Histidine is formed by several complex and distinct biochemical reactions catalysed by eight enzymes. Proteinsinvolved in steps 4 and 6 of the histidine biosynthesis pathway are contained in one family. These enzymes are calledHis6 and His7 in eukaryotes and HisA and HisF in prokaryotes. HisA is a phosphoribosylformimino-5-aminoimidazolecarboxamide ribotide isomerase (<db_xref db="EC" dbkey="5.3.1.16"/>), involved in the fourth step of histidine biosynthesis. The bacterial HisFprotein is a cyclase which catalyzes the cyclization reaction that produces D-erythro-imidazole glycerol phosphate duringthe sixth step of histidine biosynthesis. The yeast His7 protein is a bifunctional protein which catalyzes an amido-transferase reaction that generates imidazole-glycerol phosphate and 5-aminoimidazol-4-carboxamide. The latter is theribonucleotide used for purine biosynthesis. The enzyme also catalyzes the cyclization reaction that produces D-erythro-imidazole glycerol phosphate, and is involved in the fifth and sixth steps in histidine biosynthesis.