Cytosolic PPAT (phosphoribosyl pyrophosphate amidotransferase) catalyzes the reaction of 5-phospho-alpha-D-ribose 1-diphosphate (PRPP), water, and L-glutamine to form 5-phosphoribosylamine, L-glutamate, and pyrophosphate. This event is the committed step in de novo purine synthesis. The reaction itself is reversible, but it is pulled strongly in the direction of 5'-phosphoribosylamine synthesis by the irreversible hydrolysis of the pyrophosphate that is also formed in the reaction. Human and bacterial forms of PPAT have been characterized in detail: they are iron-sulfur proteins whose activity is regulated by multimerization. The homologous chicken enzyme has been identified based on sequence similarity to these other enzymes and has been shown to restore growth in purine-deficient media to cultured Chinese hamster ovary (CHO) cells whose endogenous PPAT gene was mutationally inactivated (Zhou et al. 1990). In earlier biochemical studies, the chicken enzyme was partially purified and shown to have a molecular weight consistent with tetramerization (Hartman 1963).