We report that L-threonine may substitute for L-serine in the beta-substitution reaction of an engineered subunit of tryptophan synthase from Pyrococcus furiosus, yielding (2S,3S)-beta-methyltryptophan (beta-MeTrp) in a single step. The trace activity of the wild-type beta-subunit on this substrate was enhanced more than 1000-fold by directed evolution. Structural and spectroscopic data indicate that this increase is correlated with stabilization of the electrophilic aminoacrylate intermediate. The engineered biocatalyst also reacts with a variety of indole analogues and thiophenol for diastereoselective C-C, C-N, and C-S bond-forming reactions. This new activity circumvents the 3-enzyme pathway that produces beta-MeTrp in nature and offers a simple and expandable route to preparing derivatives of this valuable building block.