Expression in yeast and tobacco of plant cDNAs encoding acyl CoA:diacylglycerol acyltransferase

Abstract

During the course of a search for cDNAs encoding plant sterol acyltransferases, an expressed sequence tag clone presenting substantial identity with yeast and animal acyl CoA:cholesterol acyltransferases was used to screen cDNA libraries from Arabidopsis and tobacco. This resulted in the isolation of two full‐length cDNAs encoding proteins of 520 and 532 amino acids, respectively. Attempts to complement the yeast double‐mutant are1 are2 defective in acyl CoA:cholesterol acyltransferase were unsuccessful, showing that neither gene encodes acyl CoA:cholesterol acyltransferase. Their deduced amino acid sequences were then shown to have 40 and 38% identity, respectively, with a murine acyl CoA:diacylglycerol acyltransferase and their expression in are1 are2 or wild‐type yeast resulted in a strong increase in the incorporation of oleyl CoA into triacylglycerols. Incorporation was 2–3 times higher in microsomes from yeast transformed with these plant cDNAs than in yeast transformed with the void vector, clearly showing that these cDNAs encode acyl CoA:diacylglycerol acyltransferases. Moreover, during the preparation of microsomes from the Arabidopsis DGAT‐transformed yeast, a floating layer was observed on top of the 100 000 g supernatant. This fraction was enriched in triacylglycerols and exhibited strong acyl CoA:diacylglycerol acyltransferase activity, whereas almost no activity was detected in the corresponding clear fraction from the control yeast. Thanks to the use of this active fraction and dihexanoylglycerol as a substrate, the de novo synthesis of 1,2‐dihexanoyl 3‐oleyl glycerol by AtDGAT could be demonstrated. Transformation of tobacco with AtDGAT was also performed. Analysis of 19 primary transformants allowed detection, in several individuals, of a marked increase (up to seven times) of triacylglycerol content which correlated with the AtDGAT mRNA expression. Furthermore, light‐microscopy observations of leaf epidermis cells, stained with a lipid‐specific dye, showed the presence of lipid droplets in the cells of triacylglycerol‐overproducer plants, thus illustrating the potential application of acyl CoA:diacylglycerol acyltransferase‐transformed plants.