2004 Volume 32 Issue 2 Pages 118-137
As whole-genome sequencing projects advance, it is becoming clear that gene exchange among organisms of the microbial consortia is constitutive. On average, 2.6 prophages per specie have been found in free-living bacteria, and between 3% and 10% of various bacterial genomes’ DNA are composed of prophages. Based upon data, it has been suggested that Escherichia coli and Salmonella separated ca 100 million years ago. Although the rRNA operon is highly conserved, E. coli has an extremely variable genome size ranging from 4.5 to 5.5 Mb, of which ca 10% consists of acquired genes as the result of horizontal gene transfer. The genome of the E. coli strain O157 Sakai has a large amount of strain-specific DNA (1.44 Mb), and contains 18 prophages or prophage remnants accounting for ca 50% of the strain-specific sequences. Moreover, six large chromosome segments, which seem to represent prophage-like genetic elements, are found in O157. Thus, phage genes are an integral part of O157 genome, indicating that virus-mediated high frequency horizontal gene transfer played a predominant role in the emergence of this strain. Direct evidence of such horizontal gene transfer is provided by prophages or prophage remnant sequences in O157 as well as in other strains.
The case of E. coli is an example of how viruses might have influenced evolutionary diversification and speciation. Therefore, it is clear that virus-mediated horizontal gene transfer would have had much more impact in the environment than previously considered. The discovery of non-specific gene transfer mediators, especially in thermal environments, is evidence that such particles might be widespread, contributing to evolutionary diversification and speciation. Viruses may have evolved to sustain ‘selfish’ genes for their coats, and consequently underwent restriction of specific host range. “Transduction” may have major effects on the genetic structure and evolution of the global population of bacteria and other organisms in the natural environment. “Broad-host range gene transporters” conserved in the environment could be descendants of an ancestral “gene transfer apparatus” that likely provided the main role of conserving genetic resources and distributing novel genes to microbial communities in early evolutionary history.
