A distinct DGAT with sn -3 acetyltransferase activity that synthesizes unusual, reduced-viscosity oils in Euonymus and transgenic seeds

Endosperm and embryo tissues from the seeds of Euonymus alatus (Burning Bush) accumulate high levels of 3-acetyl-1,2-diacyl-sn-glycerols (acTAGs) as their major storage lipids. In contrast, the aril tissue surrounding the seed produces long-chain triacylglycerols (lcTAGs) typical of most other organisms. The presence of the sn-3 acetyl group imparts acTAGs with different physical and chemical properties, such as a 30% reduction in viscosity, compared to lcTAGs. Comparative transcriptome analysis of developing endosperm and aril tissues using pyrosequencing technology was performed to isolate the enzyme necessary for the synthesis of acTAGs. An uncharacterized membrane-bound O-acyltransferase (MBOAT) family member was the most abundant acyltransferase in the endosperm but was absent from the aril. Expression of this MBOAT in yeast resulted in the accumulation of acTAGs but not lcTAG; hence, the enzyme was named EaDAcT (Euonymus alatus diacylglycerol acetyltransferase). Yeast microsomes expressing EaDAcT possessed acetyl-CoA diacylglycerol acetyltransferase activity but lacked long-chain acyl-CoA diacylglycerol acyltransferase activity. Expression of EaDAcT under the control of a strong, seed-specific promoter in Arabidopsis resulted in the accumulation of acTAGs, up to 40 mol % of total TAG in the seed oil. These results demonstrate the utility of deep transcriptional profiling with multiple tissues as a gene discovery strategy for low-abundance proteins. They also show that EaDAcT is the acetyltransferase necessary and sufficient for the production of acTAGs in Euonymus seeds, and that this activity can be introduced into the seeds of other plants, allowing the evaluation of these unusual TAGs for biofuel and other applications.