Altered nutrient content (levels of glucose) caused a drastic reduction in cell growth and triacylglycerol (TAG) production in the wild-type (WT) Rhodotorula glutinis. This was due to the decreased level of synthesis of TAG biosynthetic enzymes, reflected by a reduction in enzyme activity. A similar observation was made in the case of non-lethal mutants of TAG-deficient oleaginous yeast, namely TAG1 and TAG2, which were generated by ethyl methane sulphonate mutagenesis. Metabolic labelling of TAG-deficient cells with [14C]acetate, [32P]orthophosphate and [14C]mevalonate showed a negligible TAG formation with minimal alterations in phospholipid and sterol compositions. Assays on the activities of cytosolic TAG biosynthetic enzymes revealed that lysophosphatidic acid and diacylglycerol acyltransferases (ATs) were defective in TAG1 and TAG2 respectively. The activity of membrane-bound isoforms of TAG biosynthetic enzymes remains unaltered in the mutants. Analysis of cytosolic TAG biosynthetic enzymes by immunoblotting and immunoprecipitation indicated that the defective ATs were a part of the TAG biosynthetic multienzyme complex. Quantitatively, the cytosolic lysophosphatidic acid-AT was comparable between TAG1 and the WT. However, diacylglycerol-AT was relatively less in TAG2 than the WT. These results demonstrated that either by decreasing the nutrient content or mutating the enzymes of the soluble TAG biosynthetic pathway, TAG production was decreased with concomitant reduction in the cell growth.
Abbreviations used: ACP, acyl carrier protein; AT, acyltransferase; DAG, diacylglycerol; EMS, ethyl methane sulphonate; PA, phosphatidic acid; LPA, lysophosphatidic acid; PAPase, phosphatidic acid phosphatase; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine; SE, sterol esters; TAG, triacylglycerol; TBC, TAG biosynthetic complex; WT, wild-type.
These authors contributed equally to this work.