Recombinant polyketide synthesis in Streptomyces: engineering of improved host strains.

Efficient polyketide synthesis derived from plasmid-borne heterologous Streptomyces polyketide synthase (PKS) gene clusters necessitates a suitable host strain. Well-characterized laboratory strains such as Streptomyces coelicolor or Streptomyces lividans and their frequently used derivatives carry endogenous genes for the synthesis of actinorhodin (among other PKS genes), which might interfere with the efficient production of extrachromosomally encoded PKS proteins and the quantitative analysis of their secreted polyketide products. To circumvent this problem, a frequently used S. coelicolor derivative, designated CH999, was engineered to lack most of the actinorhodin gene cluster. However, this strain can only be transformed with methyl-free DNA. Additionally, unlike its otherwise isogenic parent CH1, CH999 exhibits low transformation efficiencies. Here, we report the construction of two S. lividans host strains, K4-114 and K4-155. With respect to the actinorhodin gene cluster, both are genotypically identical to CH999; however, both can be transformed at considerably higher frequencies and also with methylated DNA. Upon transformation with the appropriate expression vector, CH999, K4-114 and K4-155 all produce the erythromycin precursor 6-deoxyerythronolide B (6-dEB) equally well.