Rhs elements of Escherichia coli: a family of genetic composites each encoding a large mosaic protein

Abstract

The Rhs family comprises a set of composite elements found in the chromosomes of many natural Escherichia coli strains. Five Rhs elements occur in strain K‐12. The most prominent Rhs component is a giant core open reading frame (core ORF) whose features are suggestive of a cell surface ligand‐binding protein. This hypothetical protein contains a peptide motff, xxGxxxRYxYDxxGRL(I or T)xxxx, that is repeated 28 times. A similar repeated motif is found in a Bacillus subtilis wall‐associated 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 components indicate that the elements have a recent origin outside the E. coli species, and that they are composites assembled from segments with very different evolutionary histories. The Rhs cores fail 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 extension, these are highly variable. In most examples, the hypothetical product of this ORF has a candidate signal sequence for transport across 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 and Salmonella, where they are associated with loci that determine O‐antigen variation. Copyright © 1994, Wiley Blackwell. All rights reserved