The physiological role of sulfoquinovosyl diacylglycerol (SQDG) in photosynthesis was investigated with a SQDG defective mutant (hf-2) of Chlamydomonas reinhardtii that did not have any detectable amount of SQDG. The mutant showed a lower rate of photosystem II (PSII) activity by 40% and also a lower growth rate than those of the wildtype. Results of genetical analysis of hf-2 strongly suggest that the SQDG defect and the lowered PSII activity are due to a single gene mutation. The supplementation of SQDG to hf-2 cells restored the lowered PSII activity to the same level as that of wild-type cells, and also enabled the mutant to grow even in the presence of 135 nM 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Moreover, the incubation of isolated thylakoid membranes of hf-2 with SQDG raised the lowered PSII activity. Chemical modifications of SQDG impaired the recovery of PSII activity. The results suggest that SQDG is indispensable for PSII activity in Chlamydomonas by maintaining PSII complexes in their proper state.Keywords: sulfoquinovosyl diacylglycerol; photosystem II; Chlamydomonas; thylakoid membrane; glycolipid.The primary process in photosynthesis occurs in thylakoid membranes. Protein complexes such as photosystem (PS) I and PSII play a role of energy conversion from excitation energy to redox potential. As autotrophic organisms obtain energy by photosynthesis, its efficiency is a matter of survival. Thylakoid membranes consist of four glycerolipids; monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), phosphatidylglycerol (PtdG) and sulfoquinovosyl diacylglycerol (SQDG). The lipophilic matrix may promote the efficiency of the reaction by organized pigment protein complexes [1][2][3]. Thylakoid membranes adapt to environmental conditions such as the unsaturation of lipids under low temperatures [4,5]. Thus, it is important to investigate the participation of thylakoid lipids in photosynthesis.Of all known thylakoid membrane lipids, SQDG is a unique acidic glycolipid due to an unusual head group, sulfoquinovose, and is likely to be present almost exclusively in thylakoid membranes [6]. In anoxygenic photosynthetic bacteria, it is localized in the Proteobacteria a-subgroup except for Rhodopseudomonas viridis whose photosystem resembles the PSII complex of higher plants [7]. Additionally, SQDG is present in organisms that perform oxygenic photosynthesis from cyanobacteria to higher plants, except for Gloeobacter violaceus PCC7421, which is considered as one of the ancient cyanobacteria and lacks thylakoid membranes [8,9].SQDG-null mutants of Rhodobacter sphaeroides and Synechococcus sp. PCC7942 did not show any effect on the photosynthetic apparatus [10,11]. By separation of the PSII complex using detergents, it can be shown that SQDG is associated with the PSII complex in thylakoid membranes [1,[12][13][14][15]. In contrast to the SQDG-null mutants of bacteria, the SQDG-defective mutant (hf-2) obtained by UV irradiation in Chlamydomonas reinhardtii showed a slightly slower growth rate and...
Photosystem (PS) II activity of a sulfoquinovosyl diacylglycerol (SQDG)-de¢cient mutant (hf-2) of Chlamydomonas was partially decreased compared with that of wild-type. The susceptibility to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modi¢ed in the mutant. Photometric measurements in the isolated thylakoid membranes of hf-2 revealed that the lowered activity in the mutant was derived from a decrease in the e⁄ciency of the electron donation from water to tyrosine Z, not from the e⁄ciency of the electron transport from Q A to Q B . This result was con¢rmed by the decay kinetics of chlorophyll £uorescence determined in vivo. We conclude that SQDG contributes to maintaining the conformation of PSII complexes, particularly that of D1 polypeptides, which are necessary for maximum activities in Chlamydomonas. ß
Chloramphenicolin!libitcd significamly hut incomplctcly photosynthesis in leaf scgmunts of rice. Fluorcscencc and polarographic experiments indicated that chlorinnphcnicol competes with ~hc CO? reducing cycle for clcctrons from photosystcm I because it serves as an clcctron acceptor oT photosystcm I and its reduction intcrmcdiatc transfers its clcctron to molecular oxygen.
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