Expression of polyketide biosynthesis and regulatory genes in heterologous streptomycetes

There are now several examples showing that hybrid secondary metabolites can be produced as a result of interspecies cloning of antibiotic biosynthesis genes in streptomycetes. This paper reviews examples of hybrid secondary metabolite production, and examines the underlying biochemical and regulatory principles leading to the formation of hybrid anthraquinones by recombinant anthracycline-producing streptomycetes carrying actinorhodin biosynthesis genes. An anthraquinone, aloesaponarin II, was produced by cloning the actI, actIII, actIV, and actVII genes (pANT12) of actinorhodin biosynthesis pathway from Streptomyces coelicolor in anthracycline producing streptomycetes. Streptomyces galilaeus strains 31 133 and 31 671, aclacinomycin and 2-hydroxyaklavinone producers, respectively, formed aloesaponarin II as their major polyketide product when transformed with pANT12. Subcloning experiments indicated that a 2.8-kb XhoI fragment containing only the actI and actVII loci was necessary for aloesaponarin II biosynthesis by S. galilaeus 31 133. When S. galilaeus 31 671 was transformed with the actI, actVII, and actIV genes, however, the recombinant strain produced two novel anthraquinones, desoxyerythrolaccin and 1-O-methyldesoxyerythrolaccin. When S. galilaeus 31 671 was transformed with only the intact actIII gene (pANT45), aklavinone was formed exclusively. These experiments indicate a function for the actIII gene, which is the reduction of the keto group at C-9 from the carboxyl terminus of the assembled polyketide to the corresponding secondary alcohol. The effects of three regulatory loci, dauG, dnrR1, and asaA, on the production of natural and hybrid polyketides were also shown.