Tetracenomycin M, a Novel Genetically Engineered Tetracenomycin Resulting from a Combination of Mithramycin and Tetracenomycin Biosynthetic Genes

The hybrid strain Streptomyces glaucescens Tü49 (pGB7) contains recombined genes of the tetracenomycin C ( 4 ) and the mithramycin ( 6 ) biosynthesis cluster. It was designed by the trans formation of plasmid pGB7 into the tetracenomycin producer Streptomyces glaucescens Tü49. Plasmid pGB7 carries the minimal polyketide synthase (PKS) genes of mithramycin biosynthesis ( mtmP, mtmK, mtmS ) along with its upstream ( mtmX , which encodes a gene product of unknown function) and downstream flanking genes (the ketoreductasecoding mtmT1 and fragments of mtmO 1 , which encodes an oxygenase). It was assumed that early intermediates of the wellknown biosynthesis of 4 , such as tetracenomycin F 2 ( 3 ) or very similar molecules, are likely to also serve as intermediates of the biosynthesis of aureolic acid antibiotics, such as 6 . Thus, the enzymes of both parent biosynthetic pathways should be able to act on such intermediates, and several hybrid molecules were expected. Although the experiment resulted in new products, only the novel hybrid natural product tetracenomycin M ( 1 ), whose constitution was determined unambigously by spectroscopic methods, was obtained in larger amounts. The formation of 1 can be explained, if a combination of enzymes of both parent biosynthetic pathways is taken into consideration. When plasmid pGB7 is transformed into Streptomyces lividans TK21, that is, a strain which does not produce any secondary metabolites under our laboratory conditions, the production of SEK15 ( 2 ) is observed. The latter is well known as the product of the minimal PKS of decaketides, and its exclusive production indicates that the aureolic acid antibiotics are constructed via a single decaketide chain and that the enzyme products of the flanking genes mtmX, mtmT1 , and mtmO1 cannot contribute in this second experiment.