Unusual insertion elements probably involved in the evolution of the S-Layer protein genes of B.sphaericus

Unusual, S-layer-like gene carrying, mobile elements were localized on the chromosomes of the uranium mining waste pile isolate Bacillus sphaericus JG-A12 and of the closely related strain Bacillus sphaericus NCTC 9602 in the vicinity of the functional S-layer genes. Each of the two almost identical elements designated ISBsph1 (strain 9602) and ISBsph2 (strain JG-A12) possess four ORFs which encode a putative integrase /recombinase, a putative XRE-family like protein containing a DNA binding helix turn helix motif, and a S-layer-like gene copy. Interestingly, the S-layer-like gene copies of the two IS elements are analogous to the coding parts of the previously described plasmid-located, identical between each other, and siltent S-layer genes of the two strains. In the case of the strain JG-A12, however, the ISBsph2 borne s-layer-like gene is truncated.
Comparative analyses of the functional, of the plasmid-located, and of the IS-element-specific S-layer genes of the above mentioned two strains between each other and with the S-layer genes of the other Bacillus sphaericus strains studied up to date revealed that all functional S-layer genes possess mosaic structure and share regions with very high identitiy which are surrounded by regions possessing very low or almost no identity.
Our results strongly suggest that the functional S-layer protein genes in Bacillus sphaericus have evolved via horizontal transfer of genetic information followed by rearrangements which were mediated by mobile elements.
The primary structure of the functional S-layer of the uranium mining waste isolage JG-A12 and in particular its higher capability in comparison to the other S-layers to bind uranium and other metals indicate that the environmental stress factors played an important role in the evolution of its gene.