1887

Abstract

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

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2005-04-01
2019-10-21
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