Host-Mediated Repair of Discontinuities in DNA From T4 Bacteriophage

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Vol.12,No. 2 PrintedinU.S.A.

Host-Mediated Repair

of Discontinuities

in

DNA

From T4 Bacteriophage

K. CARLSON,I Z. K. LORKIEWICZ, AND A. W. KOZINSKI

Department of Human Genetics, Universityof Pennsylvania School of Medicine, Philadelphia,

Pennsylvania 19104

Received forpublication21February 1973

Discontinuities of T4 DNA whicharecaused byexcisionofUV-damagedareas,

by decay of 32p atoms, or which are present in DNA from

rII-lig.m-

phage produced inahostnonpermissive for ambermutantsareall repaired by bacterial

enzymes after infection in the presence of chloramphenicol. Escherichia coli DNA polymerase I participates in the host-mediated repair, but an approxi-mately 20-fold variation in the levels of host polynucleotide ligase does

not affect either the kinetics ortheextent of repairobserved. Upon removal of chloramphenicol, host-repaired DNA from UV-irradiated phage undergoes a secondarycycle of breakage, which ultimately results in solubilization ofmostof thephage DNA. If the cellsareco-infected withnonirradiated helper phage, the

secondary breaks arerepaired and the continuity of the polynucleotide chain is restored. The close coincidence in theextentofprimary and secondarybreakage

suggests that phage-coded enzymes recognize and excise areas improperly

repaired by the host. In contrast to host-mediated repair, repair mediated by rescuingphage probably restored functionality tothe damaged DNA.

Although bacteriophage T4, after infectionof

bacterial cells, codes for a number ofenzymes

involved in its DNA metabolism (review in

reference 19), it is quite

possible

that some of

the enzymes involved in this process may be injected into the cell together with the phage DNA upon infection or be of bacterial origin. This possibility was

explored by

Kozinski and Lorkiewicz in 1967 in a

study

of

intracellular

breakage

andrepair of

UV-damaged

DNA(15). ("Break" and

"breakage"

refer to single-stranded discontinuities in DNA with no im-plications tothe natureandsize ofthis discon-tinuity.

"Repair"

refers to the restoration of

integrity of broken

DNA.)

The authors found

that when the synthesis of new proteins was

inhibited by the addition of chloramphenicol (CM) shortly before infection, DNA from UV-irradiated phage acquiredsingle-standed inter-ruptions shortly after infection. (DNA of the radiation-sensitive mutant V,

[11]

didnot

ac-quire such breaks.) The broken strands were subsequently repairedto full integrity, still in the presence of CM. The results were

inter-preted to indicate that bacterial enzymes

per-formed both breakageandrepair.

IPresent address: McArdle Laboratory for Cancer

Re-search University of Wisconsin Medical Center Madison,

Wis.53706.

This work

provided

the first indication that DNAfrom a virulent

phage

can berepaired

by

host enzymes. More recentexperimentsin this laboratory have shown that a phage enzyme

injected with the DNA is

responsible

for the intracellular

breakage

of

UV-damaged

DNA

(R.

Shames,

Z. K.

Lorkiewicz,

and A. W.

Kozinski,

J. Virol., in press). This enzyme is the

product

of the v gene in

T4,

is found in the mature phage, and has

endonucleolytic

activity specific for

UV-damages

when

assayed

in vitro

(C.

J.

Castillo,

Ph.D.

thesis,

University

of

Pennsyl-vania, in preparation).

Theobjective ofthe present communication

is to characterize the repair event

following

breakage. Evidence will be

presented

showing

thatseveral different types of

damage

in

phage

DNA can be

repaired,

and that the enzymes

responsiblefortherestoration of

integrity

areof

bacterial origin. Escherichia coliDNA

polymer-ase Iparticipates inthis

function,

buta20-fold variation in the levels ofbacterial

polynucleo-tide ligase does not affect either kinetics or extent ofrepair. The host-repaired DNA from

UV-damaged phageisgeneticallyincompetent, and only a second round of phage-directed excision repair restores functionality to the

damagedDNA. 310

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REPAIROF PHAGE T4 DNA

MATERIALS AND METHODS

Strains. The bacterial strains E. coli W3110

su-thy- and its derivatives P3478polA- anda

spontane-ousrevertantof thismutant(6)wereobtained from J.

