Light-driven structural changes in thylakoid and cytoplasmic membranes of a cyanobacterium, Synechocystis PCC 6803

In a previous communication (Kis et al. (1998)), we showed that in Synechocystis PCC 6803 cells the expression of the genes encoding the lipid desaturases considered responsible for the temperature adaptation of the membrane fatty acid composition is photoregulated. We now analyze the morphological, functional, and structural differences between light-activated heterotrophic (LAHG) and photoautotrophic cells grown at 25 or 35 °C. In the dark, the amounts of phycobilisomes and thylakoids are lower in the cells grown at 35 °C, and even more so at 25 °C. The photosynthetic apparatus remains functional in the cells grown in LAHG condition, but is unable to acclimatize its oxygen evolution to 25 °C. The cells grown photoautotrophically at 25 °C contain more polyunsaturated fatty acids, including the α-18 : 3 and 18 : 4 species, which are not found in light grown cells at 35 °C, nor in any LAHG cells. The protein to lipid ratios are substantially higher in the thylakoid membranes of the LAHG cells, whereas these ratios are constant in the cytoplasmic membranes. In light-grown cells, the phase-transition temperatures of the membranes are lower than in LAHG-counterparts. Comparisons of the membrane dynamics at the temperatures of maximal photosynthetic oxygen evolution in the different thylakoid membranes reveal that the fatty acids α-18 : 3 and 18 : 4 are needed for the adaptation of the photosynthetic apparatus to lower temperatures. Thus, the light-regulated desaturase gene expression is manifested in two effects: the regulation of membrane dynamics in general, and the provision of special fatty acids for protein-lipid interactions in particular.