SUMMARY: A periodicity in nitrogen fixation potential with respect to the light-dark regime was studied in the filamentous non-heterocystous cyanobacterium sp. NIBB 1067. During a 12 h light/12 h dark cycle, potential nitrogenase activity measured by acetylene reduction in the light was insignificant in the dark period, but developed after illumination for 1 to 3 h. Maximum nitrogenase activity was found at the middle of the light period, and activity decreased near the end of the light period. Manipulation of the length of the light and dark periods, and use of the glutamine synthetase inhibitor L-methionine sulphoximine, led to the conclusion that (1) the periodicity in activity was not attributable to an endogenous rhythm, (2) development and maintenance of nitrogenase activity in was regulated by the light period, and (3) the decrease in activity at the end of the light period was due to the accumulation of an intermediate(s) in nitrogen metabolism. The nitrogenase Fe- and MoFe-proteins were always present despite the changes in nitrogenase activity associated with the light-dark cycle. However, a change in apparent molecular mass of the Fe-protein on SDS-PAGE correlated with the change in nitrogenase activity. The results indicate that changes of nitrogenase activity in under a light-dark regime can be attributed to activation and deactivation of the Fe-protein, and that the activation of the protein depends on light.


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