The Response Regulator Slr1588 Regulates But Is Not Crucial for Salt Acclimation of sp. PCC 6803.

Cyanobacterial sucrose biosynthesis is stimulated under salt stress, which could be used for biotechnological sugar production. It has been shown that the response regulator Slr1588 negatively regulates the gene encoding sucrose-phosphate synthase and mutation of the gene also affected the salt tolerance of (Chen et al., 2014). The latter finding is contrary to earlier observations (Hagemann et al., 1997b). Here, we observed that ectopic expression of did not restore the salt tolerance of the mutant, making the essential function of this response regulator for salt tolerance questionable. Subsequent experiments showed that deletion of the entire coding sequence of compromised the expression of the downstream situated gene, which encodes glucosylglycerol-phosphate phosphatase for synthesis of the primary osmolyte glucosylglycerol. Mutation of by deleting the N-terminal part of this protein (Δ-F976) did not affect expression, glucosylglycerol accumulation as well as salt tolerance, while the mutation of resulted in the previously reported salt-sensitive phenotype. In the Δ-F976 mutant was up-regulated but sucrose content was lowered due to increased invertase activity. Our results reveal that Slr1588 is acting as a repressor for as previously suggested but it is not crucial for the overall salt acclimation of .