Two c-Type Cytochromes, NirM and NirC, Encoded in the nir Gene Cluster of Pseudomonas aeruginosa Act as Electron Donors for Nitrite Reductase

Three c-type cytochromes, NirM, NirC, and NirN, are encoded in the nirSMCFDLGHJEN gene cluster for cytochrome cd1-type nitrite reductase (NIR) of Pseudomonas aeruginosa. nirS is the structural gene for NIR. NirM (cytochrome c551) is reported to be a physiological electron donor for nitrite reductase. The respective functions of NirC and NirN have remained unclear. In this study, we produced recombinant NirC and NirN in P. aeruginosa, and purified them from the periplasmic fraction. N-terminal amino acid sequences of the purified proteins showed that the N-terminal 31 and 18 residues of NirC and NirN precursors were cleaved, respectively, indicating that cleaved peptides act as signals for membrane translocation. In addition, the ability of NirC for electron donation to nitrite reductase was investigated. NirC, as well as NirM, was able to mediate the electron donation from the membrane electron pathway to NIR, suggesting that the structural gene for NIR is followed by the genes for two electron donors for NIR.