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

Galina Radeva

Institute of Radiochemistry, Forschungszentrum Rossendorf, D-01314 Dresden, Germany

Correspondence
Katrin Pollmann
k.pollmann{at}fz-rossendorf.de

The surface layer (S-layer) protein genes of the uranium mining waste pile isolate Bacillus sphaericus JG-A12 and of its relative B. sphaericus NCTC 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 of B. sphaericus CCM 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 of B. sphaericus CCM 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 copies sllA (9602) or sllB (JG-A12). Interestingly, both B. sphaericus strains studied were found to contain an additional, plasmid-located and silent S-layer protein gene with the same sequence as sllA and sllB. 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.

The GenBank/EMBL/DDBJ accession numbers for the sequences reported in this paper are AJ866974 (B. sphaericus NCTC 9602), AJ866975 (B. sphaericus JG-A12), AJ849547 (slfA), AJ849548 (sllA), AJ849549 (slfB), and AJ849550 (sllB). The IS elements ISBsph1 and ISBsph2 have been deposited in the ISFinder database (http://www-is.biotoul.fr).

A table of all the oligonucleotides used for sequencing and amplification is available as supplementary data with the online version of this paper.

Present address: Bulgarian Academy of Sciences, Institute of Molecular Biology, Acad. G. Bonchev Str., Bl. 21, Sofia 1113, Bulgaria.