Characterization of two polyketide methyltransferases involved in the biosynthesis of the antitumor drug mithramycin by Streptomyces argillaceus

A DNA chromosomal region of Streptomyces argillaceus ATCC 12596, the producer organism of the antitumor polyketide drug mithramycin, was cloned. Sequence analysis of this DNA region, located between four mithramycin glycosyltransferase genes, showed the presence of two genes (mtmMI and mtmMII) whose deduced products resembled S-adenosylmethionine-dependent methyltransferases. By independent insertional inactivation of both genes nonproducing mutants were generated that accumulated different mithramycin biosynthetic intermediates. The M3 Delta MI mutant (mtmMI-minus mutant) accumulated 4-demethylpremithramycinone (4-DPMC) which lacks the methyl groups at carbons 4 and 9, The M3 Delta M2 (mtmMII-minus mutant) accumulated 9-demethylpremithramycin A3 (9-DPMA3), premithramycin Al (PMA1), and 7-demethylmithramycin, all of them containing the O-methyl group at C-4 and C-1', respectively, but lacking the methyl group at the aromatic position. Both genes were expressed in Streptomyces lividans TK21 under the control of the erythromycin resistance promoter (ermEp) of Saccharopolyspora erythraea. Cell-free extracts of these clones were precipitated with ammonium sulfate (90% saturation) and assayed for methylation activity using different mithramycin intermediates as substrates. Extracts of strains MJM1 (expressing the mtmMI gene) and MJM2 (expressing the mtmMII gene) catalyzed efficient transfer of tritium from [H-3]S-adenosylmethionine into 4-DPMC and 9-DPMA3, respectively, being unable to methylate other intermediates at a detectable level. These results demonstrate that the mtmMI and mtmMII genes code for two S-adenosylmethionine-dependent methyltransferases responsible for the 4-O-methylation and 9-C-methylation steps of the biosynthetic precursors 4-DPMC and 9-DPMA3, respectively, of the antitumor drug mithramycin. A pathway is proposed for the last steps in the biosynthesis of mithramycin involving these methylation events.