Imidazole glycerol phosphate (IGP) synthase is a glutamine amidotransferase that catalyzes the formation of IGP and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) from N(1)-[(5'-phosphoribulosyl)formimino]-5-aminoimidazole-4-car boxamide ribonucleotide (PRFAR). This enzyme represents a junction between histidine biosynthesis and de novo purine biosynthesis. The recent characterization of the HIS7 gene in the yeast Saccharomyces cerevisiae IGP synthase established that this protein is bifunctional, representing a fusion between the N-terminal HisH domain and a C-terminal HisF domain. Catalytically active yeast HIS7 was expressed in a bacterial system under the control of T7 polymerase promoter. The recombinant enzyme was purified to homogeneity and the native molecular weight and steady-state kinetic constants were determined. The yeast enzyme is distinguished from the Escherichia coli IGP synthase in its utilization of ammonia as a substrate. HIS7 displays a higher K(m) for glutamine and a lower turnover in the ammonia-dependent IGP synthase activity. As observed with the E. coli IGP synthase, HIS7 shows a low basal level glutaminase activity that can be enhanced 1000-fold in the presence of a nucleotide substrate or analog. The purification and characterization of the S. cerevisiae enzyme will enable a more detailed investigation of the biochemical mechanisms that mediate the ammonia-transfer process. The fused structural feature of the HIS7 protein and the development of a high-level production system for the active enzyme elevate the potential for determination of its three-dimensional structure through X-ray crystallography.