Caims. E. coli strains N953 lac- galE-

galKam-trpam-araam- T6Rsu- and itsderivatives N1323lop8

andN1325 lop8 lig2 correspondtothe strainsN1071,

N1072 lop8, and N1252 lop8 lig2 of Gellert and

Bullock (9) which have been cured of their lambda

prophage. These strains were obtained through H.

Krisch. E. coli B23 isthenormal host for T4used in

thislaboratory.

ThebacteriophagesusedwereT4BO,r, anosmotic

shock-resistant mutant, and T4D r59 H39X, an

rII-ligam- doublemutant(2).

Phage preparation. Methods forpreparing

radio-active phage have been described (15, 16). (i) UV

irradiation of bacteriophage was performed with a

germicidal lamp (GE) without a filter, with adose

corresponding to 6 to 9 lethal hits. The phage was

suspendedin 0.15 MNaCl-0.01MTris-hydrochloride,

pH7.4, duringirradiation.

(ii) To obtain phages with DNA nicks due to 32p

decay, 32P-labeledphageofaspecific activityof 10to

20 mCi of phosphorus per mg were stored at a

concentration of 1010 particles per ml in 3 x D

medium (8)at+4C until about6to8single-stranded

scissions due to 32P decay had accumulated, as

determined by alkaline sucrose gradient

sedimenta-tion.

(iii) rII-ligam- B denotes phage which were

pre-pared byasingle growth cycleinE.coli B23 (2). This

strain isnonpermissive for the amber ligase mutation,

butpartial suppression of the amber phenotype by the

rII mutation results in productive infection. About

10%oftheresultingphagesareviable, and the phages

contain DNA with numerous single-stranded inter-ruptions (2).

Determination ofintracellularstrand integrity

ofphageDNA.Allexperimentswereperformedat37

C in TCG medium 16)at acell concentration of 3 x

108/ml.The medium wassupplementedwith

thymi-dine (TdR) at 5 ,ug/ml in experiments with

thy-strains and L-tryptophan at20/g/ml inexperiments

with trp- strains. CMwasaddedtothe culturesata

final concentration of 150jg/mlimmediately before

the cells were infected with 32P-labeled phage at a

multiplicity of 3 to 5 particles per cell. At desired

intervals after infection the samples were lysed

(together with 3H-labeled matureT4phage as

refer-ence) with lysozyme, Triton X-100, andNaOH to a

final concentration of 0.2 M (20). The lysates were

sedimented through alkaline sucrosegradients (5 to

20% in1M NaCl-0.2 MNaOH; 26,700rpmfor 3h ina

SpincoSW50rotor).Thedistance sedimentedbythe

intracellular DNA molecules relativetothe distance sedimentedbythe marker DNA(D2DJ)(4)serves as a measureof strandintegrity.

Enzymological techniques. DNA extracted from

phagewithphenolwasdialyzed against1mMEDTA,

pH 7.4, andincubated with T4polynucleotide ligase

bythe method of Weiss and Richardson(25). Incuba-tion with T4 DNApolymerase wasbythe method of

Aposhian (1). After conclusion of the enzymatic

reactions the mixtures were againdialyzed against1

mM EDTA and analyzed onalkaline sucrose

gradi-ents.

RESULTS

Intraceliular repair of different types of

single-stranded discontinuities in phage DNA. It was demonstrated previously that,

shortly after infection in the presence of CM, UV-damaged T4 DNA acquires a number of

single-stranded nicks (15) correspondingto the

number of lethal hits (17). These nicks are

repaired upon prolonged incubation (15). To test the generality of such repair, we prepared

phages with different types ofinterruptions in

their DNA.

