Novel surface layer protein genes in Bacillus sphaericus associated with unusual insertion elements

Abstract

The surface layer (S-layer) protein genes of the uranium mining waste pile isolateBacillus sphaericusJG-A12 and of its relativeB. sphaericusNCTC 9602 were analysed. The almost identical N-termini of the two S-layer proteins possess a unique structure, comprising three N-terminal S-layer homologous (SLH) domains. The central parts of the proteins share a high homology and are related to the S-layer proteins ofB. sphaericusCCM 2177 and P-1. In contrast, the C-terminal parts of the S-layer proteins of JG-A12 and NCTC 9602 differ significantly between each other. Surprisingly, the C-terminal part of the S-layer protein of JG-A12 shares a high identity with that of the S-layer protein ofB. sphaericusCCM 2177. In both JG-A12 and NCTC 9602 the chromosomal S-layer protein genes are followed by a newly identified putative insertion element comprising three ORFs, which encode a putative transposase, a putative integrase/recombinase and a putative protein containing a DNA binding helix–turn–helix motif, and the S-layer-protein-like gene copiessllA(9602) orsllB(JG-A12). Interestingly, bothB. sphaericusstrains studied were found to contain an additional, plasmid-located and silent S-layer protein gene with the same sequence assllAandsllB. The primary structures of the corresponding putative proteins are almost identical in both strains. The N-terminal and central parts of these S-layer proteins share a high identity with those of the chromosomally encoded functional S-layer proteins. Their C-terminal parts, however, differ significantly. These results strongly suggest that the S-layer protein genes have evolved via horizontal transfer of genetic information followed by DNA rearrangements mediated by mobile elements.