Mithramycin, nogalamycin and aclacinomycins are aromatic polyketide antibiotics that exhibit antitumour activity. The precursors of these antibiotics are formed via a polyketide biosynthetic pathway in which acetate (for mithramycinone and nogalamycinone) or propionate (for aklavinone) is used as a starter unit and nine acetates are used as extender units. The assembly of building blocks is catalyzed by the minimal polyketide synthase (PKS). Further steps include regiospecific reductions (if any) and cyclization. In the biosynthesis of mithramycin, however, ketoreduction is omitted and the regiospecificity of the first cyclization differs from that of anthracycline antibiotics (e.g. nogalamycin and aclacinomycins). These significant differences provide a convenient means to analyze the determinants for the regiospecificity of the first cyclization step. In order to analyze a possible role of the minimal PKS in the regiospecificity of the first cyclization in polyketide biosynthesis, we expressed the mtm locus, which includes mithramycin minimal PKS genes, in Streptomyces galilaeus, which normally makes aclacinomycins, and the sno locus, which includes nogalamycin minimal PKS genes, in Streptomyces argillaceus, which normally makes mithramycin. The host strains are defective in the minimal PKS, but they express other antibiotic biosynthesis genes. Expression of the sno minimal PKS in the S. argillaceus polyketide-deficient strain generated mithramycin production. Auramycins, instead of aclacinomycins, accumulated in the recombinant S. galilaeus strains, suggesting that the mithramycin minimal PKS is responsible for the choice of starter unit. We also describe structural analysis of the compounds accumulated by a ketoreductase-deficient S. galilaeus mutant; spectroscopic studies on the major polyketide compound that accumulated revealed a first ring closure which is not typical of anthracyclines, suggesting an important role for the ketoreductase in the regiospecificity of the first cyclization. These experiments clearly support the involvement of ketoreductase and a cyclase in the regiospecific cyclization of the biosynthetic pathway for aromatic polyketides.