presence of
the capsid protein mRNAs, using in situ RNA hybridization.
The results showed that the digoxigenin-labeled VP2 probe
hybridized with about 30% of the cells transfected with the
pVP2-3
9
VP or pVP2-3
9
SV40 plasmid (Fig. 3A). Identical data
(not shown) were obtained for cells transfected with the VP1
plasmids. No hybridization signal was detected in cells
incu-bated with RNase A prior to hybridization. The
digoxigenin-labeled NS1 probe also hybridized with about 30% of the cells
transfected with the positive-control pNS1-3
9
SV40 plasmid, a
result consistent with the immunofluorescence data. The cells
transfected with the pSG5 plasmid and the control
mock-trans-fected cells did not hybridize with the probes (data not shown).
These results indicate that the VP1 and VP2 genes are
tran-scribed, even from the transfected pVP1-3
9
VP and pVP2-3
9
VP
plasmids. Therefore, the absence of capsid protein synthesis is
not due to the inhibition of transcription but might result from
inefficient nucleocytoplasmic transport of the corresponding
mRNAs. As shown in Fig. 3A, the hybridization signal varied
from cell to cell, ranging from staining of intranuclear foci only
to overall staining of both the nucleus and the cytoplasm of
cells transfected with the pVP2-3
9
VP, pVP2-3
9
SV40, or
pNS1-FIG. 2. (A) Indirect immunofluorescence assay. Cos cells were transfected for 64 h with the pVP2-39VP, pVP2-39SV40, or pNS1-39SV40 plasmid, as indi-cated. The VP2 protein was revealed with human anti-B19 virus serum from a patient in the convalescent phase after B19 virus infection, and then with FITC-conjugated goat anti-human antibody. The NS1 protein was detected with a specific NS1 polyclonal antibody. Magnification,3400. (B) Western blot analy-sis. Proteins were extracted from Cos cells which had been transfected for 64 h with the pVP1-39VP, pVP1-39SV40, pVP2-39VP, or pVP2-39SV40 plasmid, as indicated. Proteins were separated by gel electrophoresis and blotted onto a polyvinylidene difluoride membrane. Membranes were first incubated with an anti-VP1 polyclonal antibody (left panel) or with an anti-VP1 and -VP2 poly-clonal antibody (right panel), and then with alkaline phosphatase-conjugated goat anti-rabbit antibody. The positions of the VP1 and VP2 proteins at 84 and 58 kDa, respectively, are indicated by arrows to the right of the panels. The positions of the molecular size markers are indicated in kilodaltons to the left ofthe panels.
on November 9, 2019 by guest
http://jvi.asm.org/
3
9
SV40 plasmid. However, the same percentage of
nucleocy-toplasmic staining was observed in cells transfected with the
pVP2-3
9
VP, pVP2-3
9
SV40, or pNS1-3
9
SV40 plasmid,
indicat-ing that there was no defect in the nuclear export of the capsid
protein mRNAs containing the homologous 3
9
UTR. Identical
results were obtained for cells transfected with the pVP1-3
9
VP
and pVP1-3
9
SV40 plasmids (data not shown).
To ascertain whether or not the failure to detect the capsid
proteins in Cos cells transfected with the pVP1-3
9
VP or
pVP2-3
9
VP plasmid was due to a low level of the corresponding
mRNAs, we estimated the amount of the VP2 cytoplasmic
polyadenylated RNAs in cells transfected with the pVP2-3
9
VP
or pVP2-3
9
SV40 plasmid. The pNS1-3
9
SV40 plasmid was used
as a positive control. The results of Northern blot analysis (Fig.
3B) showed that there was no obvious difference in the
amount of the VP2 cytoplasmic mRNAs between cells
trans-fected with the pVP2-3
9
VP plasmid and cells transfected
with the pVP2-3
9
SV40 plasmid. The difference in size between
the VP2 mRNAs transcribed from the pVP2-3
9
VP plasmid and
those transcribed from the pVP2-3
9
SV40 plasmid was due to
the difference in size between the corresponding 3
9
UTRs.
