NO 2 ? Efflux and its regulation in cyanobacterium Nostoc MAC

Abstract

NO2−efflux and its regulation have been studied in the cyanobacterium Nostoc MAC. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), carbonyl cyanide-m-chlorophenyl hydrazone (CCCP), sodium azide, p-chloromercuribenzoate (PCMB), and dicyclohexylcarbodiimide (DCCD), a specific inhibitor of bacterial ATPase, inhibited the NO2−efflux activity singificantly. No NO2−efflux activity was observed under dark-aerobic as well as under dark-anaerobic conditions; however, the addition of ATP resulted in NO2−efflux even under dark-aerobic condition. Maximum NO2−efflux activity was observed when NO3−served as the sole nitrogen source. However, NH4+ions inhibited the NO2−efflux activity when both NO3−and NH4+wer simulatneously available to the cells. The NO2−efflux was freed from NH4+repression by l-methionine-dl-sulfoximine (MSX), an irreversible inhibitor of glutamine synthetase (GS). Chloramphenicol, a protein synthesis inhibitor, inhibited the derepression of NO2−efflux system when NH4+-incubated cells were transferred to NO3−medium. Tungstate-treated cells lacking functional NO3−reductase but having NO3−uptake activity also lacked NO2−efflux activity. These results suggest that (i) NO2−efflux in Nostoc MAC is NO3−dependent and an energy-dependent process that can be regulated at the levels of NO3−uptake and NO3−reductase; (ii) NO2−efflux system is NH4+repressible; however, the product of NH4+assimilation via GS is being required for repression to occur; (iii) de novo protein synthesis is required for derepression of the NO2−efflux system; and (iv) the catalytic activity of NO2−reductase also seems to play an important role in the regulation of NO2−efflux system.