The alpha subunit is responsible for the aldol cleavage of indoleglycerol phosphate to indole and glyceraldehyde 3-phosphate. (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate + L-serine = D-glyceraldehyde 3-phosphate + H2O + L-tryptophan Amino-acid biosynthesis; L-tryptophan biosynthesis; L-tryptophan from chorismate: step 5/5. Tetramer of two alpha and two beta chains. Belongs to the TrpA family. This TrpA is highly divergent compared to other bacterial TrpA. As C.trachomatis seems to have lost part of the trp biosynthetic operon, it is possible that this protein is not active.