Sequencing of the eryA region of the erythromycin biosynthetic gene cluster from Saccharopolyspora erythraea has revealed another structural gene (ORF B), in addition to the previously characterised ORF A , which appears to encode a component of 6-deoxyerythronolide-B synthase, the enzyme that catalyses the first stage in the biosynthesis of the polyketide antibiotic erythromycin A. The nucleotide sequence of ORF B, whch lies immediately adjacent to ORF A , has been determined. The predicted gene product of ORF B is a polypeptide of 374417 Da (3568 amino acids), which is highly similar to the product of ORF A and which likewise contains a number of separate domains, each with substantial amino acid sequence similarity to components of known fatty-acid synthases and polyketide synthases. The order of the predicted active sites along the chain from the N-terminus is 3-oxoacyl-synthase -acyltransferase -acyl-carrier-protein -3-oxoacyl-synthase -acyltransferase -dehydratase -enoylreductase -oxoreductase -acyl-carrier-protein. The position of the dehydratase active site has been pinpointed for the first time for any polyketide synthase or vertebrate fatty-acid synthase. The predicted domain structure of 6-deoxyerythronolide-B synthase is strikingly similar to that previously established for vertebrate fatty-acid synthases. This analysis of the sequence supports the view that the erythromycin-producing polyketide synthase contains three multienzyme polypeptides, each of which accomplishes two successive cycles of polyketide chain extension. In this scheme, the role of the ORF B gene product is to accomplish extension cycles 3 and 4.