The role of pyruvate dehydrogenase and acetyl-coenzyme A synthetase in fatty acid synthesis in developing arabidopsis seeds

Acetyl-coenzyme A (acetyl-CoA) formed within the plastid is the precursor f or the biosynthesis of fatty acids and,through them, a range of important b iomolecules. The source of acetyl-CoA in the plastid is not known, but two enzymes are thought to be involved: acetyl-CoA synthetase and plastidic pyr uvate dehydrogenase. To determine the importance of these two enzymes in sy nthesizing acetyl-CoA during lipid accumulation in developing Arabidopsis s eeds, we isolated cDNA clones for acetyl-CoA synthetase and for the ptE1 al pha- and ptE1 beta-subunits of plastidic pyruvate dehydrogenase. To our kno wledge, this is the first reported acetyl-CoA synthetase sequence from a pl ant source. The Arabidopsis acetyl-CoA synthetase preprotein has a calculat ed mass of 76,678 D, an apparent plastid targeting sequence, and the mature protein is a monomer of 70 to 72 kD. During silique development, the spati al and temporal patterns of the ptE1 beta mRNA level are very similar to th ose of the mRNAs for the plastidic heteromeric acetyl-CoA carboxylase subun its. The pattern of ptE1 beta mRNA accumulation strongly correlates with th e formation of lipid within the developing embryo. In contrast, the level o f mRNA for acetyl-CoA. synthetase does not correlate in time and space with lipid accumulation. The highest level of accumulation of the mRNA for acet yl-CoA synthetase during silique development is within the funiculus. These mRNA data suggest a predominant role for plastidic pyruvate dehydrogenase in acetyl-CoA formation during lipid synthesis in seeds.