The bifunctional tryptophan synthase α2β2 complex that catalyzes the final two reactions in tryptophan biosynthesis is a classic example of a multienzyme complex that “channels” a metabolic intermediate (indole) between two active sites. The three-dimensional structure of the α2β2 complex from Salmonella typhimurium reveals that the four polypeptide sub-units are arranged in an extended αββα order forming a complex 150 Å long. The active sites of the neighboring α and β subunits are separated by about 30 Å and appear to be connected by a tunnel, which may facilitate the intramolecular transfer of indole. The active site of the α subunit, which is centrally located near one end of an eight-fold α/β barrel structure, contains the sites of most of the missense mutations which were identified as key residues by Yanofsky and colleagues in early genetic studies. Site-directed muta-genesis is being used to replace residues found in the active sites of the α and β subunits in order to probe the mechanism of catalysis. Recombinant DNA technology should also be useful in analyzing protein-protein interaction, protein folding and the channeling phenomenon.