Unusual evolution of a superoxide dismutase-like gene from the extremely halophilic archaebacterium Halobacterium cutirubrum.

The archaebacterium Halobacterium cutirubrum contains a single detectable, Mn-containing superoxide dismutase, which is encoded by the sod gene (B. P. May and P. P. Dennis, J. Biol. Chem. 264:12253-12258, 1989). The genome of H. cutirubrum also contains a closely related sod-like gene (slg) of unknown function that has a pattern of expression different from that of sod. The four amino acid residues that bind the Mn atom are conserved, but the flanking regions of the two genes are unrelated. Although the genes have 87% nucleotide sequence identity, the proteins they encode have only 83% amino acid sequence identity. Mutations occur randomly at the first, second, and third codon positions, and transversions outnumber transitions. Most of the mutational differences between the two genes are confined to two limited regions; other regions totally lack differences. These two gene sequences are apparently in the initial stage of divergent evolution. Presumably, this divergence is being driven by strong selection at the molecular level for either acquisition of new functions or partition and refinement of ancestral functions in one or both of the respective gene products.