Regulatory Systems that Quantitatively Alter Two Anionic Lipids of Chloroplasts in Chlamydomonas Reinhardtii upon Sulfur-Starvation

Abstract

Sulfoquinovosyl diacylglycerol (SQDG) and phosphatidylglycerol (PG) are commonly anionic membrane lipids of chloroplasts. Chlamydomonas reinhardtii starved for sulfur (S) degrades SQDG to ensure a sulfur-source, simultaneously elevating the PG content as if to compensate for the loss of SQDG. Similar compensation is observed even under normal growth conditions in a mutant (hf-2) of C. reinhardtii deficient in SQDG, which shows a higher content of PG than the wild type. We here investigated signaling mechanisms by which these two lipids alter in C. reinhardtii during S-starvation, with the use of two mutants (sac1 and sac3) defective in normal responses to ambient S-status, and hf-2. Compared with the wild type, both sac mutants were largely repressed in the induction of SQDG degradation, indicating involvement of SAC1 and SAC3 genes in the induction. On the other hand, the wild type increased the PG synthesis by 3.5-fold after S-starvation of 3 h whereas hf-2 has already been 2.8-fold higher in the PG synthesis than the wild type under S-replete conditions to maintain the level after the shift to S-sarved conditions. Therefore, in C. reinhardtii under S-starved conditions, the loss of SQDG seems to stimulate PG synthesis to up-regulate the PG content.