Barbara McClintock (discovery 1940s; Nobel Prize 1983)
Transposable Genetic Elements
Tansposable genetic elements are segments Of DNA that have the capacity to move from one location to another (i.e. jumping genes). They are found in viruses, bacteria and eukaryotes. They were first discovered in the 1940s by
Barbara McClintock during her studies on maize genetics (a discovery that won her the Nobel prize in 1983).
Transposable Element in Bacteria
1. Insertion sequences – carry no genetic information other than that needed to allow information other than that needed to allow them to move location
2. Transposons – larger and contain many genes
3. Special virus – Mu (mutator phage) – induces mutations in host genome in which it is integrated. Mu operates as a very large as a very large and very efficient transposable element
In the process of transposition, they may cause:
- cause mutations.
- increase (or decrease) the amount of DNA in the genome.
- promote genome rearrangements.
- regulate gene expression.
Properties of Transposble Genetic Elements
1. Random movement
Transposable genetic elements can move from any DNA molecule to any DNA other molecule or even to another location on the same molecule. The movement is not totally random; there are preferred sites in a DNA molecule at which the transposable genetic element will insert.
2. Not capable of self replication
The transposable genetic elements do not exist autonomously
(exception - some transposable phages) and thus, to be replicated they must be a part of some other replicon.
3. Transposition mediated by site-specific recombination
Transposition requires little or no homology between the current location and the new site. The transposition event is mediated by a transposase coded for by the transposable genetic element.
Recombination that does not require homology between the recombining molecules is called site-specific or illegitimate or nonhomologous recombination.
4. Transposition can be accompanied by duplication
In many instances transposition of the transposable genetic element results in removal of the element from the original site and insertion at a new site. However, in some cases the transposition event is
1. Insertion sequences – carry no genetic information other than that needed to allow information other than that needed to allow them to move location
2. Transposons – larger and contain many genes
3. Composite transposons
4. Special virus – Mu (mutator phage) – induces mutations in host genome in which it is integrated . Mu operates as a very large and very efficient transposable element
Insertion sequences (IS): Insertion sequences are transposable genetic elements that carry no known genes except those that are required for transposition. Insertion sequences are small stretches of DNA that have at their ends repeated sequences, which are involved in transposition. In between the terminal repeated sequences there are genes involved in transposition and sequences that can control the expression of the genes but no other nonessential genes are present.
IS element contains approximately 50-bp inverted repeats(IR) on either side, and in between the inverted repeats transposase enzyme is
located.
The 5’ and 3’ short direct repeats are generated from the target-site DNA during the insertion of mobile element. The length of these repeats is constant for a given IS element, but their sequence depends upon the site of insertion and is not characteristic for the IS element.
Transposition of insertion sequence (IS) elements:
1. Original copy remains in place; new copy inserts randomly.
2. IS element uses host replication enzymes for replication.
3. Transposition requires transposase, coded by the IS element.
4. Transposition initiates when transposase recognizes ITRs.
5. Site of integration = target site.
6. Staggered cuts are made in DNA at target site, IS element inserts, DNA polymerase and ligase fill the gaps.
Importance
i) Mutation
The introduction of an insertion sequence into a bacterial gene will result in the inactivation of the gene.
ii) Plasmid insertion into chromosomes
The sites at which plasmids insert into the bacterial chromosome are at or near insertion sequence in the chromosome.
2. Transposons
Transposons carry additional genes,such as drug resistance in addition to transposase , and may or may not flanked by IS elements. There are two types of prokaryotic transposons:
2.1. The composite transposons
2.2. The complex transposons (TnA transposons)
2.1. The composite transposons:
They are denoted by the symbol Tn and
Composite transposons are created when two IS sequences are inserted beside each other. The region between them can be transported along with the inserted IS. Hence the ISs have made an immobile gene mobile. These genes are nothing related to transposition they mostly code for resistance to a certain antibiotic.
For example:
Tn10: the tetracycline resistance transposonTn10, which is about 9300 bp in length, consists of a central region carrying the resistance determinants flanked by two copies of the IS10 insertion sequence in opposite orientations IS10 itself is about 1300 bp long with 23-bp inverted repeat ends and contains a
Tn5: In Tn5 the IS50R is capable of producing Transposase but IS50Lcannot due to deletion of a nucleotide pair.Tn5 when carried by non lytic bacteriophage the frequency of transposition is reduced if the virus already carries a copy on Tn5.Sinc the resident Tn5 inhibits the entry of incoming Tn5 by synthesis of a repressor
Tn9: Tn9 has direct repeats of IS1. It carries chloramphenicol resistance gene.
Composite Transposons
Direction of terminal IS element
Length(bp) Target(bp) Genetic marker
Tn5 Inverted 5700 9 KanR
Tn9 Direct 2638 9 Chloramphenicol
