Distinctive kinetics and substrate specificities of plant and fungal tRNA ligases

Single-turnover kinetics of the KlaTrl1 kinase reaction. Rapid mix-quench assays of kinase activity under conditions of enzyme excess were performed as described in Materials and Methods, with either 100 µM GTP (panel A) or 100 µM UTP (panel B) as the phosphate donor. The extents of product formation are plotted as a function of reaction time. Each datum in the graphs is the average of three separate time-course experiments ±SEM. Nonlinear regression curve fits of the data to a single exponential are shown; the rate constants are indicated.