The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes.

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Abstract
The free-living bacterium Photobacterium leiognathi is also known to be a symbiont of ponyfish. The presence of a copper/zinc superoxide dismutase in P. leiognathi has been considered to be a case of gene transfer from eukaryotes to prokaryotes because this form of superoxide dismutase is normally present only in higher eukaryotic species. However, the amino acid sequence of the enzyme from the bacterium exhibited low identities (25-30%) with the same enzyme from eukaryotes. When amino acid mutations are taken into consideration, the weighted sequence similarity increases significantly; furthermore, the bacterial enzyme has the same active site residues and similar predicted secondary structure as the eukaryotic enzymes. The possibility of convergence is ruled out and the case of divergence is considered unlikely because of the observed phylogenetic distribution of the enzyme. This indicates that the presence of the copper/zinc superoxide dismutase in P. leiognathi can indeed be considered a case of gene transfer from eukaryotic species to prokaryotic species.