| Abstract: | Nitrate uptake, nitrite efflux and their regulation have been studied in the cyanobacterium Nostoc MAC. Nitrate uptake as well as nitrite-assimilation-dependent nitrite efflux systems consisted of two distinct phases comprising an initial rapid phase followed by a slower one. Whereas, 3-(3,4-dichlorophenyl)-1-1 dimethyl urea (DCMU), an inhibitor of photosystem II inhibited both nitrate uptake and nitrite efflux significantly, exogenous supply of ATP, however, stimulated both the processes, suggesting that PS II-mediated energy generation plays vital role in regulating both nitrate uptake as well as nitrite efflux. The inhibition of both the processes by p-chloromercuribenzoate (pCMB)1, N,N dicyclohexylcarbodiimide (DCCD) and carbonyl cyanide-m-chlorophenyl hydrazone (CCCP) indicated the involvement of -SH groups and ATP hydrolysis in the regulation of both the processes. Tungstate-treated cells having nonfunctional nitrate reductase although had a significant level of nitrate uptake, yet nitrite efflux by these cells was found to be negligible. These results suggest that (i) nitrate uptake and nitrite efflux processes in Nostoc MAC are energy-dependent; (ii) assimilation of nitrate via nitrate reductase is necessarily required for nitrite efflux to occur; (iii) nitrate uptake and nitrite efflux processes are regulated at different levels by different metabolic inhibitors; and (iv) cations of larger ionic radius facilitate the nitrate uptake and nitrite efflux processes. |