Identical results were obtained for cells transfected with the
VP1 plasmids (data not shown). In addition, the results of
cycloheximide or actinomycin D treatment of the transfected
cells revealed no obvious difference between the synthesis or
degradation rates of the VP2-3
9
VP and VP2-3
9
SV40 mRNAs,
on the one hand, and those of the VP1-3
9
VP and VP1-3
9
SV40
mRNAs, on the other (data not shown).
Taken together, these results indicate that the absence of
capsid protein synthesis in Cos cells transfected with the
pVP1-3
9
VP or pVP2-3
9
VP plasmid cannot be the result of the
inhi-bition of transfected plasmid transcription, of nuclear
reten-tion, or of destabilization of the corresponding synthesized
mRNAs.
No inhibition of capsid protein mRNA translation in vitro.
To explore the ability of the pVP1-3
9
VP and pVP2-3
9
VP
plas-mids to regulate the synthesis of the VP1 and VP2 proteins,
these plasmids were used as templates in in vitro coupled
transcription and translation in the presence of reticulocyte
lysate. As shown in Fig. 4, the VP1 and VP2 proteins were of
the expected sizes, i.e., 84 and 58 kDa, respectively, and they
were synthesized from the capsid protein expression plasmids
whether they contained the DNA sequence coding for the 3
9
UTR of the capsid protein mRNAs or that coding for the 3
9
UTR of the SV40 early mRNAs. This finding indicates that in
reticulocyte lysate, the plasmids containing the DNA sequence
coding for the 3
9
UTR of the capsid protein mRNAs are
transcribed, and the corresponding mRNAs are able to
regu-late protein synthesis. It also shows that the 3
9
UTR of the
capsid protein mRNAs does not inhibit translation in an in
vitro system using erythroid cell lysate.
Inhibition of capsid protein mRNA translation in vivo.
The
results described above strongly suggest that in cells
trans-fected with the pVP1(2)-3
9
SV40 or pVP1(2)-3
9
VP plasmid,
VP1 and VP2 protein synthesis is regulated at the translational
level. To verify this hypothesis, the recruitment of the
corre-sponding mRNAs by the translational apparatus was
investi-gated. Cos cells were transfected with the pVP2-3
9
VP or
pVP2-3
9
SV40 plasmid, or with the pNS1-3
9
SV40
positive-con-trol plasmid. Forty hours after transfection, postmitochondrial
supernatants were separated into 20 fractions after
centrifuga-tion on sucrose gradients, and total RNAs were extracted from
these fractions. The distribution of the various mRNAs of
interest among the different fractions was visualized by
North-ern blot analysis and quantified by densitometry scanning of
FIG. 3. Analysis of mRNAs. (A) In situ RNA hybridization. Cos cells weretransfected for 40 h with the pVP2-39VP, pVP2-39SV40, or pNS1-39SV40 plas-mid, as indicated. Harvested cells were digested by DNase and then hybridized with a digoxigenin-labeled VP2 probe under nondenaturing conditions. Staining was performed with antidigoxigenin antibody labeled with alkaline phosphatase, with BCIP-NBT as a substrate. The hybridization signal was detected at an original magnification of3400. Cells transfected with the pNS1-39SV40 positive-control plasmid were hybridized to the NS1 probe. (B) Northern blot analysis of steady-state cytoplasmic poly(A)1RNAs. Cytoplasmic poly(A)1RNAs were
purified from Cos cells transfected for 40 h with the indicated plasmids. Each transfection was performed in duplicate, and all experiments were repeated three times. Two micrograms of each mRNA preparation was separated by electro-phoresis through a formaldehyde-agarose gel and blotted onto a nitrocellulose membrane. The membranes were first hybridized to a32P-labeled VP2 probe
(left panel) or an NS1 probe (right panel) and were submitted to autoradiogra-phy for 1 day (upper panels). The same membranes were then hybridized to a
32P-labeled GAPDH probe as a control of the mRNA level and were submitted
[image:5.612.50.286.105.313.2]to autoradiography for 1 h (lower panels). The amount of probe hybridized with the VP2 mRNAs was normalized to the extent of hybridization with the GAPDH probe. Arrows to the right of the panels indicate the corresponding mRNAs. The position of the 18S ribosomal RNA is indicated to the left of each panel.