(i) Breaks in DNA due to 32P-Decay.

Lit-win etal.(17) showed that there is aone-to-one

relationship between the numberofdecayed32P atoms and the number of resulting single-stranded interruptions, which indicates that there is a chain break adjacent to the decayed

32p.

Decayed phage were prepared by allowing 32P-labeled phage (specific activity 20 mCi of phosphorus per mg to age in 3 x D medium. Such phage acquired a number of single-stranded breaks (Fig. 1A). Bacteria were

in-fected (MOI 1) with the decayed phage in the presence ofCM. Thirtyminuteslaterthe

integ-rity of thephage DNAwasanalyzed (Fig. 1B).

Most ofthe radioactivity now coincided fairly well with the integral reference. This indicates that discontinuities due to 32p decay can be

repaired.

(ii) Discontinuities in DNA from rll-ligam- B

phage.

In a bacterial host

non-permissivefor ambermutants, partial suppres-sion ofthe amberligasemutationby rII muta-tionsallows the productionof maturephages(2, 7,13).Such phages, however, contain

discontin-uousDNA (2). Thenatureofthese

discontinui-ties is not known. Toobtain informationabout this, wetested therepair ofsuchdiscontinuous DNA invitroby subjecting the DNA to

repair-ing enzymes. DNAextractedfromrII-ligam-.B phage (Fig. 2A, insert) was incubated withT4

polynucleotide ligaseor a combination ofboth ligase andT4DNApolymerase. It was expected that asimple break not involving loss of nucleo-tides should be repaired by ligase alone, whereas a combination of the two enzymes wouldberequired if the break consisted of gaps in thepolynucleotide chain. Sampleswerethen

subjected to alkalinesucrose gradient analysis.

Ligase alone (Fig. 2A) could not effect any repair of the DNA. A combination ofthe two enzymes, however, achieved significant extent

311

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CARLSON, LORKIEWICZ, ANDKOZINSKI

A. Parental Phage

^

D2/Di

=.63

3H-

\

+

B. Intracellular at 30'

78

I I

I I I I

IDD

.

1.0

FIG. 1. Repairof breaks inflicted by 32Pdecay in T4 DNA. 3P-labeledphage (specific activity 20 mCi of

phosphoruspermg)wasstoredin3x Dmedium allowing limiteddecaytooccur.The decayedphagewasused

toinfect E. coli B in thepresenceofCM (150

gg/ml).

Thirty minutesafter infectionpartof the infected cells

were lysed and analyzed on alkaline sucrose gradients. A, Analysis of the parental phage used for the

experiment; B, intracellularDNA from thesamephage. Note significantrepair. 3Hrepresentsreference DNA

from mature T4 phage.

ofrepair(Fig. 2B).Thisresultindicatesthatthe discontinuities in rII-ligam- .B DNA are gaps, not nicks, requiring both repair synthesis and

sealing.

The extent of in vivo repair of this same

phageDNAwastesteduponinfectionofE.coli

B (MOI 3) inthe presenceofCM.Intracellular DNA was analyzed on alkaline sucrose

gradi-ents (Fig. 2C andD). The intensityofrepairis

quitesimilartowhatwasobservedin vitrowith

acombination ofligaseandpolymerase (cf.Fig.

2B andC). Note that extensive repairoccurred

already at 5 min. There is no further repairif incubation is prolonged beyond 5 min. In both

cases about 35% of the DNAmass isintegral. A

sizablefraction of the DNA isunrepairedinvivo

aswellasin vitro. This fractionmaycorrespond

to a fragmented moiety of DNA, i.e., where double-stranded scissions have occurred in

ad-dition to the single-stranded gaps. Invariably,

when DNA fromrII-lig- phageis extracted with

phenolandsedimentedthrough neutralsucrose gradients, a sizable fraction sediments more

slowly then integral DNA. It is uncertain

whether fragments are found in the phage, or

whetherthegappedDNAisunusually

suscepti-ble to fragmentation during extraction. The

extent ofgapping varies considerably from ex-periment to experiment. In other experiments wherethestartingmaterialwaslessbroken, full

restoration ofintegritywasobservedupon

infec-tion in thepresence ofCM.

