MEMBRANE LIPID UNSATURATION MODULATES PROCESSING OF THE PHOTOSYSTEM-II REACTION-CENTER PROTEIN D1 AT LOW-TEMPERATURES

The role of membrane lipid unsaturation in the restoration of photosys tem II (PSII) function and in the synthesis of the D1 protein at diffe rent temperatures after photoinhibition was studied in wildtype cells and a mutant of Synechocystis sp. PCC 6803 with genetically inactivate d desaturase genes. We show that posttranslational carboxyl-terminal p rocessing of the precursor form of the D1 protein is an extremely sens itive reaction in the PSII repair cycle and is readily affected by low temperature. Furthermore, the threshold temperature at which perturba tions in D1-protein processing start to emerge is specifically depende nt on the extent of thylakoid membrane lipid unsaturation, as indicate d by comparison of wildtype cells with the mutant defective in desatur ation of 18:1 fatty acids of thylakoid membranes. When the temperature was decreased from 33 degrees C (growth temperature) to 18 degrees C, the inability of the fatty acid mutant to recover from photoinhibitio n was accompanied by a failure to process the newly synthesized D1 pro tein, which accumulated in considerable amounts as an unprocessed prec ursor D1 protein. Precursor D1 integrated into PSII monomer and dimer complexes even at low temperatures, but no activation of oxygen evolut ion occurred in these complexes in mutant cells defective in fatty aci d unsaturation.