Inactivation of spkD, encoding a Ser/Thr kinase, affects the pool of the TCA cycle metabolites in Synechocystis sp. strain PCC 6803.

The inactivation of sll0776 (spkD), a gene encoding a protein Ser/Thr kinase in Synechocystis PCC 6803, led to a pleiotropic phenotype of the SpkD null mutant. This mutant is impaired in its growth ability under low concentration of inorganic carbon (C(i)), though its C(i)-uptake system is not affected. Addition of glucose, phosphoglyceraldehyde or pyruvate does not allow the mutant to grow under low-C(i) conditions. In contrast, this growth defect can be restored when the low-C(i) culture medium is supplemented with metabolites of the TCA cycle. Growth of the mutant is also inhibited when ammonium is provided as nitrogen source, whatever the carbon regime of the cells, due to the high demand for 2-oxoglutarate, which is the carbon skeleton for ammonium assimilation. When mutant cells are cultured under standard growth conditions, the intracellular concentration of 2-oxoglutarate is 20 % lower than is observed in the wild-type strain. However, this decrease of 2-oxoglutarate level only slightly affects the phosphorylation state of PII, a protein that regulates nitrogen and carbon metabolism according to the intracellular levels of 2-oxoglutarate. Properties of the SpkD mutant suggest that the Ser/Thr kinase SpkD could be involved in adjusting the pool of the TCA cycle metabolites according to C(i) supply in the culture medium.