FIG. 4. In vitro coupled transcription-translation assays. The pVP1-39VP, pVP1-39SV40, pVP2-39VP, and pVP2-39SV40 plasmids were used as templates in the TNT T7 coupled reticulocyte lysate system (Promega) in the presence of [35S]methionine. The pSG5 plasmid was used as a negative control. Synthesized
proteins were separated by PAGE and visualized after autoradiography of the dried gel. Arrows on the right point to the positions of the35S-labeled VP1 and
VP2 proteins of the expected sizes. The positions of the molecular size markers are indicated in kilodaltons on the left.
on November 9, 2019 by guest
http://jvi.asm.org/
the membranes. The results (Fig. 5B and C) show that the
VP2-3
9
VP mRNAs were distributed among fractions 1 to 7 and
that more than 66% of the VP2-3
9
VP mRNAs were contained
in fractions 1 and 2, which corresponded to free mRNA
mol-ecules. The remaining mRNA molecules were found in
frac-tions 3 to 7, corresponding to mRNAs associated with one 40S
ribosomal subunit (fractions 3 and 4) or with one ribosome
(fractions 6 and 7). No mRNA molecules were associated with
active polyribosomes (fractions 8 to 20). This means that the
VP2-3
9
VP mRNA molecules are not engaged in translation at
all. On the other hand, the VP2-3
9
SV40 mRNAs were
distrib-uted unevenly among fractions 3 to 20. About 27% of the
VP2-3
9
SV40 mRNA molecules were associated with the 40S
ribosomal subunit, 31% were associated with 80S ribosomes,
and 42% were associated with polyribosomes. Figure 5C
(up-per panel) clearly shows that all the VP2-3
9
SV40 mRNA
mol-ecules were actively engaged in translation, whereas the
VP2-3
9
VP mRNA molecules were not. The pNS1-3
9
SV40 plasmid
was used as a positive control, and the results in Fig. 5B show
that the NS1-3
9
SV40 mRNA molecules were unevenly
distrib-uted among fractions 4 to 20. Only 13 and 20% of the
NS1-3
9
SV40 mRNAs were associated with the 40S ribosomal
sub-unit and with 80S ribosomes, respectively. More than 67%
were associated with polyribosomes actively engaged in
trans-lation. The distribution of the GAPDH mRNAs among
poly-ribosomal subfractions was determined in each experiment as
a control of mRNA distribution. The results in Fig. 5C (lower
panel) show no significant difference in the recruitment of the
GAPDH mRNAs by the translational apparatus between Cos
cells transfected with the various plasmids. These data reveal
that the VP2 mRNA molecules which contain the homologous
3
9
UTR are not recruited at all by active polyribosomes. On the
contrary, the VP2 mRNA molecules which contain the
heter-ologous 3
9
UTR of SV40 mRNAs are efficiently translated.
This indicates that the 3
9
UTR of the capsid protein mRNAs
inhibits ribosome loading and that the replacement of the 3
9
UTR of the capsid protein mRNAs with that of the SV40
mRNAs abolishes this inhibition.
DISCUSSION
We examined the role of the 3
9
UTR of the capsid protein
mRNAs in regulating the translation of the capsid protein
mRNAs in nonpermissive cells. We used Cos cells because
even though human megakaryocytes are the main
nonpermis-sive cells in vivo, the transfection of the B19 virus genome into
Cos cells reproduces the nonpermissive viral transcriptional
and translational pattern (1, 17, 27). The data obtained from
our transient expression assays show that the 3
9
UTR of the
capsid protein mRNAs inhibits translation of the
correspond-ing mRNAs. We found that in Cos cells transfected with the
VP1 or VP2 expression plasmid harboring the sequence coding
for the homologous 3
9
UTR, the capsid proteins were not
detectable. This cannot be due to the inhibition of
transcrip-tion, because the corresponding mRNAs were synthesized. In
addition, the results of in situ hybridization showed no
signif-icant difference between the relative nucleocytoplasmic
distri-butions of the VP1 and VP2 transcripts, whether or not they
contained the 3
9
UTR of the capsid protein mRNAs.
