and PaHisGS share 43% and 69% sequence identity with their orthologues from the pathogenic bacterium Acinetobacter baumannii, AbHisZ and AbHisGS, respectively, but PaHisZ has been shown to be a potent allosteric inhibitor of AbHisGS. We thus hypothesised that AbHisZ could inhibit WT-PaHisGS. However, addition of AbHisZ activated catalysis by WT-PaHisGS (Fig. ), and data fit to equation (3) yielded an apparent KD for AbHisZ of 9  1 M. Moreover, AbHisZ also rescued catalysis by R56A-PaHisGS (Fig. ), but their interaction involved positive co-operativity as evidenced by the sigmoidal dependence of the reaction rate on the regulatory protein. The R56A-PaHisGS mutant was chosen due to its significant catalytic impairment and the proposed role in catalysis for R56.