We used freeze trapping to stabilize the Michaelis complex of wild-type tryptophan synthase and the α-subunit substrate indole-3-glycerol phosphate (IGP) and determined its structure to 1.8 Å resolution. In addition, we determined the 1.4 Å resolution structure of the complex with indole-3-propanole phosphate (IPP), a noncleavable IGP analogue. The interaction of the 3‘-hydroxyl of IGP with the catalytic αGlu49 leads to a twisting of the propane chain and to a repositioning of the indole ring compared to IPP. Concomitantly, the catalytic αAsp60 rotates resulting in a translocation of the COMM domain [βGly102-βGly189, for definition see Schneider et al. (1998) Biochemistry 37, 5394−5406] in a direction opposite to the one in the IPP complex. This results in loss of the allosteric sodium ion bound at the β-subunit and an opening of the β-active site, thereby making the cofactor pyridoxal 5‘-phosphate (PLP) accessible to solvent and thus serine binding. These findings form the structural basis for the information transfer from the α- to the β-subunit and may explain the affinity increase of the β-active site for serine upon IGP binding.