Mobile Genetic Elements of Bacterial Genome
There are several specialized genetic structures in bacteria able to move within the genome. Insertion elements (also known as insertion sequence elements or IS elements) code only their ability for intragenomic migration, i.e. carry the genes that account for transposition. The products of these genes include the transposase enzyme with endonuclease activity and regulatory proteins, controlling transposition.
IS element movement triggers a number of important genetic events, affecting bacterial genomic function. They induce mutations in the sites of their integration, being responsible for most of insertion mutations in bacteria. Also they can alter gene expression regulating the activity of promoter sequences.
Another type of mobile genetic sequences, known as transposable elements or transposons, is more complex. They also can migrate from one genetic site to another. By action of transposases, transposons are incised and further inserted into another part of the same replicon or move to integrate with another replicon (cut-and-paste mechanism). This process of transposon movement is called transposition. In some cases the sequences of transposons can be duplicated.
A special class of retrotransposons is hallmarked by the synthesis of intermediate RNA copy from primary transposon DNA sequence. Next this RNA serves as the template for synthesis of a new DNA copy of the transposon by the enzyme reverse transcriptase. Retroviruses behave as retrotransposons when infecting eukaryotic host cells.
Transposons are composed of 2500-20000 bp of DNA. Besides genetic sequences controlling their transposition, large transposons can include additional genes conferring antibiotic resistance and some other special properties. Transposons are flanked by insertion sequences, and various insertion mutations occur in places of transposon integration. Thus, these mobile elements are the potent biological mutagens.
The specificity of transposon integration is rather low. Usually transposons block the activity of genes in sites of their integration.
Unlike plasmids, transposons have no genetic information for their own self-replication. Hence, their reproduction depends on the activity of bacterial replicon, which contains the transposon. Many conjugative plasmids are transferred among bacterial cells, and insertion of a transposon into such a plasmid can lead to its dissemination throughout bacterial population.
Also plasmids and transposons may harbor genetic elements known as integrons.
Integron is the bacterial mobile genetic element that is able to incorporate and express additional genes taken from the outside. The genes predisposed to integration are usually organized as gene cassettes.
Gene cassette comprises a certain gene and site for specific recombination. It may stay as free circular DNA in bacterial cytoplasm but after recombination the cassette becomes embedded into integron.
The whole sequence of integron includes the gene of enzyme integrase, promoter and several gene cassettes predominantly encoding antibiotic resistance.
Integrase provides further insertion of integron into transposon or plasmid. The presence of promoter makes possible the expression of cassette genes thus conferring antimicrobial resistance to bacterial cell.
Overall, transposable elements play an extremely important role in gene movement within the same cell and also between different cells (horizontal gene transfer). They accelerate strongly the evolutionary process of microbial populations.