Role of bacterial enzymes in repair of

discontinuous phage DNA. Although the

oc-currenceofrepairafter infection in the absence of new protein synthesis suggested that pre-existing (bacterial) enzymes were responsible (15), the host origin of the repairenzymes was

not proven. The repair may be effected by a phage protein injectedfrom theinfecting

parti-cle or by a protein the synthesis of which is unusually resistant to CM. The notion that

repairingenzymes areindeed of bacterialorigin

is supported by the experiments described as

follows.

Role of hostpolynucleotideligaseand DNA

polymerase I in repair. Polynucleotide ligase

sealsasingle-strandednickinbihelicalDNA in

vitro(26), joininga 3'OH end toa5'P end.This

enzyme, therefore, is an obvious candidate for

performingthe last step inrepair.

E.coli mutantsdeficientinDNApolymerase

I (6) are fully viable, but display increased

sensitivity to UV (6).The increased sensitivity

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CARLSON, LORKIEWICZ, AND KOZINSKI

is probably due to diminished repair after excsion ofUV-damaged DNA (12). Both a 5' to 3' and a 3' to 5' exonuclease are firmly as-sociated with polI; the combination of

exonu-clease and polymerase activities inthe purified

enzyme will excise and repair UV-damaged

DNA in vitro (14). These findings all suggest a

role of polI in repair synthesis and not in replication per se. The involvementofpolIand

ligase in the repair of damagedphageDNA was tested in experiments analogous to those de-scribed above.

Ligasemutants. Two types ofligasemutants

inE. coli characterized by Gellert (9) andtheir

parental strain were scrutinized: N1323, which

overproduces ligase, N953 producing normal amounts, and N1325 which isdeficient in

poly-nucleotideligase.Theligaselevels differatleast 20-fold between the

overproducer

andthe

defi-cient strain (9). Intracellular repair of DNA from UV-irradiated phage is illustrated inFig.

3. Neitherdeficiency (Fig. 3E, F,G, andH)nor overproduction (Fig. 31, J, K,

1)

affected either

the extent or the kinetics of repair of

UV-damagedDNA. The same was trueforrepairof

breaks due to 32P decay and for breaks in

rll-ligag

-.B DNA(data not

shown).

DNA polymerase I mutant. Similar

experi-ments wereperformedwithbacteria deficient in

polI (Fig. 4).

Deficiency

in

polI

impaired

the repair process. While similar extent of initial

breakage of

UV-damaged

DNA was found in

both the

poll-deficient

strainandina spontane-ous polI+ revertant thereof (Fig. 4A and

D),

subsequent restoration of strand

integrity

was less efficient inpolA- cells

(Fig.

4B,

C, E,

and

F).Even 60 min ofincubationinthe presence of

CM didnot restore the

integrity

in

polA-

cells

(cf. Fig. 4C andF).

The fact that

deficiency

in

polI

reduced the extent of observed

repair

further

strengthens

the argument that bacterial enzymes

perform

this repairofphageDNA and

strongly

suggests thatpolIis

partially

responsible

forthe

repair.

While there are no doubts that

ligase

is capable of repairing

single-stranded

breaks in

DNA (22,25), we feel that its

physiological

role is still uncertain. It is

interesting

toobserve that

ligase deficiency does not

impair

the

repair

of discontinuities in

phage

DNA.This observation leaves room for

speculations

about the role of ligase inthecell.

