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The gene encoding the bacteriocuprein superoxide dismutase from Photobacterium leiognathi, American Type Culture Collection strain 25521, was cloned in a pUC12 vector and sequenced. The nucleotide sequence predicted a 22-residue leader peptide amino-terminal to the known bacteriocuprein sequence. The expected precursor bacteriocuprein was directly identified in the in vitro translation products of the cloned gene by polyacrylamide gel electrophoresis and automated Edman degradation. Enzymatically active bacteriocuprein that lacked the leader peptide was identified in sonic extracts of Escherichia coli hosts containing the cloned gene. A single transcript of 580 nucleotides was observed in blots of total P. leiognathi RNA, and a unique site of transcriptional initiation was identified by primer extension analysis. P. leiognathi bacteriocuprein is the first bacteriocuprein whose gene has been isolated and sequenced and the first copper-zinc superoxide dismutase in which a leader peptide has been found. The presence of a leader peptide suggests that the bacteriocuprein is localized in the membrane or periplasm, in contrast to the eukaryotic copper-zinc superoxide dismutases, which are cytoplasmic enzymes. Such a difference in intracellular location could be important for understanding the presence and function of the uncommon, bacteriocuprein superoxide dismutase in P. leiognathi. |
