Role of glutamine synthetase activity in the uptake and metabolism of arginine and proline in the cyanobacterium Anabaena cycadeae

Abstract

The uptake of arginine and proline and their assimilation as nitrogen source have been studied in the cyanobacterium Anabaena cycadeae and its glutamine auxotropic mutant lacking glutamine synthetase activity. The uptake pattern of arginine and proline was found to be biphasic in both wild-type and mutant strains, consisting of an initial fast phase lasting up to 60 s followed by a slower second phase. The uptake activities of both the amino acids were also found to be similar in both the strains. The wild-type strain, having normal glutamine synthetase activity, utilized arginine and proline as sole nitrogen source, whereas the mutant strain lacking glutamine synthetase activity could not do so. These results suggest that: (1) glutamine synthetase activity is necessarily required for the assimilation of arginine and proline as nitrogen source, but it is not required for the uptake of these amino acids; and (2) glutamine synthetase serves as the sole ammonia-assimilating enzyme as well as glutamine-forming route in heterocystous cyanobacteria.