Asadulghani M. and Ogura Y. et al, (2009) PLOS Pathogens 5: 1-15.
Bacteriophage is one of the major genetic elements promoting horizontal gene transfer between bacteria. Enterohemorrhagic Escherichia coli O157:H7 (Sakai isolate) contains eighteen prophages in its genome (Ohnishi et al. 1999). Two of the eighteen prophages carry Shiga toxin gene clusters; stx1AB and stx2AB; these gene products kill eukaryotic cells by inhibiting protein synthesis. Interestingly, all prophages in O157 have mutations in genes that encode basic phage functions such as site-specific recombination, replication, cell lysis, or structural proteins constituting the head and tail. Evidence obtained by genome analysis suggests that each prophage is defective. Thus, one might think this strain is not very problematic. But, it may be not true.
To assess the role of defective prophages in horizontal gene transfer of virulence determinants, the authors analyzed each basic function of the prophages in O157: excision from chromosome, phage DNA amplification in response to DNA damage, ability to be released as phage particles, and infection of new recipient cells.
Among eighteen prophages, eight prophages including the stx gene-containing phages were found to be released as phage particles, and among them four were able to infect both or either of the two E. coli recipient strains tested. These results strongly suggest that prophages in O157 are complementing defective functions with each other to make practically transferable phages.
Importantly, the phage particle produced often contains a chimeric DNA fragment made of two different prophages' fragments, indicating the recombination of prophages in host cells. Although the newly generated phage DNA were not extensively sequenced in this study, it is possible that some phage fragments reconstituted intact, functional phage through recombination.
This paper nicely explains the way bacteriophages live and propagate in a host. I think this paper is a good example of a post-genomic sequence study. In this era, we can understand how mobile genetic elements are acting in a population by combining sequence analysis and standard genetic experiments. I suspect that plasmids and transposons are also repeating cycles of inactivation and activation as shown for these phages. They will never be extinguished as long as similar mobile genetic elements exist in the world.
Ohnishi M. and Tanaka C. et al., (1991) DNA Res. 6: 361-368
Posted by H. Yano, Univerisity of Idaho