A galactolipase activated by high light helps cells acclimate to stress in cyanobacteria

In the cyanobacterium Synechococcus elongatus PCC 7942, high-light (HL) stress activates deacylation of the 4 major lipid classes in the membrane. To investigate the mechanism and the physiological relevance of the HL-activated lipid deacylation, we searched for lipase genes of S. elongatus by measuring in vitro lipase activity of recombinant proteins expressed in Escherichia coli. Three genes (lipB, lipC, and lipD) were identified as lipase genes out of 14 candidates and lipB was found to be conserved in most cyanobacteria. His-tagged LipB protein showed acyl-hydrolyzing activity against galactolipids in vitro. In a strain deficient in acyl-acyl carrier protein synthetase and hence defective in the recycling of free fatty acids (FFA), HL-induced accumulation of FFA and lysogalactolipids was reduced by 45% by lipB inactivation, verifying that LipB is a lipase involved in the HL-induced deacylation of galactolipids. Deficiency of lipB in the wild-type (WT) background had no impact on PSII photoinhibition or its subsequent recovery; however, unlike WT cells, ΔlipB cells failed to quickly resume growth when irradiated with strong light (2,000 µmol photons m-2 s-1). The HL sensitivity of growth due to lipB deficiency was more pronounced under nitrogen-limiting conditions. The phenotype was rescued by wild-type LipB expression but not by inactive LipB variant expression. These results suggest that the deacylation of galactolipids by LipB helps cells acclimate to HL conditions by regulating factors other than PSII activity.