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Reconstitution and Substrate Specificity of the Radical S-Adenosyl-methionine Thiazole C-Methyltransferase in Thiomuracin Biosynthesis

Mahanta, Nilkamal, Zhang, Zhengan, Hudson, Graham A., van der Donk, Wilfred A., Mitchell, Douglas A.
Journal of the American Chemical Society 2017 v.139 no.12 pp. 4310-4313
antibiotics, bacteria, biosynthesis, carbon, drug resistance, methyltransferases, post-translational modification, signal peptide, substrate specificity, thiazoles
Thiomuracin is a thiopeptide antibiotic with potent activity toward Gram-positive drug-resistant bacteria. Thiomuracin is biosynthesized from a precursor peptide, TbtA, by a complex array of posttranslational modifications. One of several intriguing transformations is the C-methylation of thiazole, occurring at an unactivated sp² carbon. Herein, we report the in vitro reconstitution of TbtI, the responsible radical S-adenosyl-methionine (rSAM) C-methyltransferase, which catalyzes the formation of 5-methylthiazole at a single site. Our studies demonstrate that a linear hexazole-bearing intermediate of TbtA is a substrate for TbtI whereas macrocyclized thiomuracin GZ is not. In determining the minimal substrate for TbtI, we found that the enzyme is functional when most of the leader peptide has been removed. The in vitro reconstitution of TbtI, a class C rSAM methyltransferase, further adds to the chemical versatility of rSAM enzymes, and informs on the complexity of thiomuracin biosynthesis.