DIRECT EVALUATION OF EFFECTS OF FATTY-ACID UNSATURATION ON THE THERMAL-PROPERTIES OF PHOTOSYNTHETIC ACTIVITIES, AS STUDIED BY MUTATION AND TRANSFORMATION OF SYNECHOCYSTIS PCC6803

Glycerolipids of thylakoid membranes isolated from the cyanobacterium Synechocytis PCC6803 contained high levels of dienoic and trienoic C18 fatty acids, in addition to saturated C16 and monoenoic C18 fatty acids. A mutant (Fad12) of this cyanobacterium was defective in the desaturation of C18 fatty acids at the DELTA-12 position, and its thylakoid membranes lacked trienoic acids and contained a very reduced level of dienoic acids. A derivative strain of Fad12 (Fad12/desA), which had been transformed with a gene for desaturation at the DELTA-12 position, fully recovered the ability to desaturate the fatty acids in the glycerolipids of thylakoid membranes. The thermal properties of the photosynthetic activities of the mutant and the transformant were compared with those of the wild-type strain. Despite great diversity in the extent of unsaturation of fatty acids between the wild-type, Fad12, and Fad12/desA strains, no significant differences were found either in the temperature dependence of photosynthesis or in the heat stability of photosynthetic, photosystem II and photosystem I activities. These results demonstrate that the trienoic fatty acids and, probably, the dienoic acids of the lipids in the thylakoid membrane do not affect the thermal properties of the above-mentioned activities of photosynthesis.