A ROLE FOR BACTERIOPHAGES IN THE EVOLUTION AND TRANSFER OF BACTERIAL VIRULENCE DETERMINANTS

A virulence-associated region in the genome of Dichelobacter nodosus h as been shown to contain an integrase gene which is highly related to the integrases of Shigella flexneri phage Sf6 and coliphages P4 and ph i R73, together with open reading frames (vapB, C and D) related to ge nes borne on plasmids in Neisseria gonorrhoeae, Escherichia coli, Acti nobacillus actinomycetemcomitans and Treponema denticola. Similar to P 4 and phi R73, the vap region is bracketed by putative bacteriophage a ff sites and is adjacent to a tRNA gene, which suggests that the vap r egion has been derived by the integration of a bacteriophage, or a pla smid carrying a bacteriophage-related integrase gene. Many similaritie s in genes and genes clusters encoding virulence determinants have bee n found in distantly related bacteria. These genes are often located o n plasmids in one organism but on the chromosome in others, implying t hat transmission of the genes has been followed by integration, Thus, the events which have generated the vap regions of D. nodosus may repr esent a common mechanism for transfer of virulence determinants. A num ber of genes involved in the virulence of bacterial pathogens are foun d on integrated bacteriophages, and we suggest that others will prove to be associated with tRNA genes and/or integrase genes derived from b acteriophages. The use of tRNA genes as integration sites for many bac teriophages and plasmids may favour intergeneric transmission, as tRNA genes are highly conserved.