The amino acid sequences of the copper/zinc superoxide dismutases from swordfish and Photobacter leiognathi confirm the predictions made from the compositions.

Recent suggestions that the amino acid sequence of the copper/zinc superoxide dismutases of swordfish and Photobacter leiognathi do not support the theory that the bacterium obtained the gene for the enzyme by transfer from its eucaryotic symbiont [Rocha, H. A., Bannister, W. H. and Bannister, J. V. (1984) Eur. J. Biochem. 145, 477-484] are examined. The amount of difference between the sequence is in good agreement with expectation from the amino acid compositions. Moreover, the gene-transfer hypothesis cannot be discarded without postulating an enormous increase in the rate at which the superoxide dismutase gene has accumulated amino acid substitutions since the divergence of the swordfish and cattle lineages.