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A bacteriocuprein is a copper- and zinc-containing superoxide dismutase isolated from a bacterium. Until recently, the first and only documented bacteriocuprein was that from the marine bacterium Photobacterium leiognathi, which lives symbiotically with Leiognathid fishes. A new bacteriocuprein has been discovered, purified, and characterized from the free living, non-symbiotic bacterium, Caulobacter crescentus CB15. In its native molecular weight, homodimeric subunit structure, specific activity, and metal content, Caulobacter bacteriocuprein is very similar to the copper-zinc superoxide dismutases isolated from eukaryotes, just as the bacteriocuprein from Photobacterium has been shown to be. However, isolation and compositional analysis of tryptic peptides from Caulobacter bacteriocuprein has suggested that it contains amino acid substitutions at a number of sites which have been strictly conserved among the sequences of the eukaryote copper-zinc dismutases, from yeast to human. Consequently, Caulobacter bacteriocuprein may not be as closely related to the eukaryote enzymes as Photobacterium bacteriocuprein appears to be. Thus, the hypothesis of eukaryote to prokaryote gene transfer, proposed for the origin of the Photobacterium protein, may not be applicable for it. Alternative evolutionary mechanisms may therefore be necessary to explain the presence of the rare bacteriocuprein branch in the family tree of copper-zinc superoxide dismutases. |
