Conjugative transposons are genetic elements resembling the self-transmissible plasmids which, like the F-plasmid, can integrate into the bacterial chromosome, or may remain free in the cell. They have also the ability to mobilize non-transmissible plasmids from cell to cell when they fuse with such plasmids.

Normally, the conjugative transposons remain integrated with the bacterial chromosome. Occasionally, they are excised from the chromosome to form a circular intermediate, a copy of which is transferred to a recipient cell, presumably through a mating bridge, just like a self-transmissible plasmid.

The copy transferred to the recipient cell is eventually integrated into the chromosome. A conjugative transposon possesses an attachment site with which it is inserted into the chromosome. Replication of the circularized free transposon occurs by rolling circle model originating from a site, OriT, as in case of a self-transmissible plasmid, like F. Conjugative transposons have been found in several bacteria where they act as potential carriers of bacterial genes.

The insertion sequences are the simplest form of transposable elements found in prokaryotes. They were first discovered in connection with genes controlling galactose utilization in E. coli. The IS-elements are normal constituents of the bacterial genome. They may be present in the chromosome or extra-chromosomal genetic elements, called plasmids.

For example, IS-elements present in the F-plasmid of E. coli are involved in the integration of the F-plasmid with the bacterial chromosome producing Hfr strains. During the process, genetic exchange between IS-elements of the plasmid and the chromosome takes place.

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