Host-repaired areas may be

recognized by

phage-codedrepairingenzymes. Kozinski and Lorkiewicz (15) showed

that,

despite

a

nearly

perfect restoration of

integrity

of

UV-damaged

DNAupon removal ofCM,therewas no

accom-panying restoration of

phage

viability,

as if

repair wasincompetent. UV excisions in E. coli DNA produce quite large gaps (3, 24). If the same is true for UVexcisions in T4 DNA (23), the filling of such gaps would lead toinsertion of cytosine instead of hydroxymethylcytosine (HMC) (since phage enzymes required to syn-thesize HMC and glucosylate it are not pro-duced in the presence of CM). Phage-coded excision (and repair) enzymes may be able to recognize such inadequately repaired areas of the DNA. If this were the case, expression of phage functions after removal of CM from the culture may result in a second round of excision and possibly repair. This was tested in the following experiment. A culture of B23 was divided into two parts. One part was infected with 32P-labeled, UV-irradiated T4 phage (MOI 0.1), and the second part was mixedly infected with the same UV-irradiated phage (MOI 0.1) and nonirradiated helper phage (MOI 3). CM was added to both parts at the moment of infection. Sixty minutes later the infected bac-teriaweresedimented and resuspended in fresh

medium without CM, and incubation was con-tinued.Figure5 showsalkaline sucrose gradient analysis of samples from the mixedly infected culture at different times after infection. Prompt nicking occurs at 3 min after infection (Fig. 5A), and repair in the polynucleotide chain has occurredby60 min (Fig.5B).Twenty minutes after the removal of CM, the DNA repaired by the host has undergone a second

cycle of nicking (compare Fig. 5C and A and note that the average DJ/Dl of secondarily

nicked DNA is virtually identical). The pat-ternsofdistributiondepictedinFig.5A, B, and

C are, for all practical purposes, identical also for bacteria which were singly infected with

UV-irradiated phage without any unirradiated

helper phage (notdocumentedhere). Figure5D

represents the fate of DNA fromUV-irradiated

phage 100 min after infection together with helper phage and shows that there is secondary repair. This is a typical pattern only for the

suspensions co-infected with rescuingcold, via-ble phage. In the case of infection with UV-irradiated phage alone, there is no secondary repair, but gradualsolubilization of DNA (up to 80% of the parental label, not documented

here). In a typical experiment the secondary nicking is first detectable at 10 min after removal of CM, reaching its maximum at ap-proximately 20 min after removal of CM. The close coincidence in the extents ofprimary and secondary nicking indicates that the phage enzymes may recognize excise and repair those areas that were previously subjected to host-mediatedrepair. This experiment also indicates

314 J.VIROL.

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FIG. 3. RepairofWV-damagedDNA inligasemutantsofE.coli. E. coli N953(wild-typeligaselevels), Nl325 (deficientinligase),orN1323overproducing ligase) wasinfectedwithUV-irradiated phagein thepresenceof

CM.Atintervalssampleswerewithdrawn andanalyzedasdescribed inpreceding legends.Note thestriking

similarityin thepatternof occurringrepairas afunction oftime and theseeminglackofeffect of ligaselevelson

thekinetics oftherepair. 3H represents referenceDNA frommature T4phage.

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P 3478

0.5

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3110 Thy- Pol

Rb

E

F

'- BOTTOM

FRACTION

OF THE

LENGTH

OF

THE

GRADIENT

FIG. 4. Repair of UV-damagedDNA inaDNApolymerasemutantofE. coli. E. coli P3478polA-orapolA+

revertantofthis strainwereinfectedwith UV-irradiatedphageasdescribed inlegendtoFig. 3,andtheinfected

cellsweresampledatintervals. Both strains allow theinfliction ofnicks at 3min;however,adrasticdifference

is observedintheextentof subsequent repairinpol+andpol-mutants.A, B,andC, polA-;D, E,andF,polA+

revertant.

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REPAIROF PHAGE T4 DNA

A

3 min.(+ CM)

B

60min.(+CM)

C

80min.(-CMat60min.)

D

110min.(-CMat 60min.)

