<p>Indole-3-glycerol phosphate synthase (<db_xref db="EC" dbkey="4.1.1.48"/>) (IGPS) catalyses the fourth step in the biosynthesis of tryptophan, the ring closure of 1-(2-carboxy-phenylamino)-1-deoxyribulose into indol-3-glycerol-phosphate. In some bacteria, IGPS is a single chain enzyme. In others, such as <taxon tax_id="562">Escherichia coli</taxon>, it is the N-terminal domain of a bifunctional enzyme that also catalyses N-(5'-phosphoribosyl)anthranilate isomerase (<db_xref db="EC" dbkey="5.3.1.24"/>) (PRAI) activity (see <db_xref db="INTERPRO" dbkey="IPR001240"/>), the third step of tryptophan biosynthesis. In fungi, IGPS is the central domain of a trifunctional enzyme that contains a PRAI C-terminal domain and a glutamine amidotransferase (<db_xref db="EC" dbkey="2.4.2"/>) (GATase) N-terminal domain (see <db_xref db="INTERPRO" dbkey="IPR000991"/>).</p> <p> A structure of the IGPS domain of the bifunctional enzyme from the mesophilicbacterium E. coli (eIGPS) has been compared with the monomeric indole-3-glycerol phosphatesynthase from the hyperthermophilic archaeon <taxon tax_id="2287">Sulfolobus solfataricus</taxon> (sIGPS). Both are single-domain(beta/alpha)8 barrel proteins, with one (eIGPS) or two (sIGPS) additional helices inserted before the first beta strand [<cite idref="PUB00007146"/>]. </p>