This enzyme, found in NAD supression mutants of Escherichia coli, synthesizes 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) without the participation of EC
2.7.6.1, ribose-phosphate diphosphokinase. Ribose, ribose 1-phosphate and ribose 5-phosphate are not substrates, and GTP cannot act as a phosphate donor.