Evolutionary aspects of the S-layers of Bacillus sphaericus

The primary structures of the S-layer proteins of the uranium mining waste pile isolate Bacillus sphaericus JG-A12 and of Bacillus sphaericus NTCC9602 were analysed. The amino acid sequences of the two proteins are almost identical but possess an abrupt homology to some of the other B. sphaericus S-layers studied up to date. This is an indication that a horizontal transfer of genetic information was involved in the evolution of the genes of the mentioned S-layers. An extensive comparative analysis of the S-layer proteins deposited to the GeneBanks demonstrated that several additional S-layers of B. sphaericus and of other Bacillus species were also subjected to late evolutionary events via lateral DNA transfer. The possible mechanism of these events in the case of B. sphaericus will be discussed.
Interestingly, the slight amino acid differences between the S-layer proteins of the strains JG-A12 and NTCC9692 lead to dramatic differences in their ability to interact with metals and also in their stability against proteases. As estimated by using EXAFS spectroscopy phosphorous residues, in addition to the carboxyl groups of the proteins, are involved in the binding of uranium (VI). ICP-MS analysis demonstrated that the two purified and recrystalized S-layer proteins contain phosphorous which indicates that they are phosphorylated. Moreover, the amount of phosphorous found in the S-layer of the strain JG-A12 was six times higher than those found in the S-layer of the reference strain NTCC 9602. This explains the higher ability of the S-layer of the uranium mining waste isolate to complex uranium and other metals. The latter seems to give an advantage to the strain JG-A12 to survive in the heavily polluted with uranium and other toxic metals environment from which it was recovered.