Trasposable elements
Trasposable elements or Mobile genetic elements were discovered in bacteria in the late 1960s. They were also recognized
because they caused a particular type of mutation which was unstable. The first type of element that was discovered (what
we now know as an insertion element) will block the expression of any gene into which it inserts and it will also block the
expression of any downstream genes that are expressed as part of the same operon. This kind of effect is called a polar effect
At more or less the same time research into the structure, properties and origins of broad host-range plasmids and the
spread of antibiotic resistance in bacteria revealed the presence of more complex, related, elements that were also potentially mobile
their are four types of bacterial transposon
Insertion Sequences
Insertion Sequences or IS elements are the simplest mobile element. They consist of a fairly short
(700 - 1500 bp) DNA segment flanked by a 10 - 40 bp inverted repeat sequence. The segment codes
for the protein (transposase) that catalyses the transposition event:
Transposons
Simple transposons are similar to IS elements. They contain DNA segments flanked by short inverted repeat
sequences. The DNA segments, however, usually code for a number of gene products. In addition to a transposase
they may also code for a resolvase (this will depend on their mechanism of transposition) and they
may contain for one or more antibiotic resistance genes:
Composite Transposons
Composite transposons are DNA segments that are flanked by an IS element at either end. In other words
instead of each IS element moving independently, they now act in concert and move together along with the intervening DNA
Each IS element is a typical IS element although only one of the two elements typically retains a functional transposase
activity. The IS elements may be in the same or in the opposite orientation with respect to one another.
The intervening segment often carries the genetic determinants for a number of antibiotic or other toxin resistances:
Bacteriophage Elements
The E. coli bacteriophage Mu is an unusual phage. It can infect E. coli as a normal phage. It is a temperate phage
so it has both lytic and lysogenic growth cycles. The Mu prophage integrates into the host genome. However, it does
not do so by a site-specific recombination - rather, it integrates randomly into the host genome by a transpositional
mechanism. When Mu replicates, it does so by replicative transposition events in which each new copy of Mu is inserted
somewhere else in the chromosome Mu is, in effect, a transposon that happens to have genes that package
its DNA as well as genes that regulate its transposition
تعليق