0 0.5 1.0

3BOTTOM

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FRACTION OF THE LENGTH OF THE GRADIENT

FIG. 5. Primary and secondary breakdown and

repair of UV-damaged phageDNA. E. coli B23was

infectedinthepresenceofCM withanMOIof0.1of

32P-labeled, UV-inactivated (6lethalhits) T4phage

andanMOIof3.0of cold,nonirradiated T4phage. A,

Pattern obtained at 3 min after infection in the

presenceofCM; notethe peaking ofthe breakdown

product at DJ/D, = 0.62. This is the product of

primary nicking.B, Fateof injfectedDNA at60min

after infection; note the extensive repair (primary

repair). The reference 3H DNA (broken line) is

that host repair of UV damages in phage DNA doesnotaffect thecoding for excision enzyme(s)

recognizing host-repaired areas, but that

pro-duction ofenzyme(s) restoring strand integrity

areprevented.

DISCUSSION

Experiments documented here support our

earlier conclusion that there is host-mediated

repair ofdiscontinuous phage DNA under

con-ditions when the synthesis of phage-induced

proteins is prevented. Physiological (rHIligam-.B) and physical (32P decay)

discon-tinuitiesin phage DNA, aswell as

discontinui-ties inflicteduponthe DNAafterithasentered the cell (UV damage), are promptly repaired

upon infection of E. coli B in the presence of

CM. The three types of damage are quite

different: rH-ligam- B phage DNA has single-stranded gaps, 32p decay causes a single-stranded nickwhere the newly exposedend may

have an S terminus, which would be a rather

unyielding candidate for repair, and UV-irradiated phage have damaged DNA which is not discontinuous to begin with but becomes

broken after infection ofthe cells.

It is possible that the repair itself it quite

similarinall threecases. Ithas beenshown that

excision ofUV-irradiated DNA both invivo in

E. coli (3, 24) andinvitro using purified E. coli polymerase I (14) or a mixture of T4

endonu-cleaseI and T4"excision enzyme" (23) results inthereleaseof oligonucleotides in size exceed-ing a thymine-thymine dimer by 10 to 30

residues. The terminus at the nick arising due to 32P decay provides a veryunusual substrate

for repairing enzymes. It appears simpler to

postulate that in thiscaserepair is preceded by a preparatory, "editing" enzyme which recog-nizes that the DNA termini are somehow

un-usual, and excises adjacent nucleotidesin

"an-ticipation" of repair ofthe damage. Possibly,

this excision ofnucleotides may proceed by a

similar mechanism as the UVexcision, maybe utilizing the same enzyme(s). The

discontinu-ousDNAofrII-ligam-

.B

phagewasshown here to contain single-stranded gaps of undefined

partially degraded, therefore after repairthe

distribu-tionof 32ptendstobemorehomogeneousthan thatof

thereference. C, Secondary nicking occurring at20

min after the removalofCM; note that theD2/D, of

thefragmented moiety = 0.67; this isverysimilarto

the product of primary nicking shown in A. D,

Secondary repair which occurs at 40min after the

removal ofCM; note therepair in the continuity of

polynucleotidechain andcomparewith therepairin

B. I0

1--4% 0

U

U.

0.

I--CL

1Lu

I--z

0.

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VOL.12, 1973

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CARLSON, LORKIEWICZ,AND KOZINSKI

length, sincethe discontinuities are repairable

only byacombination of DNApolymerase and ligase. In all three cases, then, repair may involve fillingof a single-stranded gap.

