Studies on the Regulation of Arginine Metabolism in Cyanobacteria Should Include Mixotrophic Conditions

摘要

In most bacteria ammonium assimilation takes place through the very efficient and ATP-consuming glutamine synthetase-glutamate synthase pathway, which produces two amino acids that are general distributors of nitrogen in cellular metabolism, glutamine and glutamate (1). A very direct use of these amino acids takes place in arginine biosynthesis, which starts with glutamate as precursor and in which another glutamate molecule, aspartate, and carbamoyl phosphate provide the three nitrogen atoms included in the guanidine group; carbamoyl phosphate is synthesized from glutamine, bicarbonate, and ATP (2). Given its high energetic demand, this pathway is usually subjected to feedback inhibition by arginine of one of its first enzymatic steps. In cyanobacteria, N-acetylglutamate kinase (NAGK) is inhibited by arginine, but under sufficient nitrogen, the C/N balance and energy status indicator PII protein (glnB gene product) binds to NAGK, relieving its inhibition by arginine, with the effect of increasing the production of arginine by the biosynthetic pathway (3). Bolay et al. (4) now add another player to this regulatory system that they have identified in the unicellular cyanobacterium Synechocystis, the 51-amino-acid protein PirA. In response to ammonia upshifts, PirA binds to PII in an ADP-dependent manner, preventing binding of PII to NAGK and making this enzyme susceptible again to inhibition by arginine (Fig. 1).