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

Acyl-acyl carrier protein synthetase (Aas), involved in free fatty acid (FFA) recycling, is essential for growth of Synechocystis sp. PCC 6803 at 30 °C under the high-light (HL) stress conditions that activates lipid deacylation at the sn-2 position (400 μmol photons m−2 s−1), but not under low-light (LL) conditions (50 μmol photons m−2 s−1) Kojima et al. et al. (High-light-induced stress activates lipid deacylation at the sn-2 position in the cyanobacterium Synechocystis sp. PCC 6803. Plant Cell Physiol. 2022;63:82-91). Even under the LL conditions, however, Aas-deficient mutant cells grew much more slowly than WT cells at 22 °C. The mutant accumulated several times 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 suggested 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 accumulation of the toxic products.