Functional roles in S-adenosyl-l-methionine binding and catalysis for active site residues of the thiostrepton resistance methyltransferase

•Tsr exhibits equivalent SAM binding at two structurally non-equivalent sites.•An arginine residue is implicated in the Tsr catalytic mechanism.•A central asparagine is integral to Tsr structure, SAM binding and catalysis.•Implications for a dynamic link in substrate/cosubstrate binding by SPOUT enzymes.