Dephosphorylation of the phosphoprotein P-II in Synechococcus PCC 7942: identification of an ATP and 2-oxoglutarate-regulated phosphatase activity

The phosphorylation state of the putative Signal transduction protein P-II from the cyanobacterium Synechococcus sp. strain PCC 7942 depends on the cellular state of nitrogen and carbon assimilation. In this study, dephosphorylation of phosphorylated P-II protein (P-II-P) was investigated both in vivo and in vitro. The in vivo studies implied that P-II-P dephosphorylation is regulated by inhibitory metabolites involved in the glutamine synthetase-glutamate synthase pathway of ammonium assimilation. An in vitro assay for P-II-P dephosphorylation was established that revealed a Mg2+-dependent P-II-P phosphatase activity. P-II-P phosphatase and P-II kinase activities could be separated biochemically. A partially purified P-II-P phosphatase preparation also catalysed the dephosphorylation of phosphoserine/phosphothreonine residues on other proteins in a Mg2+-dependent manner. However, only dephosphorylation of P-II-P was regulated by synergistic inhibition by ATP and 2-oxoglutarate. As the same metabolites stimulate the P-II kinase activity, it appears that the phosphorylation state of P-II is determined by ATP and 2-oxoglutarate-dependent reciprocal reactivity of P-II towards its phosphatase and kinase.