The active site of methionyl-tRNA synthetase (MetRS) possesses two functions: synthetic, which provides Met-tRNA for protein synthesis, and editing, which rejects inadvertently misactivated homocysteine. During editing, the side chain -SH group of homocysteine reacts with its activated carboxyl group forming a cyclic thioester, homocysteine thiolactone. As shown here, the side chain -SH and the activated carboxyl groups do not need to be present on the same molecule for the editing to occur. Thioester formation occurs when a thiol and activated methionine, in the form of Met-tRNA, are incubated with MetRS. Depending on the structure of thiols, methionine thioesters may undergo secondary acyl transfer reactions to cis amino, hydroxy, or carboxyl groups which yield methionine dipeptides, esters, or anhydrides, respectively. At saturating thiol concentrations, formation of some thiol derivatives of methionine is as fast as formation of homocysteine thiolactone. Thiol specificity of the reaction and noncompetitive inhibition by the cognate methionine, as well as structure-function studies of active site MetRS mutants, all indicate that there is a specific -SH binding subsite, distinct from the methionine binding subsite, in the synthetic/editing active site of MetRS.