THE CONTROL OF INTERSPECIES GENETIC EXCHA NGES AND GENETIC-VARIABILITY IN BACTERIA

Inquiring about the genetic barriers between different species amounts to asking how populations become genetically isolated. The genetic ba rrier which separates Salmonella typhimurium from Escherichia coli is primarily recombinational. The structural component of this barrier is genomic sequence divergence. The mismatch repair enzymes act as poten t inhibitors of interspecies recombination, whereas the SOS system act s as an inducible positive regulator. These genetic systems also contr ol genomic stability in bacterial populations. The mismatch repair con stitutively maintains genetic stability, while the SOS system is induc ible and generates genetic variability. These opposing activities enab le mismatch repair and SOS systems to determine both the rate of accum ulation of sequence divergence and the extent of genetic isolation, wh ich are the key components of the speciation process. This review pres ents a compilation of present knowledge about mechanisms of gene trans fer in bacteria, homologous recombination, the SOS system and the mism atch repair system, all necessary for understanding the control of evo lution of bacterial species.