MOLECULAR EVOLUTION OF A PATHOGENICITY ISLAND FROM ENTEROHEMORRHAGIC ESCHERICHIA-COLI O157-H7

We report the complete 43,359-bp sequence of the locus of enterocyte e ffacement (LEE) from EDL933, an enterohemorrhagic Escherichia coli O15 7:H7 serovar originally isolated from contaminated hamburger implicate d in an outbreak of hemorrhagic colitis. The locus was isolated from t he EDL933 chromosome with a homologous-recombination-driven targeting vector. Recent completion of the LEE sequence from enteropathogenic E. coli (EPEC) E2348/69 afforded the opportunity for a comparative analy sis of the entire pathogenicity island. We have identified a total of 54 open reading frames in the EDL933 LEE. Of these, 13 fall within a p utative P4 family prophage designated 933L, The prophage is not presen t in E2348/69 but is found in a closely related EPEC O55:H7 serovar an d other O157:H7 isolates, The remaining 41 genes are shared by the two complete LEEs, and we describe the nature and extent of variation amo ng the two strains for each gene. The rate of divergence is heterogene ous along the locus. Most genes show greater than 95% identity between the two strains, but other genes vary more than expected for clonal d ivergence among E, coli strains, Several of these highly divergent gen es encode proteins that are known to be involved in interactions with the host cell. This pattern suggests recombinational divergence couple d with natural selection and has implications for our understanding of the interaction of both pathogens with their host, for the emergence of O157:H7, and for the evolutionary history of pathogens in general.