Cyanophycin and arginine metabolism in cyanobacteria

Cyanobacteria are oxygenic phoautotrophs that can utilize inorganic nitrogen salts, atmospheric nitrogen and some amino acids such as arginine as nitrogen source. Under unbalanced growth in the presence of sufficient nitrogen, many cyanobacteria accumulate cyanophycin, a co-polymer of aspartate and arginine that serves as a nitrogen reservoir. Cyanophycin metabolism enzymes include cyanophycin synthetases, cyanophycinase and isoaspartyl dipeptidase. The latter splits β‑aspartyl arginine released from cyanophycin by cyanophycinase into aspartate and arginine. The arginine catabolic pathway of cyanobacteria has been recently elucidated and consists of two bifunctional enzymes, arginine-guanidine removing enzyme (AgrE) and proline oxidase (PutA). This pathway makes available to metabolism the four nitrogen atoms of arginine, three as ammonia and one as glutamate. A variant of the pathway cycles ornithine (an intermediate in the AgrE-catalyzed reactions) back to arginine incorporating aspartate and, hence, recovering its nitrogen atom for metabolism. Many cyanobacteria also make use of this pathway to utilize arginine taken up from the outer medium through a high-affinity ABC transporter. An analysis of co-occurrence in cyanobacteria of genes encoding cyanophycin metabolism and arginine catabolism enzymes and arginine and aspartate transporters indicates a strong correlation between the presence of cyanophycin and the AgrE/PutA pathway.