SaTrmK cofactor binding site.A, key polar and nonpolar interactions between SaTrmK residues and SAM. The dashed lines depict polar contacts. B, overlay of the cofactor-binding site of SaTrmK apoenzyme and the binary complexes with SAM and SAH. In both panels, oxygen is depicted in red, nitrogen in blue, sulfur in yellow, and carbon in either gray (SAM), teal (residues in SaTrmK:SAM), purple (residues in SaTrmK:SAH), or green (residues in SaTrmK apoenzyme). Water molecules are shown as red spheres. SaTrmK, S. aureus TrmK; TrmK, m1A22-tRNA methyltransferase.