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 50kb 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.