Evidence for the nitrate assimilationdependent nitrite excretion in cyanobacterium Nostoc MAC

Nitrate uptake and nitrite efflux patterns in Nostoc MAC showed a rapid phase followed by their saturation. Nitrite efflux was maximum in nitrate medium whereas the cells incubated in N and NH media exhibited a decreased nitrite efflux activity. The simultaneous presence of NH and nitrate significantly decreased nitrite efflux. L-Methionine-DL-sulphoximine (MSX) prevented inhibition of nitrite efflux by NH . In the dark there was negligible nitrite efflux, whereas illumination increased the rate of nitrite efflux significantly. The nitrite efflux system was maximally operative at pH 8.0, 30°C and a photon fluence rate of 50μmol m. s. These results confirm that (i) the nitrite efflux system in Nostoc MAC is dependent upon nitrate uptake and assimilation and is repressible by NH; (ii) NH itself is not the actual represser of nitrite efflux; a product of NH assimilation via glutamine synthetase (GS) is required for repression to occur; (iii) the catalytic function of GS does not appear to be involved in nitrate assimilation-dependent nitrite efflux, and (iv) the optimum pH, temperature and illumination for maximum nitrite efflux were found to be 8.0, 30°C and 50 μmol m s respectively.