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

Enrique Flores, Eduardo A. Groisman, Ray Dixon
2021 mBio  
KEYWORDS arginine biosynthesis, arginine catabolism, mixotrophy, ornithineammonium cycle n 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
more » ... utamate 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 P II 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 P II in an ADP-dependent manner, preventing binding of P II to NAGK and making this enzyme susceptible again to inhibition by arginine (Fig. 1 ). The ammonia upshift has a general effect in Synechocystis, increasing the levels of many amino acids, notably glutamine, aspartate, arginine, and two arginine biosynthesis intermediates, ornithine and citrulline (4). Inactivation of pirA exacerbates the increase of aspartate, ornithine, and citrulline in response to the ammonia upshift, an effect that is counteracted by overexpression of pirA. Additionally, inactivation of pirA increases, and overexpression of pirA decreases, the levels of glutamate, proline, and some other amino acids, and overexpression of pirA decreases glutamine and arginine levels. These effects are consistent with a role of PirA in regulation of NAGK as shown by Bolay et al. ( 4 ), so that PirA appears to contribute to keep low levels of arginine biosynthesis intermediates downstream of NAGK as well as low levels of arginine and the arginine catabolism products proline and glutamate (see next paragraph). Nonetheless, the proposed mode of action of PirA, through ADP-dependent binding to P II , points to a possible more general effect of PirA in regulation of the C/N balance in cyanobacteria. Bolay et al. (4) interpret their results as an effect of PirA controlling "flux into the ornithine-ammonia cycle." This cycle (5) is a side activity of the arginine catabolism pathway and was defined before the whole activity of the key enzyme in cyanobacterial arginine catabolism, AgrE (arginine-guanidine-removing enzyme [6, 7]), was known. In the arginine catabolism pathway, AgrE (bifunctional arginine dihydrolase/ornithine cyclodeaminase) produces proline from arginine with ornithine as an intermediate, releasing one CO 2 and three NH 4 1 molecules, and proline is further catabolized to glutamate by PutA (Fig. 1 ). It is possible that some ornithine is released from AgrE reentering the arginine biosynthesis pathway, hence the "ornithine-ammonia cycle" (5), Citation Flores E. 2021. Studies on the regulation of arginine metabolism in cyanobacteria should include mixotrophic conditions. mBio 12:e01433-21.
doi:10.1128/mbio.01433-21 pmid:34154397 fatcat:iljuvoevorc2vodl2oqvkv5zai