CORRELATION OF RHS ELEMENTS WITH ESCHERICHIA-COLI POPULATION-STRUCTURE

The Rhs family of composite genetic elements was assessed for variatio n among independent Escherichia coli strains of the ECOR reference col lection. The location and content of the RhsA-B-C-F subfamily correlat es highly with the clonal structure of the ECOR collection. This corre lation exists at several levels: the presence of Rhs core homology in the strain, the location of the Rhs elements present, and the identity of the Rhs core-extensions associated with each element. A provocativ e finding was that an identical 1518-bp segment, covering core-extensi on-b1 and its associated downstream open reading frame, is present in two distinct clonal groups, but in association with different Rhs elem ents. The sequence identity of this segment when contrasted with the d ivergence of other chromosomal segments suggests that shuffling of Rhs core extensions has been a relatively recent variation. Nevertheless the copies of core-extension-b1 were placed within the respective Rhs elements before the emergence of the clonal groups. In the course of t his analysis, two new Rhs elements absent from E. coli K-12 were disco vered: RhsF, a fourth member of the RhsA-B-C-F subfamily, and RhsG, th e prototype of a third Rhs subfamily.