UDP-Glucose Pyrophosphorylase Responsible for Sulfolipid Synthesis in a Green Alga Chlamydomonas Reinhardtii

摘要

Sulfoquinovosyl diacylglycerol (SQDG) is ubiquitous in thylakoid membranes of oxygenic photosynthetic organisms. Recently, a gene for UDP-glucose pyrophosphorylase (UGP3) was identified as participating specifically in SQDG synthesis in Arabidopsis thaliana. We here found that a mutant of Chlamydomonas reinhardtii disrupted in the LPB1 gene, a homologue of UGP3, lacked SQDG. It was concluded that LPB1 is the gene for UDP-glucose pyrophosphorylase involved in SQDG synthesis, based upon its responsibility for SQDG synthesis together with its similarity to the UGP3 protein in the amino acid sequence. We then examined how LPB1 expression is regulated at the transcript level in C. reinhardtii under S-starved conditions where SQDG synthesis capacity is known to increase, for protection of the PSⅠ complex, with elevation in the mRNA level of the SQD1 gene for SQDG synthesis. Semiquantitative analysis of transcripts by RT-PCR showed that the level of LPB1 mRNA was remarkably increased 4 h after onset of S-starvation, thereafter the enhanced level kept for the next 20 h. The results indicated cooperative up-regulation of the transcript level of LPB1 with that of SQD1, coinciding with the increase in the SQDG synthesis capacity.