RHS ELEMENTS OF ESCHERICHIA-COLI - A FAMILY OF GENETIC COMPOSITES EACH ENCODING A LARGE MOSAIC PROTEIN

The Rhs family comprises a set of composite elements found in the chro mosomes of many natural Escherichia coli strains. Five Rhs elements oc cur in strain K-12. The most prominent Rhs component is a giant core o pen reading frame (core ORF) whose features are suggestive of a cell s urface ligand-binding protein. This hypothetical protein contains a pe ptide motif, xxGxxxRYxYDxxGRL(I or T)xxxx, that is repeated 28 times. A similar repeated motif is found in a Bacillus subtilis wall-associat ed protein. The Rhs core ORFs consist of two distinct parts: a large N -terminal core that is conserved in all Rhs elements, and a smaller C- terminus that is highly variable. Distinctive G+C contents of Rhs comp onents indicate that the elements have a recent origin outside the E. coli species, and that they are composites assembled from segments wit h very different evolutionary histories. The Rhs cores fall into three sub-families that are mutually more than 20% divergent. Downstream of the core ORF is a second, much shorter ORF. Like the adjacent core ex tension, these are highly variable. In most examples, the hypothetical product of this ORF has a candidate signal sequence for transport acr oss the cytoplasmic membrane. Another Rhs component, the 1.3 kb H-rpt, has features typical of insertion sequences. Structures homologous to H-rpt have been detected in other bacterial genera, such as Vibrio an d Salmonella, where they are associated with loci that determine O-ant igen variation.