Three acyltransferases and a nitrogen responsive regulator are implicated in nitrogen starvation-induced triacylglycerol accumulation in Chlamydomonas

Three acyltransferases, DGAT1, DGTT1, and PDAT1, are induced by nitrogen starvation and are likely to have a role in TAG accumulation based on their patterns of expression. Each gene also shows increased mRNA abundance in other TAG-accumulating conditions (-S, -P, -Zn, -Fe). Insertional mutants, pdat1-1 and pdat1-2, accumulate 25% less TAG compared to the parent strain, D66+, which demonstrates the relevance of the trans-acylation pathway in Chlamydomonas. The biochemical functions of DGTT1 and PDAT1 were validated by rescue of oleic acid sensitivity and restoration of TAG accumulation in a yeast strain lacking all acyltransferase activity. Time course analysis suggest than an SBP domain transcription factor protein, whose mRNA increases precede that of other genes like DGAT1, is a candidate regulator of the N-deficiency response. An insertional mutant, nrr1-1, accumulates only 50% of the TAG compared to the parental strain in N-starvation conditions and is unaffected by other nutrient stresses, suggesting the operation of multiple signaling pathways leading to stress-induced TAG accumulation. Sampling of Chlamydomonas 2137 cultivated in TAP, N-Free TAP or TAP with varying amounts of N.