Comparison of the effects of His6-GFRP on wt-GTPCH and Δ45-GTPCH activity. GTPCH assays were conducted in the presence or absence of His6-GFRP, in 100 mm Tris-HCl (pH 7.8) at 37 °C, using the kinetic microplate assay described under “Experimental Procedures.” A, 0.1 μm wt-GTPCH or Δ45-GTPCH activity was assayed in the absence or presence of 1 μm His6-GFRP in reactions containing 5–500 μm GTP. Shown is a typical plot of the reaction rate versus substrate concentration from a single experiment. Values are mean ± S.E. of triplicate measurements. Nonlinear regression analysis was used to fit a curve to the data points. B, the activity of 0.1 μm wt-GTPCH and Δ45-GTPCH were assayed in the absence or presence of either 1.0 or 0.1 μm His6-GFRP, in reactions containing 1 mm GTP. Values are mean ± S.E. of quintuplicate measurements, and compared by analysis of variance. ***, p < 0.001; **, p < 0.01. ns, not significant.