Biochem. 269, 2353Biochem. 269, -2358. To understand the spread of the taxa in which PSII interacts with SQDG, especially in cyanobacteria, we produced a mutant defective in the putative sqdB gene responsible for SQDG synthesis from two cyanobacteria, Synechocystis sp. PCC6803 and Synechococcus sp. PCC7942. The mutant of PCC6803, designated SD1, lacked SQDG synthetic ability and required SQDG supplementation for its growth. After transfer from SQDG-supplemented to SQDG-free conditions, SD1 showed decreased net photosynthetic and PSII activities on a chlorophyll (Chl) basis with a decrease in the SQDG content. Moreover, the sensitivity of PSII activity to 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine was increased in SD1. However, SD1 maintained normal amounts of cytochrome b 559 and D1 protein (the subunits comprising the PSII complex) on a Chl basis, indicating that the PSII complex content changed little, irrespective of a decrease in the SQDG content. These results suggest that the role of SQDG is the conservation of the PSII properties in PCC6803, consistent with the results obtained with C. reinhardtii. In contrast, the SQDG-null mutant of PCC7942 showed the normal level of PSII activity with little effect on its sensitivity to PSII herbicides. Therefore, the difference in the SQDG requirement for PSII is species-specific in cyanobacteria; this could be of use when investigating the molecular evolution of the PSII complex.Keywords: cyanobacteria; glycolipid; photosystem; sulfoquinovosyl diacylglycerol; thylakoid membranes.Biomembranes constructed predominantly from lipids and proteins are sites of energy production, metabolism such as lipid synthesis, and the transportation of substances such as metabolites between the inside and outside of membrane systems. Thylakoid membranes in plant chloroplasts and cyanobacterial cells are unique in possessing photosynthetic electron transport and photophosphorylation systems for the conversion of light to chemical energy. The lipid composition of thylakoid membranes is highly conserved among higher plants, algae, and cyanobacteria, comprised mainly of the following four glycerolipids, monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), phosphatidylglycerol (PG), and sulfoquinovosyl diacylglycerol (SQDG) [1][2][3]. PG and SQDG possess negatively charged head groups, whereas MGDG and DGDG are noncharged lipids.Lipid analyses of subfractions and protein complexes of thylakoid membranes have shown specific lipid compositions, implying roles of some lipid classes in photosynthesis [4]. Significant differences in lipid composition and the fatty acid saturation state were observed, e.g. in extraction of photosystem II (PSII) from photosynthetic membranes with different detergents [5][6][7]. On the other hand, reconstitution of photosynthetic activity with the addition or removal of thylakoid lipids in vitro has also been performed with thylakoid membranes, subfractions of thylakoid membranes and purified protein complexes, with important observ...