Salt-dependent expression of glucosylglycerol-phosphate synthase, involved in osmolyte synthesis in the cyanobacterium Synechocystis sp. strain PCC 6803.

The cyanobacterium Synechocystis sp. strain PCC 6803 is able to acclimate to levels of salinity ranging from freshwater to twice the seawater concentrations of salt by accumulating the compatible solute glucosylglycerol (GG). Expression of the ggpS gene coding for the key enzyme (glucosylglycerol-phosphate synthase) in GG synthesis was examined in detail. Under control conditions, the GgpS protein is stable, so that weak constitutive transcription of the ggpS gene resulted in a significant protein content. However, the enzyme activity was biochemically switched off, and no GG was detectable. After a salt shock, an immediate increase in mRNA content proportional to the salt content occurred, while the GgpS protein and GG contents rose in a linear manner. Furthermore, the stability of the ggpS mRNA increased transiently. In salt-acclimated cells expression of the ggpS gene, the GgpS protein content, and the amount of accumulated GG depended linearly on the external salt concentration. Mapping of the 5' end of the ggpS transcript revealed a long nontranslated 5' sequence and a putative typical cyanobacterial promoter, which did not show any obvious salt-regulatory element. The alternative sigma factor sigma(F) was found to be involved in salt-dependent regulation of ggpS, since in a sigma(F) mutant induction of this gene was strongly reduced. The present study demonstrated that in addition to biochemical regulation of GgpS activity, alterations of ggpS expression are involved in regulation of GG synthesis in Synechocystis sp. strain PCC 6803. A model showing the interaction of the two regulatory levels is presented.