The mtmVUC genes of the mithramycin gene cluster in Streptomyces argillaceus are involved in the biosynthesis of the sugar moieties

Mithramycin is a glycosylated aromatic polyketide produced by Streptomyces argillaceus, and is used as an antitumor drug. Three genes (mtmV, mtmU and mtmC) from the mithramycin gene cluster have been cloned, and characterized by DNA sequencing and by analysis of the products that accumulate in nonpr oducing mutants, which were generated by insertional inactivation of these genes. The mtmV gene codes for a 2,3-dehydratase that catalyzes early and c ommon steps in the biosynthesis of the three sugars found in mithramycin (D -olivose, D-oliose and D-mycarose); its inactivation caused the accumulatio n of the nonglycosylated intermediate premithramycinone. The mtmU gene code s for a 4-ketoreductase involved in D-oliose biosynthesis, and its inactiva tion resulted in the accumulation of premithramycinone and premithramycin A l, the first glycosylated intermediate which contains a D-olivose unit. The third gene, mtmC, is involved in D-mycarose biosynthesis and codes for a C -methyltransferase. Two mutants with lesions in the mtmC gene accumulated m ithramycin intermediates lacking the D-mycarose moiety but containing D-oli vose units attached to C-12a in which the 4-keto group is unreduced. This s uggests that mtmC could code for a second enzyme activity, probably a D-oli vose 4-ketoreductase, and that the glycosyltransferase responsible for the incorporation of D-olivose (MtmGIV) shows some degree of flexibility with r espect to its sugar co-substrate, since the 4-keto-analog is also transferr ed. A pathway is proposed for the biosynthesis of the three sugar moieties in mithramycin.