Methylmalonyl Coenzyme A Selectivity of Cloned and Expressed Acyltransferase and b-Ketoacyl Synthase Domains of Mycocerosic Acid Synthase from Mycobacterium bovis BCG

Methyl-branched fatty acids and polyketides occur in a variety of living organisms. Previous studies have established that multifunctional enzymes use methylmalonyl coenzyme A (CoA) as the substrate to generate methyl-branched products such as mycocerosic acids and polyketides. However, we do not know which of the component activities show selectivity for methylmalonyl-CoA in any biological system. A comparison of homologies of the domains of the multifunctional synthases that selectively use malonyl-CoA or methylmalonyl-CoA suggested that the acyltransferase (AT) and b-ketoacyl synthase (KS) domains might be responsible for the substrate selectivity. To test this hypothesis, we expressed the AT and KS domains of the mycocerosic acid synthase (MAS) gene from Mycobacterium bovis BCG in Escherichia coli and examined whether they confer to synthases that normally do not use methylmalonyl-CoA the ability to incorporate methylmalonyl-CoA into fatty acids. Both the AT and the KS domains of MAS showed selectivity for methylmalonyl-CoA over malonylCoA. Acyl carrier protein (ACP)-dependent elongation of the n-C12 acyl primer mainly by one methylmalonylCoA unit was catalyzed by an E. coli fatty acid synthase preparation only in the presence of the expressed MAS domains. An ACP-dependent elongation of the n-C20 acyl primer by one methylmalonyl-CoA extender unit was catalyzed by fatty acid synthase from Mycobacterium smegmatis only in the presence of the expressed MAS domains. These results show methylmalonyl-CoA selectivity for the AT and KS domains of MAS. These domains may be useful in producing novel polyketides by genetic engineering.