Mobile DNA and Genome Plasticity
Microbial genomes are highly dynamic, resulting in diverse variations in genetic information, which are the substrates for adaptation and selection. Multiple types of genome alterations and rearrangements underlie such genome plasticity. DNA amplification provides changes in gene copy number that can lead to changes in gene expression and, hence, alter cell growth capabilities. Such amplification occurs by alternative replication mechanisms, leading to amplification of particular DNA segments or to changes in chromosome copy number that lead to genetic diversity. DNA rearrangements also occur by excision and integration of gene cassettes often found in the genetic elements called integrons. The best-known of these cassettes encode antibiotic resistance genes, but the functions of many other diverse cassettes remain to be determined. Induction of the cellular SOS system in response to single-stranded DNA from horizontal gene transfer or DNA damage causes induction of the integrase that mobilizes gene cassettes, thus providing genetic diversity. Genetic diversity also results from genetic elements that move from place to place. Introns are RNA segments, sometimes autocatalytic, that can be excised (spliced) from a precursor RNA to form the mature RNA and Group II introns are also mobile elements whose movement between DNA molecules is mediated by both autocatalytic RNA and reverse transcriptase. Some introns also encode an endonuclease which contributes to intron mobility. Endonucleases also underlie the movement of genes encoding inteins, which are autocatalytic protein segments that can excise from a host protein. These are but a few of many strategies for genome plasticity and diversification.