beta-ketoacyl-acyl carrier protein synthase III (FabH) is a determining factor in branched-chain fatty acid biosynthesis

A universal set of genes encodes the components of the dissociated, type II , fatty acid synthase system that is responsible for producing the multitud e of fatty acid structures found in bacterial membranes. We examined the bi ochemical basis for the production of branched-chain fatty acids by gram-po sitive bacteria. Two genes that mere predicted to encode homologs of the be ta-ketoacyl-acyl carrier protein synthase LII of Escherichia coli (eFabH) w ere identified in the Bacillus subtilis genome. Their protein products were expressed, purified, and biochemically characterized. Both B. subtilis Fab H homologs, bFabH1 and bFabH2, carried out the initial condensation reactio n of fatty acid biosynthesis with acetyl-coenzyme A (acetyl-CoA) as a prime r, although they possessed lower specific activities than eFabH, bFabH1 and bFabH2 also utilized iso- and anteiso-branched-chain acyl-CoA primers as s ubstrates. eFabH was not able to accept these CoA thioesters. Reconstitutio n of a complete round of fatty acid synthesis in vitro with purified E. col i proteins showed that eFabH was the only E, coli enzyme incapable of using branched-chain substrates. Expression of either bFabH1 or bFabH2 in E. col i resulted in the appearance of a branched-chain 17-carbon fatty acid, Thus , the substrate specificity of FabH is an important determinant of branched -chain fatty acid production.