Among several candidates for the role of

bacterialrepairing enzyme(s),DNApolymerase I and polynucleotide ligase attract special

at-tention, being well characterized enzymes

which havebeen shown to repair DNA

discon-tinuities in vitro (18, 22, 25). Our finding that

deficiencyinpolIinterferes withtherepair of T4

DNAis well in line with thediminishedrepair

activities in this strain reported from another

laboratory(12). Deficiency inpolI hasalso been

reported to interfere with replication and

re-combination of T4 DNA (21). In some repeti-tions of our polA experiments (notdocumented here), some residual repair was observed,

though this repair was significantly less than

whatwasfoundinthepol+revertant. The origin of the residual repair activity in the

polI-mutant is not known.recA- bacteria lack

abil-ity to repairdiscontinuities inphage DNA (15)

andarealso UVsensitive (5). Thissuggeststhat

therecA protein maybe involvedinthe residual

repair observed inpolA- cells. It isof interest

here to note that it is not possible to obtain a

doublemutantdeficientinbothrecA andpolA,

suggesting that the two enzymes perform

similar complementary functions (10).

More surprising was the

discovery

that

nei-ther deficiency nor overproduction of ligase affected therate or extentofrepair. The

ligase-deficient

mutant employed inthisstudy shows increased UV sensitivity, suggesting impaired repair activity, although there is no effect on DNAreplication or onthe

viability

ofthe cells (9). It is possible that the small amounts of

ligase found in the ligase-deficient mutant are

sufficientto"seal" thegaps in

phage

DNAonce

they

are filled. Another

possibility

is that the

"sealing"

maybeperformed

by

a

highly

special-ized, asyet undescribed ligatingenzyme, orby thegap-filling enzyme itself.

It wasshown

by

Kozinskiand Lorkiewicz

(15)

that functionality can not be restored by the host-mediated repair of phage DNA.

Experi-mentsdescribedinthis paperprovidean

expla-nation for this observation. Upon removal of

CM there is a

secondary

breakdown of

UV-irradiated, host-repaired phage DNA. The ex-tent of this secondary breakdown coincides

closelywith that of theprimary

breakdown,

and proceeds with an identical

pattern

both in

singly and in

mixedly

(UV-damaged

and

non-UV-damaged

phage)

infected bacteria. After

completion ofthe

secondary

round of

nicking,

secondary repair takes

place,

but

only

if the

cells have been co-infected with non-UV-damaged "rescuer". In the absence of unir-radiated helper phage the irunir-radiated DNA is gradually solubilized. Thus, host-mediated re-pair not only did not improve the DNA, but

actually provedvery detrimental.

This suggests that secondary nicking and repair are caused bynewlysynthesized

phage-codedenzymes which atfirst areable to

recog-nize areas improperly repaired by the host

enzyme(s) and thenrestorethedamagedarea to its integrity. Theformer, but not the latter, may

be coded from UV-irradiated DNA. There are reasons to believe that viability is also restored,

since it was demonstrated that the parental

labelofsuchrescued phageisbeingtransferred to progeny phage as segments of parental

strandswhich underwentsemiconservative

rep-lication (E. Shahn, Ph.D. thesis, 1965,

Univer-sityofPennsylvania, Philadelphia).

Stringent

proof that the

host-repaired

areas

are recognized, excised, and repaired

by

phage

enzymescould beobtained

if,

forinstance, 3H-TdR was incorporated during the host-medi-ated repair in CM. Upon removal of CM one

should then observe solubilization of this 3H

label coincidentally with the appearance of

secondary breaks. However, after infection in

thepresence ofCM mostofthe 3H-TdR

uptake

will go to bacterial DNA. This bacterial DNA

synthesis, and the phage-mediated breakdown oflabeled hostDNA afterremovalof

CM,

would overshadow any label entering and

being

re-leased from the

repaired

areas of the phage DNA.

ACKNOWLEDGMENTS

We thankD. T.Denhadtforperforming theinvitrorepair ofrIl-liga,&- BphageDNA,and J. Cairns and H. M. Kirsch forgenerousgifts of strains. P. B. Kozinski and M. Mitchell provided excellent assistance with the experiments. This investigationwassupportedbyNationalScience Foundation grantGB-29637,andbyPublic Health Service grantCA-10055 andinpartbygrant 1-F02-CA-51268-01,the lattertwofrom the National Cancer Institute.

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VOL. 12, 1973