Branched-chain fatty acid biosynthesis in Escherichia coli

beta -Ketoacyl-acyl carrier protein (ACP) synthase III (KASIII) catalyzes t he first elongation step in straight-chain fatty acid (SCFA) biosynthesis i n Escherichia coli. Overproduction of the corresponding KASIII gene, or the Brassica napus KASIII gene has previously been observed to lead to an incr ease in the amount of shorter-chain fatty acids produced by E. coli. In thi s study it is shown that overexpression of the KASIII gene, which initiates branched-chain fatty acid (BCFA) in Streptomyces glaucescens, does not lea d to a change in the fatty acid profiles of E. coli. E. coli produces trace levels of BCFAs when grown in the presence of isobutyric acid, but the amo unts of these are not significantly altered by expression of the S. glauces cens KASIII gene. In contrast, the amounts of BCFAs produced from isobutyry l CoA in vitro by E. coli cell-free extracts can be increased at least four -fold by the presence of the S. glaucescens KASIII. These observations sugg est that in vivo production of isopalmitate by E. coli expressing the S. gl aucescens KASIII is limited by availability of the appropriate BCFA biosynt hetic primers.