Isolation of a novel NAD(P)H-quinone oxidoreductase from the cyanobacterium Synechocystis PCC6803.

A novel NAD(P)H-quinone oxidoreductase (NQR) was isolated from the cyanobacterium Synechocystis PCC6803 by ion-exchange, affinity and gel-filtration chromatographies. Isolated NQR was found to be a drgA gene product that was a homodimer composed of 23-kDa subunits. It showed NAD(P)H-plastoquinone oxidoreductase activity with Km values for NADPH and NADH of 12 and 48 microM, respectively. The activity was inhibited by thiolmodifying reagents, but not by rotenone, amobarbital, salicylhydroxamic acid, dicumarol, flavone, or diphenyleneiodonium chloride. Therefore, the Cys-147 residue is probably involved in the catalytic reaction. The amino acid sequence of the purified NQR had some homology with those of NADH oxidase, NAD(P)H-flavin oxidoreductase, and nitroreductase but did not contain either an adenine-binding motif or a phosphate-binding motif, thus, it is a new type of NQR.