Investigating Photosynthetic Stability: Relation Between Thylakoid Lipid Content and the Stability of the Cytochrome b6f Complex

The cytochrome b6f complex is an enzyme found in plants, cyanobacteria, and green algae that catalyzes the transport of electrons in the rate-limiting step of oxygenic photosynthesis. This dimeric complex has an extensive lipid architecture that is primarily composed of five distinct lipid classes: monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), phosphatidyl glycerol (PG), monoglucosyl diacylglycerol (GlcDG), and sulfoquinovosyl diacylglycerol (SQDG). While these lipid classes have been identified, their precise role in the function of the cytochrome complex are only beginning to be understood. Mechanisms describing the relation between thylakoid lipid content on the stability of the b6f complex are not known. This study validates the importance of the lipids on cytochrome b6f dimer formation and stability by showing that SQDG and the synthetic lipids 1,2-dioleoylphosphatidylglycerol and 2-dioleoyl-sn-glycero-3-phosphocholine reduce the temperature dependent rate of monomerization (denaturation) of the native dimer. A novel method of growing the cyanobacterium Synechococcus PCC 7002 anaerobically to test the relation between thylakoid lipid content and growth temperature was developed. This method of growing Synechococcus greatly reduces the relative SQDG content and increases the relative PG content in thylakoid membranes. The analysis of MGDG, DGDG, PG, GlcDG, and SQDG content in Synechococcus cultures grown at 30℃ and 33℃ revealed that the MGDG content depends inversely on the growth temperature.