THE LOADING DOMAIN OF THE ERYTHROMYCIN POLYKETIDE SYNTHASE IS NOT ESSENTIAL FOR ERYTHROMYCIN BIOSYNTHESIS IN SACCHAROPOLYSPORA-ERYTHRAEA

6-Deoxyerythronolide B synthase (DEBS) is a large multifunctional enzy me that catalyses the biosynthesis of the erythromycin polyketide agly cone. DEBS is organized into six modules, each containing the enzymic domains required for a single condensation of carboxylic acid residues which make up the growing polyketide chain. Module 1 is preceded by l oading acyltransferase (AT-L) and acyl carrier protein (ACP-L) domains , hypothesized to initiate polyketide chain growth with a propionate-d erived moiety. Using recombinant DNA technology several mutant strains of Saccharopolyspora erythraea were constructed that lack the initial AT-L domain or that lack both the AT-L and ACP-L domains. These strai ns were still able to produce erythromycin, although at much lower lev els than that produced by the wild-type strain. In addition, the AT-L domain expressed as a monofunctional enzyme was able to complement the deletion of this domain from the PKS, resulting in increased levels o f erythromycin production. These findings indicate that neither the in itial AT-L nor the ACP-L domains are required to initiate erythromycin biosynthesis; however, without these domains the efficiency of erythr omycin biosynthesis is decreased significantly. It is proposed that in these mutants the first step in erythromycin biosynthesis is the char ging of KS1 with propionate directly from propionyl-CoA.