Genomics and Recombination
John K. Davies
from: Neisseria: Molecular Mechanisms of Pathogenesis (Edited by: Caroline Genco and Lee Wetzler). Caister Academic Press, U.K. (2010)
Analyses of the available genome sequences of Neisseria species and strains have considerably increased our knowledge of the recombination processes occurring within this genus. The accumulation of multiple copies of mobile genetic elements such as insertion sequences and bacteriophages has obviously contributed to genomic diversity. However these elements have also served as substrates for homologous recombination reactions that, at a low frequency, have resulted in various genomic rearrangements. Numerous examples of the horizontal acquisition of DNA segments are also obvious. The largest of these acquired DNA segments have been termed Islands of Horizontal Transfer, and appear to have been acquired at a low frequency via an as yet unknown process. A second class of horizontally acquired DNA segments, involving smaller fragments termed Minimal Mobile Elements, seem to be moving through the bacterial population at a higher frequency. These elements consist of a series of mutually exclusive cassettes, containing just a few genes, which target specific positions on the genomes. It is unclear how these elements evolved, but their movement appears to be facilitated by the natural transformation system, and integration occurs through homologous recombination involving conserved flanking genes. Each sequenced genome also contains a remarkable amount of non-coding repetitive DNA sequences. The most common of these repeats is involved in recognition and uptake of genus-specific DNA during transformation. Other repeats appear to be the target for site-specific recombinases or, especially when occurring in repeat arrays, the homologous recombination system. Finally, the genome sequences have also revealed numerous, previously unknown, examples of genes that appear to be subject to phase or antigenic variation. While the systems operating on a few of these genes have been known for some time, the magnitude of the variable gene repertoire was much greater than expected read more ...