Diacylglycerol acyltransferase (DGAT) activity is an essential enzymatic step in the formation of neutral lipid i.e.,
triacylglycerol in all living cells capable of accumulating storage lipid. Previously, we characterized an oleaginous yeast
Candida tropicalis SY005 that yields storage lipid up to 58% under a specific nitrogen-stress condition, when the DGATspecific
transcript is drastically up-regulated. Here we report the identification, differential expression and function of two
DGAT2 gene homologues- CtDGAT2a and CtDGAT2b of this C. tropicalis. Two protein isoforms are unique with respect to the
presence of five additional stretches of amino acids, besides possessing three highly conserved motifs known in other
reported DGAT2 enzymes. Moreover, the CtDGAT2a and CtDGAT2b are characteristically different in amino acid sequences
and predicted protein structures. The CtDGAT2b isozyme was found to be catalytically 12.5% more efficient than CtDGAT2a
for triacylglycerol production in a heterologous yeast system i.e., Saccharomyces cerevisiae quadruple mutant strain H1246
that is inherently defective in neutral lipid biosynthesis. The CtDGAT2b activity rescued the growth of transformed S.
cerevisiae mutant cells, which are usually non-viable in the medium containing free fatty acids by incorporating them into
triacylglycerol, and displayed preferential specificity towards saturated acyl species as substrate. Furthermore, we document
that the efficiency of triacylglycerol production by CtDGAT2b is differentially affected by deletion, insertion or replacement
of amino acids in five regions exclusively present in two CtDGAT2 isozymes. Taken together, our study characterizes two
structurally novel DGAT2 isozymes, which are accountable for the enhanced production of storage lipid enriched with
saturated fatty acids inherently in C. tropicalis SY005 strain as well as in transformed S. cerevisiae neutral lipid-deficient
mutant cells. These two genes certainly will be useful for further investigation on the novel structure-function relationship
of DGAT repertoire, and also in metabolic engineering for the enhanced production of lipid feedstock in other organisms