NITROGENASE AND ASPECTS OF ITS REGULATION IN CYANOBACTERIA

ABSTRACT The ability of various cyanobacteria (blue-green algae) to fix N2 is now well established. The most studied unicellular N2-fixing form is Gloeothece; about half the non-hetero- cystous forms fix N2 anaerobically, and heterocystous forms usually fix N2 aerobically and anaerobically. Cyanobacterial nitrogenase is little different to that of other N2- fixing prokaryotes, and the enzyme is extremely O2 sensitive. This O2 sensitivity can be partially overcome by H2; the reductant source for nitrogenase is fixed carbon from the vetative cells and H2 utilized by an uptake hydrogenase in heterocysts can also support nitrogenase activity. There is evidence that Φψ may be involved in regulation of nitrogenase activity. ATP can be generated by photophosphorylation and respiration in heterocysts, and the primary route of NH+ 4 assimilation is the glutamine synthetase-glutamate synthase pathway. Characteristics of purified glutamine synthetase from heterocystous and non-heterocystous cyanobacteria are present...