Fig. 7.

Δ578–626 is defective in facilitating splicing of the Neurospora mt LSU intron. A, Disappearance of precursor RNA (20 nM) was followed at 25°C in the presence of 100 nM CYT-18 dimer in the absence of CYT-19 (○) or with 100 nM CYT-19 (∇), 100 nM Δ578–626 (◇), or 500 nM Δ578–626 (□). The RNA was also incubated without any proteins (×) as a negative control. RNA splicing was performed as described in Materials and Methods. B, Dependence of the splicing rate constant on the concentrations of Δ578–626 (○, λ) and wild-type CYT-19 (□, ν). Equivalent reactions in the presence of CYT-18 and absence of CYT-19 are also shown (Δ, σ). The data are from the experiment shown in panel A (filled symbols) and an analogous experiment using independent preparations of proteins (open symbols). Linear fits to the data shown gave k_cat/K_M values of 2.9 (± 0.8) × 10⁵ M⁻¹ min⁻¹ and 1.9 (± 0.2) × 10⁴ M⁻¹ for the wild-type and Δ578–626 proteins, respectively.