BACTERIAL TRANSPOSONS

TRANSPOSONS “Transposable elements” “Jumping genes” Mobile DNA able to move from one place to another within a cell’s genome sometimes a copy is made and the copy moves insertion requires target DNA sequences

Transposon inverted terminal repeat (ITR)

In the process, they may - cause mutations. - increase (or decrease) the amount of DNA in the genome. - promote genome rearrangements. - regulate gene expression. - induce chromosome breakage and rearrangement.

Discovery of transposons Barbara McClintock 1950’s Ac Ds system in maize influencing kernel color unstable elements changing map position promote chromosomal breaks. Rediscovery of bacterial insertion sequences source of polar mutations

These mobile segments of DNA are sometimes called "jumping genes" There are two distinct types of transposons: 1) DNA transposons -transposons consisting only of DNA that moves directly from place to place 2) Retrotransposons - first transcribe the DNA into RNA and then - use reverse transcriptase to make a DNA copy of the RNA to insert in a new location

BACTERIAL TRANSPOSONS In bacteria, transposons can jump from chromosomal DNA to plasmid DNA and back. Transposons in bacteria usually carry an additional gene for function other than transposition---often for antibiotic resistance. Bacterial transposons of this type belong to the Tn family. When the transposable elements lack additional genes, they are known as insertion sequences (IS family).

Insertion sequences Insertion sequences – IS1 and IS186, present in the 50-kb segment of the E. coli DNA, are examples of DNA transposons. Single E. coli genome may contain 20 of them. Most of the sequence is taken by one or two genes for transposase enzyme that catalyses transposition. IS elements transpose either replicatively or conservatively.

Bacterial IS element Central region encodes for one or two enzymes required for transposition. It is flanked by inverted repeats of characteristic sequence. 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. Arrows indicate orientation.

Mechanism of transposition Two distinct mechanisms of transposition: Replicative transposition – direct interaction between the donor transposon and the target site, resulting in copying of the donor element Conservative transposition – involving excision of the element and reintegration at a new site.

Mechanism of transposition 1. Replicative transposition Copy of transposon sequence Transposase enzyme cut target DNA Transposition Duplication of target sequence

2. Non-replicative (conservative) transposition - Cannot copy transposon sequence - Transposition by cut and paste model Cut transposon sequence from donor molecule attach to target site Ex. IS10, Tn10

Evolution of Transposons Transposons are found in all major branches of life. Duplications and DNA rearrangements contributed greatly to the evolution of new genes.

Transposons causing diseases Transposons are mutagens. They can damage the genome of their host cell in different ways: 1. A transposon or a retroposon that inserts itself into a functional gene will most likely disable that gene. 2.After a transposon leaves a gene, the resulting gap will probably not be repaired correctly. 3.Multiple copies of the same sequence, such as Alu sequences can hinder precise chromosomal pairing during mitosis and meiosis, resulting in unequal crossovers, one of the main reasons for chromosome duplication.

Cont… Diseases caused by transposons include -hemophilia A and B -severe combined immunodeficiency -Porphyria -Cancer -Duchenne muscular dystrophy

Applications Researchers use transposons as a means of mutagenesis. To identifying the mutant allele. To study the chemical mutagenesis methods. To study gene expression.