Further-more, the results of Northern blotting revealed that the VP1
and VP2 mRNAs synthesized from plasmids, with or without
the sequence coding for the homologous 3
9
UTR, were
de-graded at the same rate, as shown by their very similar
steady-state amounts. Therefore, the absence of the capsid proteins in
cells transfected with the pVP1-3
9
VP or pVP2-3
9
VP plasmid
cannot result from the inhibition of the nucleocytoplasmic
transport of the transcripts or from a decrease in the stability
of the VP1-3
9
VP or pVP2-3
9
VP mRNAs. Taken together,
these results strongly suggest that in transfected Cos cells, the
capsid protein mRNAs are differentially translated, depending
on their 3
9
UTR. In this connection, polyribosome analysis
FIG. 5. Recruitment of the VP2 or NS1 mRNAs by the translationalappa-ratus. Cos6 cells were transfected for 40 h with the pVP2-39VP, pVP2-39SV40, or pNS1-39SV40 plasmid. The postmitochondrial supernatants were layered onto 10 to 40% linear sucrose gradients. After centrifugation, 20 fractions of identical volume were collected. RNAs were extracted from each fraction, and the distri-bution of VP2 or NS1 mRNAs among the different fractions was revealed by Northern blotting with the relevant32P-labeled VP2- or NS1-specific DNA
probe. Membranes were sequentially hybridized with the corresponding VP2 or NS1 probe and were submitted to autoradiography for 1 day. All membranes were then hybridized to the32P-labeled GAPDH probe as a control for mRNA
distribution and were submitted to autoradiography for 1 day. (A) Typical UV absorbance profile of polyribosomes from Cos cells transfected with the pVP2-39SV40 plasmid. Fraction numbers are indicated under the profile, with fraction 1 at the top of the gradient. The 40S, 60S, and 80S positions, as well as the positions of polyribosomes, are specified. (B) Autoradiograms of membranes after hybridization with the VP2 probe (top and middle panels) or with the NS1 probe (bottom panel). On the right, VP2-39VP, VP2-39SV40, and NS1-39SV40 refer to mRNAs transcribed from the pVP2-39VP, pVP2-39SV40, and pNS1-39SV40 transfected plasmids, respectively. (C) Quantification of the VP2-39VP, VP2-39SV40, and NS1-39SV40 mRNAs among the polyribosomal fractions (up-per panel) and of control GAPDH mRNAs from each ex(up-periment (lower panel). After autoradiography, the indicated mRNAs were quantified in each fraction by scanning densitometry of the corresponding membranes on a molecular imager system. Results are given as the amount of total radioactivity in each fraction expressed as a percentage of the sum of the radioactivity in all 20 fractions.
on November 9, 2019 by guest
http://jvi.asm.org/
indeed showed that the capsid protein mRNA molecules which
contained the 3
9
UTR of SV40 early mRNAs were associated
with a large number of ribosomes, indicating that they are
efficiently translated. On the other hand, the capsid protein
mRNA molecules containing the 3
9
UTR of the VP1 and VP2
mRNAs had no connection whatever with any ribosomal
sub-unit. This suggests that the 3
9
UTR of the VP1 and VP2
mRNAs suppresses capsid protein mRNA translation by
in-hibiting one of the steps required to bind the 40S ribosomal
subunit. If true, this would suffice to explain the complete
inhibition of capsid protein synthesis in transfected Cos cells.
