Electron Transport Coupled to Nitrate Reduction in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

The electron transport system involved in nitrate reductionand its relationship to photosynthetic cyclic electron transportin a photodenitrifier, Rhodopseudomonas sphaeroides forma sp.denitrificans, were studied. Nitrate oxidized only b-type cytochromein the presence of cyanide, which inhibits nitrite reductase.Heptylhydroxyquinoline-N-oxide (HOQNO) inhibited the oxidationof b-type cytochrome by nitrate, but not the oxidation of b-and c-type cytochrome by nitrite. The inhibition by HOQNO wasovercome by phenazine methosulfate (PMS). Absorption changesof b-type cytochrome induced by illumination were in just theopposite directions for oxygen- and nitrate-oxidized cells;the cytochrome was reduced in oxygen-oxidized cells and oxidizedin nitrate-oxidized cells. Antimycin enhanced the reductionand inhibited the oxidation, but had no inhibitory effect onthe oxidation of b-type cytochrome by nitrate. Dithionite-reducedminus ferricyanide-oxidized difference spectra of cells at 77?Kshowed two b-type cytochrome components with a bands at 556.5and 562 nm. The proportion of the b-562 component decreasedin cells grown under denitrifying conditions. It was concludedthat a b-type cytochrome is involved in the nitrate reduction.The b-type cytochrome was presumed to be an alternative to thecytochrome b in the photosynthetic cyclic electron transport. ¹ Present address: Japanese Red Cross Tokyo-to Komagome BloodCenter, Komagome 2-2-2, Toshima-ku, Tokyo 170, Japan. (Received August 13, 1981; Accepted December 5, 1981)