Characterization of the icfG Gene Cluster Implicated in the Regulation of Carbon Metabolism in the Cyanobacterium Synechocystis Sp. PCC 6803

Abstract

Both IcfG and PII are involved in the regulation of carbon metabolism in the cyanobacterium Synechocystis sp. PCC 6803. The PII protein was known to be phosphorylated on a Ser residue in response to both carbon and nitrogen supply. The icfG gene, which encodes a protein phospho-Ser phosphatase, is found within a gene cluster that also includes the slr1861 gene encoding a Ser kinase and the slr1859 and slr1856 genes whose products are substrates of Slr1861. While icfG null mutant was lethal when grown in the presence of glucose under inorganic-carbon-limiting conditions, slr1859 and slr1856 mutants were viable under different carbon regimes. However, the growth of a slr1859 null mutant was severely affected when cultured under inorganic-carbon-limiting conditions supplemented with glucose. The inactivation of slr1861 was unsuccessful under all conditions tested so far. Both Slr1861 and Slr1859, analyzed by immunodetection, were present in similar amounts in cells cultured with high- or low-concentrations of inorganic carbon, in the presence or absence of glucose. Slr1861 could phosphorylate Slr1856 and Slr1859 in vitro, but it could not phosphorylate PII under similar conditions. Among members of the icfG gene cluster, at least icfG and slr1859, and possibly also slr1861, were involved in the regulation of inorganic carbon assimilation in response to the presence of glucose. But this regulation is unlikely achieved though direct phosphorylation of PII by Slr1861.