The cyanophycin granule polypeptide (CGP) is known to serve as a nitrogen reserve protein in cyanobacteria and is mobilized during nitrogen starvation. An exopeptidase, provisionally called cyanophycinase, which degraded CGP in vitro has been studied, and its product characterized, in two N-fixing species. The enzyme had a pH optimum of 85, had no requirement for monovalent or divalent cations and was inhibited by -arginine and -aspartic acid. The product of this enzyme was an aspartic acid-arginine dipeptide. A higher activity of both arg-poly(asp) synthetase and cyanophycinase was observed in extracts of heterocysts of both species than in vegetative cells. This supports the view that CGP has a dynamic role in nitrogen metabolism of cyanobacteria as well as a storage function.


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