Other 3
9
UTRs have been shown to inhibit protein synthesis,
and some were found to affect mRNA nucleocytoplasmic
transport specifically (9, 32, 33). However, this was not the case
here for the 3
9
UTR of the B19 virus capsid protein mRNAs in
transfected Cos cells. Many of the elements inside transcripts
that control mRNA stability have been localized in the 3
9
UTR
(5). Analysis of the 3
9
UTR sequence of the B19 virus capsid
protein mRNAs revealed the presence of three AUUUA
se-quences (AU-rich elements [AREs]) already described in the
3
9
UTRs of several cellular and viral mRNAs (34). These
sequences are known to promote mRNA degradation by
stim-ulating rapid deadenylation, at speeds depending on the
cel-lular context (23, 36). However, these specific elements did not
seem to affect VP1 or VP2 mRNA turnover in transfected Cos
cells. Other 3
9
UTRs have been shown to affect translational
initiation, for example, during the early development or
differ-entiation of somatic cells (12). Translational repression via the
3
9
UTR can be achieved by several mechanisms (45). One
example is the 3
9
UTR of the rabbit erythroid 15-lipoxygenase
mRNA, which helps to inhibit translation during the early
stages of erythropoiesis (25). The present findings for in vitro
coupled transcription-translation show that capsid proteins are
synthesized from VP1 and VP2 mRNAs containing the 3
9
UTR of the capsid protein mRNAs, indicating that the 3
9
UTR
of the capsid protein mRNAs does not inhibit translation in
rabbit reticulocyte lysate. This suggests that cellular factors are
involved either in inhibiting the translation of the capsid
pro-tein mRNAs in Cos cells or in stimulating the translation in
erythroid cell lysate. It also suggests that the susceptibility of
erythroid cells to B19 virus infection depends on one or several
of the cellular factors involved in the translation of viral
mR-NAs.
Our results demonstrate that the inhibition of capsid protein
synthesis in Cos cells transfected with the pVP1(2)-3
9
VP
plas-mids is due to the 3
9
UTR of the capsid protein mRNAs, which
prevents the translation of the corresponding mRNAs.
How-ever, the 3
9
UTR of these mRNAs is not the only element in
the VP1 and VP2 mRNAs which contributes to the inhibition
of capsid protein synthesis. Under our experimental
condi-tions, the NS1 and the VP1 as well as the VP2 mRNAs
syn-thesized from the pNS1-3
9
SV40 and pVP1(2)-3
9
SV40
plas-mids, which contain the sequence coding for the 3
9
UTR of the
SV40 early mRNAs, were detected in about 30% of the
trans-fected Cos cells. Nevertheless, the NS1 protein was detected in
30% of the cells, whereas the VP1 and VP2 proteins were
detected in only 5% of the cells. This was not due to the
presence of an inefficient translation initiation codon in the
capsid protein mRNAs, because the replacement of the
weak AUG initiation codon with the GCCACCAUGG Kozak
consensus sequence (13) did not increase VP1 or VP2 protein
synthesis (data not shown). Furthermore, polyribosome
anal-ysis showed that the NS1-3
9
SV40 mRNA molecules were more
efficiently translated than the VP2-3
9
SV40 mRNA molecules.
This could suggest that the ORF is also involved in regulating
capsid protein mRNA translation. Although it has been
dem-onstrated that the AAV rep gene acts in cis or in trans as a
translational inhibitor of mRNAs from the AAV p40 promoter
(42), the NS1 protein of the B19 virus could stimulate
trans-lation of mRNAs containing the 3
9
UTR of the SV40 early
mRNAs. However, we failed to detect any stimulation in
dou-ble transfection experiments with the pNS1-3
9
SV40 and
pVP2-3
9
SV40 plasmids (data not shown).
Taken together, these findings could indicate that the
inhib-itory effect of the 3
9
UTR of the capsid protein mRNAs on the
translation of the VP1(2)-3
9
VP mRNAs is due to a mechanism
which involves the 3
9
UTR and possibly the homologous ORF.
Computer analysis using the RNA folding form program by M.
