A Critical Role for the cccA Gene Product, Cytochrome c2, in Diverting Electrons from Aerobic Respiration to Denitrification in Neisseria gonorrhoeae

Neisseria gonorrhoeae is a microaerophile that, when oxygen availability is limited, supplements aerobic respiration with a truncated denitrification pathway, nitrite reduction to nitrous oxide. We demonstrate that the cccA gene of Neisseria gonorrhoeae strain F62 (accession number NG0292) is expressed, but the product, cytochrome c(2), accumulates to only low levels. Nevertheless, a cccA mutant reduced nitrite at about half the rate of the parent strain. We previously reported that cytochromes c(4) and c(5) transfer electrons to cytochrome oxidase cbb(3) by two independent pathways and that the CcoP subunit of cytochrome oxidase cbb(3) transfers electrons to nitrite. We show that mutants defective in either cytochrome c(4) or c(5) also reduce nitrite more slowly than the parent. By combining mutations in cccA (Delta c(2)), cycA (Delta c(4)), cycB (Delta c(5)), and ccoP (ccoP-C368A), we demonstrate that cytochrome c(2) is required for electron transfer from cytochrome c(4) via the third heme group of CcoP to the nitrite reductase, AniA, and that cytochrome c(5) transfers electrons to nitrite reductase by an independent pathway. We propose that cytochrome c(2) forms a complex with cytochrome oxidase. If so, the redox state of cytochrome c(2) might regulate electron transfer to nitrite or oxygen. However, our data are more consistent with a mechanism in which cytochrome c(2) and the CcoQ subunit of cytochrome oxidase form alternative complexes that preferentially catalyze nitrite and oxygen reduction, respectively. Comparison with the much simpler electron transfer pathway for nitrite reduction in the meningococcus provides fascinating insights into niche adaptation within the pathogenic neisseriae.