Multi-factorial engineering of heterologous polyketide production in Escherichia coli reveals complex pathway interactions

Polyketides represent a significant fraction of all natural products. Many possess pharmacological activity which makes them attractive drug candidates. The production of the parent macrocyclic aglycones is catalyzed by multi-modular polyketide synthases utilizing short-chain acyl-CoA monomers. When producing polyketides through heterologous hosts, one must not only functionally express the synthase itself, but activate the machinery used to generate the required substrate acyl-CoA's. As a result, metabolic engineering of these pathways is necessary for high-level production of heterologous polyketides. In this study, we over-express three different pathways for provision of the two substrates (propionyl-CoA and (2S)-methylmalonyl-CoA) utilized for the biosynthesis of 6-deoxyerythronolide B (6-dEB; the macrolactone precursor of erythromycin): (1) a propionate→propionyl-CoA→(2S)-methylmalonyl-CoA pathway, (2) a methylmalonate→methylmalonyl-CoA→propionyl-CoA pathway, and (3) a succinate→succinyl-CoA→(2R)-methylmalonyl-CoA→(2S)-methylmalonyl-CoA→propionyl-CoA pathway. The current study revealed that propionate is a necessary component for greater than 5mgL^-1 titers. Deletion of the propionyl-CoA:succinate CoA transferase (ygfH) or over-expression of the transcriptional activator of short chain fatty acid uptake improved titer to over 100mgL^-1, while the combination of the two improved titer to over 130mgL^-1. The addition of exogenous methylmalonate could also improve titer to over 100mgL^-1. Expression of a Streptomyces coelicolor A3(2) methylmalonyl-CoA epimerase, in conjunction with over-expression of Escherichia coli's native methylmalonyl-CoA mutase, allowed for the incorporation of exogenously fed succinate into the 6-dEB core.