Copper-zinc superoxide dismutase of Caulobacter crescentus: cloning, sequencing, and mapping of the gene and periplasmic location of the enzyme

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Abstract
Although widely found in the cytoplasm of eucaryotes, the copper-zinc form of superoxide dismutase (CuZnSOD) has been identified in only a small number of bacterial species. One species is the freshwater bacterium Caulobacter crescentus, which also contains an SOD with iron as the metal cofactor (FeSOD). To investigate the function of this CuZnSOD and its structural relationship to the eucaryotic CuZnSODs, the gene encoding CuZnSOD (sodC) of C. crescentus CB15 was cloned and sequenced. By hybridization to pulsed-field electrophoresis gels, sodC was mapped near cysE in the C. crescentus chromosome. Through analysis of spheroplasts, the two SODs of C. crescentus were shown to be differently localized, CuZnSOD in the periplasm and FeSOD in the cytoplasm. In its natural habitat, C. crescentus is frequently associated with blue-green algae (cyanobacteria). The oxygen evolved by these photosynthetic algae may create an extracellular oxidative stress against which the periplasmic CuZnSOD may defend more effectively than the cytoplasmic FeSOD. Amino acid sequence alignments of C. crescentus CuZnSOD with eucaryotic CuZnSODs and with CuZnSOD of Photobacterium leiognathi (the only other bacterium from which CuZnSOD has been isolated and sequenced) suggest similar supersecondary structures for bacterial and eucaryotic CuZnSODs but reveal four novel substitutions in C. crescentus CuZnSOD: a phenylalanine critical to intrasubunit hydrophobic bonding replaced by alanine, a histidine ligand of zinc replaced by aspartate, and substitutions of two other previously invariant residues that stabilize zinc or both copper and zinc. These amino acid substitutions in C. crescentus CuZnSOD may have implications for its catalysis and stability.