Summary: The regulation of nitrogenase derepression, plus the catalytic activity and protein concentration of glutamine synthetase (GS), nitrate reductase (NR), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phycoerythrin (PE) were studied in the filamentous non-heterocystous cyanobacterium PCC 73110. Both nitrogen limitation and microaerobic incubation were essential for the derepression of nitrogenase. Oxygen caused irreversible inactivation of nitrogenase, as well as repression of its synthesis. A temporal separation of N fixation and net photosynthetic O evolution was observed under a N/CO (95:5, v/v) atmosphere. Repeated peaks of nitrogenase and growth were observed. Immunogold localization showed that in N-fixing cultures, all cells, including those undergoing division, contained nitrogenase, and that the nitrogenase antigen was uniformly distributed throughout the cells without any preferential association with cellular structures. Rubisco was mainly located in carboxysomes of both N-fixing and NO -grown cells. Both N-fixing and NO -grown cells showed similar levels of PE, which was associated with the thylakoid membranes. GS antigen was distributed throughout the cells and the relative amounts of this enzyme, as well as its activity, were 20% higher in N-fixing than in NO -grown cultures. NO uptake and NR systems were found to be NO inducible, with very low activities in N-fixing cultures. The latter may be important in avoiding competition for Mo between nitrogenase and NR.


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