TsrM crystal structure and reaction (IMAGE) Penn State Caption The synthesis of the potent antibiotic thiostrepton uses a radical SAM protein TsrM, whose crystal structure is shown at left while bound to an iron-sulfur cluster and cobalamin. New images of this crystal structure allowed researchers from Penn State to infer the chemical steps during the antibiotic's synthesis (right), as a methyl group moves from a molecule called S-adenosyl-L-methionine (SAM) to the cobalamin in TsrM to the substrate tryptophan. Credit Booker Lab, Penn State Usage Restrictions Credit must be provided License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.