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Cyanobacteria are the only organisms on earth known to synthesize cyanophycin, a polypeptide storage polymer composed of arginines linked to ß-carboxy groups of aspartates. The polypeptide is made not by ribosomes but by the enzyme cyanophycin synthetase. Karl Zeigler and others in the lab of Wolfgang Lockau have made a considerable advance in the study of cyanophycin by cloning the gene from Synechocystis PCC 6803 encoding the synthetic enzyme.
Cyanophycin synthetase was first purified from Anabaena variabilis owing to the relatively poor yield and stability of the same enzyme from Synechocystis. Six partial amino acid sequences were sufficient to identify an unidentified open reading frame (slr2002) in CyanoBase potentially encoding a protein of approximately the same molecular weight (96 kDa) as cyanophycin synthetase from Anabaena. The gene was cloned by PCR and expressed in E. coli.
The resulting production of cyanophycin in E. coli shows unequivocally that the protein product of a single gene is sufficient for the synthesis of cyanophycin. Both the biochemical characteristics of the enzyme and the sequence of the gene that encodes it suggests a relationship with peptide synthetases responsible for peptidoglycan biosynthesis.
Cyanophycin has long inspired speculation as to its role in nitrogen metabolism. Is it a dynamic store, as suggested by Noel Carr? Perhaps its metabolism contributes to the spacing of heterocysts? It clearly will not be long before we hear about mutants defective in cyanophycin metabolism and these speculations can be put to the ultimate test.
This work has recently appeared in Eur J Biochem (1998) 254:154-159.