PII signalling in unicellular cyanobacteria: analysis of redox‐signals and energy charge

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Abstract
This communication presents a short outline of the current knowledge on the molecular basis of PII signal transduction in unicellular cyanobacteria with respect to the perception of environmental stimuli. First, the general characteristics of the PII signalling system in unicellular cyanobacteria are presented, the hallmark of which is modification by serine‐phosphorylation, as compared to the paradigmatic PII signal transduction system in proteobacteria, which is based on tyrosyl‐uridylylation. Then, the focus is turned on the signals controlling PII phosphorylation state. Recently, the cellular phosphatase (termed PphA), which specifically dephosphorylates phosphorylated PII (PII‐P) was identified in Synechocystis sp. strain PCC 6803. With the availability of a PphA‐deficient mutant and the purified components for in vitro assay of PphA mediated PII‐P dephosphorylation, novel insights into the signals, to which PII‐P dephosphorylation responds, can be obtained. Here we present an investigation of the response of PII‐P dephosphorylation towards treatments that affect the redox‐balance of the cells. Furthermore, a possible role of varying ATP/ADP ratios on PII‐P dephosphorylation was examined. From these studies, together with previous investigations, we conclude that PII‐P dephosphorylation specifically responds to changes in the levels of central metabolites of carbon metabolism, in particular 2‐oxoglutarate.