An unusually large multifunctional polypeptide in the erythromycin-producing polyketide synthase of Saccharopolyspora erythraea

Abstract

ERYTHROMYCIN A, a clinically important polyketide antibiotic, is produced by the Gram-positive bacterium Saccharopolyspora erythraea.. In an arrangement that seems to be generally true of antibiotic biosynthetic genes in Streptomyces and related bacteria like S. erythraea¹, the ery genes encoding the biosynthetic pathway to erythromvein are clustered around the gene (ermE) that confers self-resistance on S. erythraea^2–6. The aglycone core of erythro-mycin A is derived from one propionyl-CoA and six methylmalonyl-CoA units, which are incorporated head-to-tail^7–10 into the growing polyketide chain, in a process similar to that of fatty-acid biosynthesis¹, to generate a macrolide intermediate, 6-deoxyeryth-ronolide B¹⁰. 6-Deoxyerythronolide B is converted into erythro-mycin A through the action^2–5,10 of specific hydroxylases, glycosyItransferases and a methyltransferase. We report here the analysis of about 10 kilobases of DNA from S. erythraea, cloned by chromosome 'walking' outwards from the erythromycin-resistance determinant ermE, and previously shown to be essential for erythromycin biosynthesis^5,11. Partial sequencing of this region¹² indicates that it encodes the synthase. Our results confirm this, and reveal a novel organization of the erythromycin-producing polyketide synthase, which provides further insight into the mechanism of chain assembly.