HisA. Phosphoribosylformimino-5-aminoimidazole carboxamide ribonucleotide (ProFAR) isomerase catalyzes the fourth step in histidine biosynthesis, an isomerisation of the aminoaldose moiety of ProFAR to the aminoketose of PRFAR (N-(5'-phospho-D-1'-ribulosylformimino)-5-amino-1-(5''-phospho-ribosyl)-4-imidazolecarboxamide). In bacteria and archaea, ProFAR isomerase is encoded by the HisA gene.