ROLE OF GLUTAMINE-SYNTHETASE ACTIVITY IN THE ARGININE AND PROLINE REGULATION OF HETEROCYST AND NITROGENASE FORMATION IN THE CYANOBACTERIUM ANABAENA-CYCADEAE

Growth and arginine and proline regulation of heterocyst and nitrogenase formation have been studied in the diazotrophic cyanobacterium Anabaena cycadeae and its glutamine auxotrophic mutant strain lacking glutamine synthetase (GS) activity. The wild-type strain having normal GS activity utilized arginine and proline as sole nitrogen source without producing heterocyst and nitrogenase activity. In contrast, the glutamine auxotrophic mutant lacking GS activity could not utilize these amino acids as sole nitrogen source. However, similar to the wild-type strain it showed complete repression of heterocyst and nitrogenase activity in the presence of both amino acids. Both wild-type and mutant strain showed high level of arginase and proline oxidase activities but arginine- and proline-dependent NH4+ generation was confined to only the mutant strain lacking GS activity. These results suggest that (1) GS does not control the formation of N-2-fixing heterocysts; and (2) GS activity is necessarily required for the assimilation of arginine and proline as sole nitrogen source.