Enhancing the atom economy of polyketide biosynthetic processes through metabolic engineering

Polyketides, a large family of bioactive natural products, are synthesized from building blocks derived from a-carboxylated Coenzyme A thioesters such as malonyl-CoA and (2S)-methylmalonyl-CoA. The productivity of polyketide fermentation processes in natural and heterologous hosts is frequently limi ted by the availability of these precursors in vivo. We describe a metaboli c engineering strategy to enhance both the yield and volumetric productivit y of polyketide biosynthesis. The genes matB and matC from Rhizobium trifol ii encode a malonyl-CoA synthetase and a putative dicarboxylate transport p rotein, respectively. These proteins can directly convert exogenous malonat e and methylmalonate into their corresponding CoA thioesters with an ATP re quirement of 2 mol per mol of acyl-CoA produced. Heterologous expression of matBC in a recombinant strain of Streptomyces coelicolor that produces the macrolactone 6-deoxyerythronolide B results in a 300% enhancement of macro lactone titers. The unusual efficiency of the bioconversion is illustrated by the fact that approximately one-third of the methylmalonate units added to the fermentation medium are converted into macrolactones. The direct con version of inexpensive feedstocks such as malonate and methylmalonate into polyketides represents the most carbon- and energy-efficient route to these high value natural products and has implications for cost-effective fermen tation of numerous commercial and development-stage small molecules.