Characterization of two glycosyltransferases involved in early glycosylation steps during biosynthesis of the antitumor polyketide mithramycin by Streptomyces argillaceus

A 2580-bp region of the chromosome of Streptomyces argillaceus, the produce r of the antitumor polyketide mithramycin, was sequenced. Analysis of the n ucleotide sequence revealed the presence of two genes (mtmGIII and mtmGIV) encoding proteins that showed a high degree of similarity to glycosyltransf erases involved in the biosynthesis of various antibiotics and antitumor dr ugs. Independent insertional inactivation of both genes produced mutants th at did not synthesize mithramycin but accumulated several mithramycin inter mediates. Both mutants accumulated premithramycinone, anon-glycosylated int ermediate in mithramycin biosynthesis. The mutant affected in the mtmGIII g ene also accumulated premithramycin A1, which contains premithramycinone as the aglycon unit and a D-olivose attached at C-12a-O. These experiments de monstrate that the glycosyltransferases MtmGIV and MtmGIII catalyze the fir st two glycosylation steps in mithramycin biosynthesis. A model is proposed for: the glycosylation steps in mithramycin biosynthesis.