Involvement in denitrification of the napKEFDABC genes encoding the periplasmic nitrate reductase system in the denitrifying phototrophic bacterium Rhodobacter sphaeroides f. sp. denitrificans

Seven genes, napKEFDABC, encoding the periplasmic nitrate reductase system were cloned from the denitrifying phototrophic bacterium Rhodobacter sphaer oides f. sp. denitrificans IL106, Two transmembrane proteins, NapK and NapE , an iron-sulfur protein NapF, a soluble protein NapD, a catalytic subunit of nitrate reductase precursor NapA, a soluble c-type diheme cytochrome pre cursor NapB, and a membrane-anchored c-type tetraheme cytochrome NapC were deduced as the gene products. Every mutant in which each nap gene was disru pted by ba-cassette insertion lost nitrate reductase activity as well as th e ability of cells to grow with nitrate under anaerobic-dark conditions. A transconjugant of the napD-disrupted mutant with a plasmid bearing the napK EFDABC genes recovered both nitrate reductase activity and nitrate-dependen t anaerobic-dark growth of cells. Denitrification activity, which was not o bserved in the napD mutant, was also restored by the conjugation, These res ults indicate that the periplasmic nitrate reductase encoded by the napKEF- DABC genes Is the enzyme responsible for denitrification in this phototroph , although the presence of a membrane-bound nitrate reductase has been repo rted in the same strain.