Recombinant Escherichia coli host cells that comprise recombinant DNA expression vectors that drive expression of methylmalonyl CoA mutase from Propionibacterium shermanii or Streptomyces cinnamonensis as well as Propionibacterium shermanii epimerase can produce S-methylmalonyl CoA, a required substrate for the production of polyketides by most modular polyketide synthases not present in wild-type E. coli host cells.