FIGURE 4.

Determination of energy efficiency in the presence of serine and threonine. The formation of [α-³²P]AMP (filled circles) and formation of [¹⁴C]aminoacyl-tRNA (filled squares) at pH 8.0 and 37 °C were followed in parallel using 100 μm [α-³²P]ATP, 1 mm ¹⁴C-labeled amino acid substrate, and 15 μm tRNA^Thr. A, 0.1 μm wild type ThrRS and threonine as substrate; B, 2 μm H73A ThrRS and serine as substrate; C, 0.5 μm H73A,H309A ThrRS and threonine as substrate; D, 5 μm H73A,H309A ThrRS and serine as substrate. The error bars associated with the amount of reaction product formed at each time point represent the mean ± S.E. from three independent experiments.