β-Hydroxydecanoyl Thio Ester Dehydrase Does Not Catalyze a Rate-Limiting Step in Escherichia coli Unsaturated Fatty Acid Synthesis

The intracellular level of β-hydroxydecanoyl thio ester dehydrase, the product of the fabA gene of Escherichia coli, was increased by isolation of a putative promotor mutant (termed fabAup) or by molecular cloning of the wild-type fabA gene into plasmid pBR322. The fabAup and plasmid-carrying strains overproduced dehydrase by about 15- and 10-fold, respectively. The phospholipids of all strains that overproduced the dehydrase contained significantly higher levels of saturated fatty acids than isogenic strains producing a normal level of dehydrase. No increased levels of unsaturated fatty acids were observed. This result indicates that, although the dehydrase is required for unsaturated fatty acid synthesis, the level of dehydrase activity in wild-type cells does not limit the rate of unsaturated fatty acid synthesis. The introduction of a plasmid carrying the structural gene for β-ketoacyl acyl carrier protein synthase I into a fabAup strain overcame the effect of dehydrase overproduction on fatty acid composition.