DGAT2, a gene encoding type-2 diacylglycerol acyltransferase, is involved in hyper-accumulation of triacylglycerol in a Chlamydomonas reinhardtii starchless mutant

Yantao Li, Milton Sommerfeld, and Qiang Hu

Laboratory for Algae Research and Biotechnology (LARB), Arizona State University Polytechnic campus, 7001 E. Williams Field Road, Mesa, AZ 85212

Many unicellular microalgae use starch and triacylglycerol (TAG) as the major carbon and energy storage compounds under stress conditions (e.g., excessive light, nutrient starvation). Both starch and TAG are promising feedstocks for biofuels, but TAG is more preferable for high energy density fuels (e.g., gasoline, diesel and jet fuel). Since synthesis of starch and TAG shares common precursors from the Calvin cycle, we hypothesized that overproduction of TAG may be accomplished by shunting photosynthetic carbon flow from the starch synthesis pathway to TAG formation. Using Chlamydomonas reinhardtii as a model system, we demonstrated that a starchless mutant defective in ADP-glucose pyrophosphorylase (BAFJ5) was able to hyper-accumulate TAG in cytosolic lipid bodies under high-light and nitrogen-depleted conditions. Metabolic engineering of the starch synthesis pathway represents an alternative, yet more effective way to over produce TAG than direct manipulation of the lipid synthesis pathway. Our comparative transcriptomic and lipidomic studies revealed that the gene DGAT2, one of six copies of genes encoding diacylglycerol acyltransferases (DGAT), was most sensitive to the stimuli inducing TAG synthesis. DGAT2 transcripts increased 4-fold and 69-fold in the wild type and BAFJ5 mutant after 12 h of stress, respectively, compared to that measured prior to the onset of stress. As a result, the mutant accumulated up to 10-fold TAG of that present in the wild type. Application of transcriptional and translational inhibitors demonstrated that the DGAT2 gene was regulated at the transcriptional level. Collectively, these results suggest that DGAT2 is a key gene in regulating TAG synthesis in Chlamydomonas.

e-mail address of presenting author: yantaoli@asu.edu