Involved in the biosynthetic pathway of the antibiotic thiostrepton A. First, TsrM catalyzes the transfer of a methyl group from S-adenosyl methionine (SAM) to cobalamin, leading to the formation of methylcobalamin (CH3-cobalamin) and S-adenosyl-L-homocysteine (SAH). Then the methyl group is transferred to the C2 position of tryptophan (Trp) with the concerted action of the radical SAM [4Fe-4S] center, leading to the production of methyltryptophan. L-tryptophan + S-adenosyl-L-methionine = 2-methyl-L-tryptophan + H(+) + S-adenosyl-L-homocysteine Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. TsrM has a totally new mechanism among methyltransferases and radical SAM enzymes. TsrM is not able to use methylcobalamin directly but instead coordinates free cobalamin. SAM serves exclusively as a methyl donor and not as a radical source, and the methyl group of SAM is not directly transferred to tryptophan.