The Synechococcus elongatus PII signal transduction protein controls arginine synthesis by complex formation with N-acetyl-L-glutamate kinase

This communication identifies, for the first time, a receptor protein for signal perception from the P-II signal transduction protein in the cyanobacterium Synechococcus elongatus. P-II, a phosphoprotein that signals the carbon/nitrogen status of the cells, forms a tight complex with the key enzyme of the arginine biosynthetic pathway, N-acetylglutamate (NAG) kinase. In complex with P-II, the catalytic activity of NAG kinase is strongly enhanced. Complex formation does not require the effector molecules of P-II, 2-oxoglutarate and ATP, but it is highly susceptible to modifications at the phosphorylation site of P-II, Ser-49. Stable complexes were only formed with the non-phosphorylated form of P-II but not with Ser-49 mutants. In accordance with these data, NAG kinase activity in S. elongatus extracts correlated with the phosphorylation state of P-II, with high NAG kinase activities corresponding to non-phosphorylated P-II (nitrogen-excess conditions) and low activities to increased levels of P-II phosphorylation (nitrogen-poor conditions), thus subjecting the key enzyme of arginine biosynthesis to global nitrogen control.