R32A/R56A/K57A-PaHisGS cannot be rescued by PaHisZ. The hypothesis that R32 and R56 can compensate to a certain extent for the absence of the other in the presence of PaHisZ to restore the electrostatic preorganisation of PaHisGS active site predicts that removal of both arginine residues would prevent allosteric rescue of catalysis. To test this prediction, the R32A/R56A/K57A-PaHisGS triple mutant was produced (Supplementary Fig. ) and ESI/TOF-MS analysis resulted in the expected mass (Supplementary Fig. ).