This review will detail the motivations, experimental approaches, and growing list of successful cases associated with the heterologous production of complex natural products.

6-Deoxyerythronolide B (6dEB) is the macrocyclic aglycone precursor of the antibiotic natural product erythromycin. Heterologous production of 6dEB in Escherichia coli was accomplished, in part, by designed over-expression of a native prpE gene (encoding a propionyl-CoA synthetase) and heterologous pcc genes (encoding a propionyl-CoA carboxylase) to supply the needed propionyl-CoA and (2S)-methylmalonyl-CoA biosynthetic substrates. Separate E. coli metabolism includes three enzymes, Sbm (a methylmalonyl-CoA mutase), YgfG (a methylmalonyl-CoA decarboxylase), and YgfH (a propionyl-CoA:succinate CoA transferase), also involved in propionyl-CoA and methylmalonyl-CoA metabolism. In this study, the sbm, ygfG, and ygfH genes were individually deleted and over-expressed to investigate their effect on heterologous 6dEB production. Our results indicate that the deletion and over-expression of sbm did not influence 6dEB production; ygfG over-expression reduced 6dEB production by fourfold while ygfH deletion increased 6dEB titers from 65 to 129 mg/L in shake flask experiments. It was also found that native E. coli metabolism could support 6dEB biosynthesis in the absence of exogenous propionate and the substrate provision pcc genes. Lastly, the effect of the ygfH deletion was tested in batch bioreactor cultures in which 6dEB titers improved from 206 to 527 mg/L.

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An S-adenosylmethionine synthetase gene (metK) from Streptomyces spectabilis was cloned into an expression plasmid under the control of an inducible T7 promoter and introduced into a strain of Escherichia coli (BAP1(pBP130/pBP144)) capable of producing the polyketide product 6-deoxyerythronolide B (6-dEB). The metK coexpression in BAP1(pBP130/pBP144) improved the specific production of 6-dEB from 10.86 to 20.08 mg l(-1) OD(600)(-1). In an effort to probe the reason for this improvement, a series of gene deletion and expression experiments were conducted based on a metK metabolic pathway that branches between propionyl-CoA (a 6-dEB precursor) and autoinducer compounds. The deletion and expression studies suggested that the autoinducer pathway had a larger impact on improved 6-dEB biosynthesis. Supporting these results were experiments demonstrating the positive effect conditioned media (the suspected location of the autoinducer compounds) had on 6-dEB production. Taken together, the results of this study show an increase in heterologous 6-dEB production concomitant with heterologous metK gene expression and suggest that the mechanism for this improvement is linked to native autoinducer compounds.