Difference in SQDG Metabolism Between Green Algae and Cyanobacteria Under the Sulfur-Starved Condition

Abstract

A sulfolipid sulfoquinovosyl diacylglycerol (SQDG) is included predominantly in thylakoid membranes, having been shown in a green alga, Chlamydomonas reinhardtii, to contribute to the functioning of photosystem (PS) II through associating with the PSII complex for its normal conformation (Sato et al. 1995). Impaired photosystem II was found in a mutant of Chlamydomonas reinhardtii defective in sulfoquinovosyl diacylglycerol (Aoki et al. 2004). Besides its crucial role as a membrane lipid, we have recently found that SQDG is subject to degradation for providing the major S-source for synthesis of proteins in C. reinhardtii, upon transfer of the cells to sulfur (S)-starved conditions, thereby playing a role as a sulfur (S)-storage lipid. Here, it was investigated whether or not this regulation system operates also in other green alga, Chlorella kessleri, and in two strains of cyanobacteria, Synechocystis sp. PCC6803 and Synechococcus sp. PCC7942. The cells of the respective species when pre-incubated in the presence of [³⁵S]sulfate for universal labeling of cellular S-compounds were transferred to S-starved conditions for chasing of the radioactivity of SQDG. C. kessleri showed a decrease in the radioactivity of SQDG, in line with that in the SQDG content, whereas little decrease was evident in either strain of the cyanobacteria. These results indicated that SQDG degradation is induced in C. kessleri as well as in C. reinhardtii, but definitely not in either cyanobacterial species. The induction system of SQDG degradation thus seems to occur after the evolutionary appearance of green algae at latest, but not in cyanobacteria before the primary endosymbiosis.