-!- The alpha-subunit catalyzes the conversion of 1-C-(indol-3- yl)glycerol 3-phosphate to indole and D-glyceraldehyde 3-phosphate (this reaction was listed formerly as EC 4.1.2.8). -!- The indole migrates to the beta-subunit where, in the presence of pyridoxal 5'-phosphate, it is combined with L-serine to form L-tryptophan. -!- In some organisms this enzyme is part of a multifunctional protein that also includes one or more of the enzymes EC 2.4.2.18, EC 4.1.1.48, EC 4.1.3.27 and EC 5.3.1.24. -!- In thermophilic organisms, where the high temperature enhances diffusion and causes the loss of indole, a protein similar to the beta subunit can be found (EC 4.2.1.122). -!- That enzyme cannot combine with the alpha unit of EC 4.2.1.20 to form a complex.

-!- Forms part of the defense mechanism against insects and microbial pathogens in the grass family, Gramineae, where it catalyzes one of the steps in the formation of the cyclic hydroxamic acids 2,4- dihydroxy-2H-1,4-benzoxazin-3(4H)-one (DIBOA) and 2,4-dihydroxy-7- methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA). -!- Resembles the alpha-subunit of EC 4.2.1.20 but, unlike tryptophan synthase, its activity is independent of the beta-subunit and free indole is released.