Zuker revealed the formation of a structured region which
might participate in the inhibition of capsid protein mRNA
translation in nonpermissive cells. Mutants in the 3
9
UTR and
the ORF will be useful for determination of the sequence
involved in inhibiting capsid protein mRNA translation and for
identification of the cellular factor(s) necessary for structural
protein mRNA translation. In conclusion, the present results
showed that the translation of the mRNAs synthesized by our
plasmids containing the sequence coding for the 3
9
UTR of the
capsid protein mRNAs was inhibited. The same might apply to
the infection of nonpermissive cells by the B19 virus.
Conse-quently, in nonpermissive cells, the true B19 virus mRNAs
might not be translated because of their 3
9
UTR.
ACKNOWLEDGMENTS
We thank J.-J. Madjar (CNRS UMR 5537, Lyon, France) for critical
reading of the manuscript and C. Gazin (INSERM U 462, Paris,
France) and U. Hazan (INSERM U 380, Paris, France) for helpful
advice and discussion. We are also grateful to B. Cohen (Colindale,
London, United Kingdom) and S. Kajigaya (National Institutes of
Health, Bethesda, Md.) for generous gifts of capsid protein antibodies,
and to C. Astell (University of British Columbia, Vancouver, Canada)
for the generous gift of NS1 antibody, gifts without which this study
would not have been possible. Lastly, we thank M. Santillana-Hayat
(CNRS UPR 9051, Paris, France) for technical assistance in antibody
production, the Laboratoire Photographique d’He´matologie for
pho-tographic work, and M. Schmid and C. Doliger for scanning
densitom-etry analysis.
The work of C. Pallier was supported by FRM, AP-HP/CNRS, and
INSERM fellowships.
REFERENCES
1. Beard, C., J. St. Amand, and C. R. Astell. 1989. Transient expression of B19 parvovirus gene products in COS-7 cells transfected with B19–SV40 hybrid vectors. Virology 172:659–664.
2. Blundell, M. C., C. Beard, and C. R. Astell. 1987. In vitro identification of a B19 parvovirus promoter. Virology 157:534–538.
3. Brown, K. E., J. R. Hibbs, G. Gallinella, S. M. Anderson, E. D. Lehman, P.
McCarthy, and N. S. Young.1994. Resistance to parvovirus B19 infection due to lack of virus receptor (erythrocyte P antigen). N. Engl. J. Med.
330:1192–1196.
4. Cotmore, S. F., V. C. McKie, L. J. Anderson, C. R. Astell, and P. Tattersall. 1986. Identification of the major structural and nonstructural proteins en-coded by human parvovirus B19 and mapping of their genes by procaryotic expression of isolated genomic fragments. J. Virol. 60:548–557.
5. Decker, C. J., and R. Parker. 1994. Mechanisms of mRNA degradation in eukaryotes. Trends Biochem. Sci. 19:336–340.
6. Diaz, J. J., D. Simonin, T. Masse, P. Deviller, K. Kindbeiter, L. Denoroy, and
J. J. Madjar.1993. The herpes simplex virus type 1 US11 gene product is a phosphorylated protein found to be non-specifically associated with both ribosomal subunits. J. Gen. Virol. 74:397–406.
7. Doerig, C., B. Hirt, J.-P. Antonietti, and P. Beard. 1990. Nonstructural protein of parvoviruses B19 and minute virus of mice controls transcription. J. Virol. 64:387–396.
8. Fourney, R. M., J. Miyakoshi, R. S. Day, and M. C. Paterson. 1988. Northern blotting: efficient RNA staining and transfer. Focus 10:5–7.
9. Furth, P. A., and C. C. Baker. 1991. An element in the bovine papillomavirus late 39untranslated region reduces polyadenylated cytoplasmic RNA levels. J. Virol. 65:5806–5812.
10. Giron, M.-L., F. Rozain, M.-C. Debons-Guillemin, M. Canivet, J. Peries, and
on November 9, 2019 by guest
http://jvi.asm.org/
R. Emanoil-Ravier.1993. Human foamy virus polypeptides: identification of
env and bel gene products. J. Virol. 67:3596–3600.
