Sulfoquinovosyldiacylglycerol has an Essential Role inThermosynechococcus elongatusBP-1 Under Phosphate-Deficient Conditions

Abstract

Anionic lipids, sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), are major classes of the thylakoid membrane lipids in cyanobacteria and plant chloroplasts. PG is essential for growth and photosynthesis of cyanobacteria, algae and plants, but the requirement for SQDG differs even among cyanobacterial species. Although SQDG and PG can compensate each other in part presumably to maintain proper balance of anionic charge in lipid bilayers, the functional relationship of these lipids is largely unknown. In this study, we inactivated the sqdB gene, encoding a UDP-sulfoquinovose synthase and involved in SQDG biosynthesis, in Thermosynechococcus elongatus BP-1. In wild-type cells, PG accounted for only approximately 3.5 mol% of total membrane lipids, but its content was substantially increased along with complete loss of SQDG in the sqdB mutant. Under phosphate (Pi)-sufficient conditions, the growth rate and PSII activity were slightly lower in sqdB than in wild-type cells. In addition, the formation of PSI trimers and PSII dimers and energy transfer in phycobilisomes were perturbed in the mutant. Under Pi-deficient conditions, the growth of sqdB cells was severely impaired, with a decrease in PSII activity. PG supplementation could partially rescue the defective growth and PSII activity of Pi-deficient sqdB cells but fully recovered the impaired growth of the pgsA mutant of T. elongatus, which is deficient in PG biosynthesis. These data suggest that SQDG has a specific role in the growth and photosynthesis of T. elongatus, which cannot be complemented by PG, particularly under Pi-deficient conditions.

Thylakoid membrane lipid sulfoquinovosyl-diacylglycerol (SQDG) is required for full functioning of photosystem II in Thermosynechococcus elongatus

“…The results showed that the oxygen-evolving activity of the mutant samples decreased by about 19 -32% in the thylakoid membranes and purified PSII dimers. This decrease is comparable with those observed with the mutant cells of T. elongatus reported previously (3) and also with the previous report in the C. reinhardtii mutant with the SQDG synthesis gene inactivated (15)(16)(17). To examine the causes for the decrease of PSII activity in detail, we performed spectroscopic analyses with the WT and ⌬sqdB samples.…”

“…The roles of lipids can be classified in two ways: one is to form the lipid bilayer, and the other one is to associate with some specific sites of membrane protein(s) to function as an essential part of that protein(s). MGDG and DGDG are nonionic lipids and are considered to function mainly as structural lipids for the formation of the lipid bilayer, whereas SQDG and PG are acidic lipids and are probably required to maintain the balance of negative charges in the thylakoid membranes (3,4). In addition, all of these lipids are found to bind directly to photosynthetic membrane-protein complexes, such as photosystem II (PSII), photosystem I (PSI), and cytochrome b 6 f (5)(6)(7)(8), and therefore may be required to maintain the integral functions of these protein complexes.…”

“…PCC 7002), inactivation of the genes for SQDG synthesis resulted in very little effects on their growth, oxygen-evolving activity, and the sensitivity to herbicides such as 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) under normal growth conditions (11)(12)(13). The effects of inactivation of the gene for SQDG synthesis, however, became much more severe under PG-limiting conditions caused by phosphate (P i ) deficiency or by genetic disruption of the PG biosynthesis pathway (3,11,14). In Chlamydomonas reinhardtii, deletion of the gene for SQDG synthesis resulted in a decrease of oxygen-evolving activity by 30 -40% and an increase in the sensitivity to DCMU (15)(16)(17).…”

“…PCC 6803 but not in other cyanobacteria, and how this inhibition occurs. Recently, the gene sqdB responsible for SQDG synthesis was disrupted in a thermophilic cyanobacterium Thermosynechococcus elongatus BP-1, and the resulting mutant (⌬sqdB) was characterized in terms of growth and photosynthetic activities (3). It was found that the ⌬sqdB mutant had a slightly decreased growth rate and O 2 -evolving activity than the WT under normal growth conditions, and both PSII dimer and PSI trimer were monomerized, with the PSII dimer almost disappearing based on the blue native-PAGE analysis.…”

“…The ⌬sqdB mutant cells have been shown to have slightly decreased oxygen-evolving activity in the presence of PSII electron acceptors (3). We further compared the oxygen-evolving activities of isolated thylakoid membranes and PSII dimers between WT and ⌬sqdB with 0.4 mM phenyl p-benzoquinone and 1 mM potassium ferricyanide as electron acceptors.…”