Low-temperature-induced stress activates lipid deacylation at the <i>sn</i> -1 position in the cyanobacterium <i>Synechocystis</i> sp. PCC 6803

Acyl-acyl carrier protein synthetase (Aas), involved in free fatty acid (FFA) recycling, is essential for the growth of Synechocystis sp. PCC 6803 at 30°C specifically under high-light (HL) stress conditions that activate lipid deacylation at the sn-2 position (400 μmol photons m-2 s-1). We found that even under low-light (LL) conditions (50 μmol photons m-2 s-1), however, Aas-deficient mutant cells grew much more slowly than wild-type cells at 22°C. The mutant accumulated much larger amounts of FFAs when cultivated at 22°C than when cultivated at 30°C, with C18 polyunsaturated FFAs comprising >90% of the total FFAs. At 22°C, the mutant cells also accumulated lysolipids derived from all four major lipid classes of cyanobacteria. The lysolipids were found to carry a C16 fatty acyl moiety. Since C18 and C16 fatty acids are esterified to the sn-1 and sn-2 positions, respectively, of membrane lipids in Synechocystis, the findings indicated that the low-temperature conditions activated lipid deacylation at the sn-1 position. In Synechococcus elongatus PCC 7942, whose membrane lipids carry mainly C16 fatty acids as acyl moieties and do not produce polyunsaturated fatty acids, low-temperature activation of lipid deacylation was detected in an Aas-deficient mutant, but the amount of resulting FFAs was small, and no growth inhibition was observed at 22°C. These results suggest that accumulation of toxic polyunsaturated FFAs was the cause of growth inhibition of the Synechocystis Aas mutant at 22°C and that Aas is normally preventing the accumulation of toxic products.