<jats:title>ABSTRACT</jats:title> <jats:p> Mithramycin is an antitumor polyketide drug produced by <jats:italic>Streptomyces argillaceus</jats:italic> that contains two deoxysugar chains, a disaccharide consisting of two <jats:sc>d</jats:sc> -olivoses and a trisaccharide consisting of a <jats:sc>d</jats:sc> -olivose, a <jats:sc>d</jats:sc> -oliose, and a <jats:sc>d</jats:sc> -mycarose. From a cosmid clone (cosAR3) which confers resistance to mithramycin in streptomycetes, a 3-kb <jats:italic>Pst</jats:italic> I- <jats:italic>Xho</jats:italic> I fragment was sequenced, and two divergent genes ( <jats:italic>mtmGI</jats:italic> and <jats:italic>mtmGII</jats:italic> ) were identified. Comparison of the deduced products of both genes with proteins in databases showed similarities with glycosyltransferases and glucuronosyltransferases from different sources, including several glycosyltransferases involved in sugar transfer during antibiotic biosynthesis. Both genes were independently inactivated by gene replacement, and the mutants generated (M3G1 and M3G2) did not produce mithramycin. High-performance liquid chromatography analysis of ethyl acetate extracts of culture supernatants of both mutants showed the presence of several peaks with the characteristic spectra of mithramycin biosynthetic intermediates. Four compounds were isolated from both mutants by preparative high-performance liquid chromatography, and their structures were elucidated by physicochemical methods. The structures of these compounds were identical in both mutants, and the compounds are suggested to be glycosylated intermediates of mithramycin biosynthesis with different numbers of sugar moieties attached to C-12a-O of a tetracyclic mithramycin precursor and to C-2-O of mithramycinone: three tetracyclic intermediates containing one sugar (premithramycin A1), two sugars (premithramycin A2), or three sugars (premithramycin A3) and one tricyclic intermediate containing a trisaccharide chain (premithramycin A4). It is proposed that the glycosyltransferases encoded by <jats:italic>mtmGI</jats:italic> and <jats:italic>mtmGII</jats:italic> are responsible for forming and transferring the disaccharide during mithramycin biosynthesis. From the structures of the new metabolites, a new biosynthetic sequence regarding late steps of mithramycin biosynthesis can be suggested, a sequence which includes glycosyl transfer steps prior to the final shaping of the aglycone moiety of mithramycin.