Shewanella frigidimarina NCIMB400 is a non-fermenting, facultative anaerobe from the gamma group of proteobacteria. When grown anaerobically this organism produces a wide variety of periplasmic c-type cytochromes, mostly of unknown function. We have purified a small, acidic, low-potential tetrahaem cytochrome with similarities to the cytochromes c3 from sulphate-reducing bacteria. The N-terminal sequence was used to design PCR primers and the cctA gene encoding cytochrome c3 was isolated and sequenced. The EPR spectrum of purified cytochrome c3 indicates that all four haem irons are ligated by two histidine residues, a conclusion supported by the presence of eight histidine residues in the polypeptide sequence, each of which is conserved in a related cytochrome c3 and in the cytochrome domains of flavocytochromes c3. All four haems exhibit low midpoint redox potentials that range from -207 to -58 mV at pH 7; these values are not significantly influenced by pH changes. Shewanella cytochrome c3 consists of a mere 86 amino acid residues with a predicted molecular mass of 11780 Da, including the four attached haem groups. This corresponds closely to the value of 11778 Da estimated by electrospray MS. To examine the function of this novel cytochrome c3 we constructed a null mutant by gene disruption. S. frigidimarina lacking cytochrome c3 grows well aerobically and its growth rate under anaerobiosis with a variety of electron acceptors is indistinguishable from that of the wild-type parent strain, except that respiration with Fe(III) as sole acceptor is severely, although not completely, impaired.

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