A SORANGIUM-CELLULOSUM (MYXOBACTERIUM) GENE-CLUSTER FOR THE BIOSYNTHESIS OF THE MACROLIDE ANTIBIOTIC SORAPHEN A - CLONING, CHARACTERIZATION, AND HOMOLOGY TO POLYKETIDE SYNTHASE GENES FROM ACTINOMYCETES

A 40-kb region of DNA from Sorangium cellulosum So ce26, which contain s polyketide synthase (PKS) genes for synthesis of the antifungal macr olide antibiotic soraphen A, was cloned. These genes were detected by homology to Streptomyces violaceoruber genes encoding components of gr anaticin PKS, thus extending this powerful technique for the identific ation of bacterial PKS genes, which has so far been applied only to ac tinomycetes, to the gram-negative myxobacteria. Functional analysis by gene disruption has indicated that about 32 kb of contiguous DNA of t he cloned region contains genes involved in soraphen A biosynthesis. T he nucleotide sequence of a 6.4-kb DNA fragment, derived from the regi on with homology to granaticin PKS genes, was determined. Analysis of this sequence has revealed the presence of a single large open reading frame beginning and ending outside the 6.4-kb fragment. The deduced a mino acid sequence indicates the presence of a domain with a high leve l of similarity to beta-ketoacyl synthases that are involved in polyke tide synthesis. Other domains with high levels of similarity to region s of known polyketide biosynthetic functions were identified, includin g those for acyl transferase, acyl carrier protein, ketoreductase, and dehydratase. We present data which indicate that soraphen A biosynthe sis is catalyzed by large, multifunctional enzymes analogous to other bacterial PKSs of type I.