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Protein phosphatase PphA from Synechocystis sp. PCC 6803: the physiological framework of P_II-P dephosphorylation

Institut für Mikrobiologie und Molekularbiologie, Justus-Liebig-Universität Giessen, 35392 Giessen, Germany

Correspondence
Karl Forchhammer
Karl.Forchhammer{at}mikro.bio.uni-giessen.de

The phosphorylated signal transduction protein P_II (P_II-P) in the cyanobacterium Synechocystis sp. strain PCC 6803 is dephosphorylated by PphA, a protein phosphatase of the 2C family (PP2C). In this study, the physiological conditions of P_II-P dephosphorylation were investigated with respect to the in vivo specificity of P_II-P towards PphA and the cellular abundance of PphA in cells growing under different nitrogen regimes. Furthermore, the consequences of impaired P_II-P dephosphorylation with respect to short-term inhibition of glutamine synthetase (GS) were studied. With a contribution of approximately 15 % of total Mn²⁺-dependent p-nitrophenyl phosphate hydrolysis activity, PphA has only a minor impact on the total PP2C activity in Synechocystis extracts. Nevertheless, residual P_II-P dephosphorylation in PphA-deficient cells could only be observed after prolonged incubation in the presence of ammonium. The abundance of PphA correlates with the phosphorylation state of P_II under nitrogen-replete conditions and is specifically enhanced by nitrite. Regulation of pphA expression operates at the post-transcriptional level. In the presence of nitrate/nitrite, PphA is present in molar excess over P_II-P, enabling the cells to rapidly dephosphorylate P_II-P in response to changing environmental conditions. A PphA-deficient mutant is not impaired in short-term inhibition of GS activity following ammonium treatment. Down-regulation of GS occurs by induction of gif genes (encoding GS inactivating factors 7 and 17), which is controlled by NtcA-mediated gene repression. Thus, impaired P_II-P dephosphorylation does not affect ammonium-prompted inactivation of NtcA.

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