11. Greco, A., D. Simonin, J. J. Diaz, L. Barjhoux, K. Kindbeiter, J. J. Madjar,
and T. Masse.1994. The DNA sequence coding for the 59 untranslated region of herpes simplex virus type 1 ICP22 mRNA mediates a high level of gene expression. J. Gen. Virol. 75:1693–1702.
12. Hentze, M. W. 1995. Translational regulation: versatile mechanisms for met-abolic and developmental control. Curr. Opin. Cell Biol. 7:393–398. 13. Kozak, M. 1991. Structural features in eucaryotic mRNAs that modulate the
initiation of translation. J. Biol. Chem. 266:19867–19870.
14. Laemmli, U. K. 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680–685.
15. Leruez-Ville, M., I. Vassias, C. Pallier, A. Cecille, U. Hazan, and F. Morinet. 1997. Establishment of a cell line expressing human parvovirus B19 non-structural protein from an inducible promoter. J. Gen. Virol. 78:215–219. 16. Liu, J. M., H. Fujii, S. W. Green, N. Komatsu, N. S. Young, and T. Shimada.
1991. Indiscriminate activity from the B19 parvovirus p6 promoter in non-permissive cells. Virology 182:361–364.
17. Liu, J. M., S. W. Green, T. Shimada, and N. S. Young. 1992. A block in full-length transcript maturation in cells nonpermissive for B19 parvovirus. J. Virol. 66:4686–4692.
18. Luo, W., and C. R. Astell. 1993. A novel protein encoded by small RNAs of parvovirus B19. Virology 195:448–455.
19. Momoeda, M., S. Wong, M. Kawase, N. S. Young, and S. Kajigaya. 1994. A putative nucleoside triphosphate-binding domain in the nonstructural pro-tein of B19 parvovirus is required for cytotoxicity. J. Virol. 68:8443–8446. 20. Morey, A. L., J. W. Keeling, H. J. Porter, and K. A. Fleming. 1992. Clinical
and histopathological features of parvovirus B19 infection in the human fetus. Br. J. Obstet. Gynaecol. 99:566–574.
21. Morinet, F., L. D’Auriol, J. D. Tratschin, and F. Galibert. 1989. Expression of the human parvovirus B19 protein fused to protein A in Escherichia coli: recognition by IgM and IgG antibodies in human sera. J. Gen. Virol. 70: 3091–3097.
22. Mortimer, P. P., R. K. Humphries, J. G. Moore, R. H. Purcell, and N. S.
Young.1983. A human parvovirus-like virus inhibits haematopoietic colony formation in vitro. Nature 302:426–429.
23. Muhlrad, D., C. J. Decker, and R. Parker. 1994. Deadenylation of the unstable mRNA encoded by the yeast MFA2 gene leads to decapping fol-lowed by 59339digestion of the transcript. Genes Dev. 8:855–866. 24. Naeger, L. K., J. Cater, and D. J. Pintel. 1990. The small nonstructural
protein (NS2) of the parvovirus minute virus of mice is required for efficient DNA replication and infectious virus production in a cell-type-specific man-ner. J. Virol. 64:6166–6175.
25. Ostareck-Lederer, A., D. H. Ostareck, N. Standart, and B. J. Thiele. 1994. Translation of 15-lipoxygenase mRNA is inhibited by a protein that binds to a repeated sequence in the 39untranslated region. EMBO J. 13:1476–1481. 26. Ozawa, K., J. Ayub, Y. S. Hao, G. Kurtzman, T. Shimada, and N. Young. 1987. Novel transcription map for the B19 (human) pathogenic parvovirus. J. Virol. 61:2395–2406.
27. Ozawa, K., J. Ayub, S. Kajigaya, T. Shimada, and N. Young. 1988. The gene encoding the nonstructural protein of B19 (human) parvovirus may be lethal in transfected cells. J. Virol. 62:2884–2889.
28. Ozawa, K., J. Ayub, and N. Young. 1988. Translational regulation of B19 parvovirus capsid protein production by multiple upstream AUG triplets.
