previously assigned to the phosphorus in the N 1 -5-phospho-β-D-ribose moiety of PRATP, was only detected here when PaHisZ was present in the reaction, confirming the rescue of the catalytically compromised mutants by the regulatory protein. Histidine binds to PaHisZ and allosterically inhibits PaATPPRT catalysis, and the suppression of the burst in product formation in the presence of histidine suggests that allosteric inhibition directly affects the chemical step of the reaction. Histidine also inhibits the reaction catalysed by the rescued PaATPPRT mutants (Fig. ; Supplementary Fig. ),