ABSTRACT
When the
sta6
(starch-null) strain of the green microalga
Chlamydomonas reinhardtii
is nitrogen starved in acetate and then “boosted” after 2 days with additional acetate, the cells become “obese” after 8 days, with triacylglyceride (TAG)-filled lipid bodies filling their cytoplasm and chloroplasts. To assess the transcriptional correlates of this response, the
sta6
strain and the starch-forming
cw15
strain were subjected to RNA-Seq analysis during the 2 days prior and 2 days after the boost, and the data were compared with published reports using other strains and growth conditions. During the 2 h after the boost, ∼425 genes are upregulated ≥2-fold and ∼875 genes are downregulated ≥2-fold in each strain. Expression of a small subset of “sensitive” genes, encoding enzymes involved in the glyoxylate and Calvin-Benson cycles, gluconeogenesis, and the pentose phosphate pathway, is responsive to culture conditions and genetic background as well as to boosting. Four genes—encoding a diacylglycerol acyltransferase (
DGTT2
), a glycerol-3-P dehydrogenase (
GPD3
), and two candidate lipases (Cre03.g155250 and Cre17.g735600)—are selectively upregulated in the
sta6
strain. Although the bulk rate of acetate depletion from the medium is not boost enhanced, three candidate acetate permease-encoding genes in the
GPR1/FUN34/YaaH
superfamily are boost upregulated, and 13 of the “sensitive” genes are strongly responsive to the cell's acetate status. A cohort of 64 autophagy-related genes is downregulated by the boost. Our results indicate that the boost serves both to avert an autophagy program and to prolong the operation of key pathways that shuttle carbon from acetate into storage lipid, the combined outcome being enhanced TAG accumulation, notably in the
sta6
strain.
Identification and expression analysis of two inorganic C- and N-responsive genes encoding novel and distinct molecular forms of eukaryotic phosphoenolpyruvate carboxylase in the green microalga Chlamydomonas reinhardtii