The Primary Structure of Cu-Zn Superoxide Dismutase fromPhotobacterium leiognathi:Evidence for a Separate Evolution of Cu-Zn Superoxide Dismutase in Bacteria

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Abstract
The complete amino-acid sequence of the copper-zinc superoxide dismutase of the P. leiognathi was determined. The fragmentation strategy employed included cyanogen bromide cleavage at its Met residues and the only Try residue. The S-carboxymethylated chain was further cleaved by means of trypsin, in order to obtain overlapping fragments. For sequence determination automated solid or liquid-phase techniques of Edman degradation were used. C-Terminal amino acids of the entire chain were determined after treatment with carboxypeptidase A. Comparison of the primary structure of this bacterial Cu-Zn superoxide dismutase with the established amino-acid sequences of the other eukaryotic Cu-Zn superoxide dismutases revealed clear homologies. Correspondingly, the Cu-Zn-binding amino-acid residues of the active center were localized: His45, His47, His70, His79, His125 and Asp91. The 2 Cys residues in position 52 and 147 were homologous to the Cys residues, modelling the essential intrachain disulfide bridge of the corresponding bovine enzyme. As only 25-30% of aligned sequence positions were found to be identical, the enzyme of P. leiognathi shows only a remote phylogenetic relationship towards eukaryotic Cu-Zn superoxide dismutases. When compared to the established phylogenetic tree of the cytochrome c family, this indicates a separate evolution of Cu-Zn superoxide dismutase in Photobacterium. A natural gene transfer from the eukaryotic host (ponyfish) to the prokaryotic photobacterium, which Martin and Fridovich postulated 1981 on the basis of amino-acid compositions, can be excluded.