J. Biol. Chem. 263:10922–10926.
29. Ozawa, K., G. Kurtzman, and N. Young. 1986. Replication of the B19 parvovirus in human bone marrow cell cultures. Science 233:883–886. 30. Ozawa, K., and N. Young. 1987. Characterization of capsid and noncapsid
proteins of B19 parvovirus propagated in human erythroid bone marrow cell cultures. J. Virol. 61:2627–2630.
31. Palmer, P., C. Pallier, M. Leruez-Ville, M. Deplanche, and F. Morinet. 1996. Antibody response to human parvovirus B19 in patients with primary infec-tion by immunoblot assay with recombinant proteins. Clin. Diagn. Lab. Immunol. 3:236–238.
32. Patzel, V., and G. Sczakiel. 1997. The hepatitis B virus post-transcriptional regulatory element contains a highly stable RNA secondary structure. Bio-chem. Biophys. Res. Commun. 231:864–867.
33. Pavlakis, G. N., and B. K. Felber. 1990. Regulation of expression of human immunodeficiency virus. New Biol. 2:20–31.
34. Ross, J. 1996. Control of messenger RNA stability in higher eukaryotes. Trends Genet. 12:171–175.
35. Shade, R. O., M. C. Blundell, S. F. Cotmore, P. Tattersall, and C. R. Astell. 1986. Nucleotide sequence and genome organization of human parvovirus B19 isolated from the serum of a child during aplastic crisis. J. Virol. 58: 921–936.
36. Shyu, A. B., J. G. Belasco, and M. E. Greenberg. 1991. Two distinct desta-bilizing elements in the c-fos message trigger deadenylation as a first step in rapid mRNA decay. Genes Dev. 5:221–231.
37. Snyder, R. O., D.-S. Im, and N. Muzyczka. 1990. Evidence for covalent attachment of the adeno-associated virus (AAV) Rep protein to the ends of the AAV genome. J. Virol. 64:6204–6213.
38. Sol, N., F. Morinet, M. Alizon, and U. Hazan. 1993. Trans-activation of the long terminal repeat of human immunodeficiency virus type 1 by the parvo-virus B19 NS1 gene product. J. Gen. Virol. 74:2011–2014.
39. Srivastava, A., E. Bruno, R. Briddell, R. Cooper, C. Srivastava, K. van
Besien, and R. Hoffman.1990. Parvovirus B19-induced perturbation of hu-man megakaryocytopoiesis in vitro. Blood 76:1997–2004.
40. St. Amand, J., and C. R. Astell. 1993. Identification and characterization of a family of 11-kDa proteins encoded by the human parvovirus B19. Virology
192:121–131.
41. Takahashi, T., K. Ozawa, K. Takahashi, S. Asano, and F. Takaku. 1990. Susceptibility of human erythropoietic cells to B19 parvovirus in vitro in-creases with differentiation. Blood 75:603–610.
42. Trempe, J. P., and B. J. Carter. 1988. Regulation of adeno-associated virus gene expression in 293 cells: control of mRNA abundance and translation. J. Virol. 62:68–74.
43. Van Der Vlied, P. 1991. The role of cellular transcription factors in the enhancement of adenovirus DNA replication. Semin. Virol. 2:271–280. 44. Vassias, I., S. Perol, L. Coulombel, M. C. Thebault, P. H. Lagrange, and F.
Morinet.1993. An in situ hybridization technique for the study of B19 human parvovirus replication in bone marrow cell cultures. J. Virol. Methods
44:329–338.
45. Wickens, M., J. Kimble, and S. Strickland. 1996. Monograph 15, p. 411–450.
In J. Hershey, M. Mathews, and N. Sonenberg (ed.), Translational control.
Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. 46. Wisdom, R., and W. Lee. 1991. The protein-coding region of c-myc mRNA
contains a sequence that specifies rapid mRNA turnover and induction by protein synthesis inhibitors. Genes Dev. 5:232–243.
