Evidence for ammonia as an inhibitor of heterocyst and nitrogenase formation in the cyanobacterium Anabaena cycadeae.

Growth and regulation of heterocyst and nitrogenase by fixed nitrogen sources were studied comparatively in parent and glutamine auxotrophic mutant of Anabaena cycadeae. The parent strain grew well on N2, NH+4 or glutamine while the mutant strain grew on glutamine but not on N2 or NH+4. The total lack of active glutamine synthetase in the mutant strain thus appears to be the reason for its observed lack of growth in N2 or NH+4, which explains why it is a glutamine auxotroph and at the same time shows glutamine synthetase to be the sole primary ammonia assimilating enzyme. NH+4 repression of heterocyst and nitrogenase in the mutant and the parental strains and their derepression by L-methionine-DL-sulfoximine suggest that NH+4 per se and not glutamine synthetase mediated pathway of ammonia assimilation is the initial repressor signal of heterocyst and nitrogenase in A. cycadeae.