Figure 6.

How m⁶A modification of cognate stop codon UAA to Um⁶AA affects RF2-dependent termination. Ribosomal termination complex RT₀ (Figure 1B), at concentration 0.02 μM with native UAA codon (A) or 0.05 μM with modified Um⁶AA codon (B) in A site and with ³H-labeled fMet-Phe-Tyr-tRNA^Tyr in P site were reacted with RF2 at indicated concentrations. The fractional extents (y-axis) of terminated ribosomes, RT₃ (Figure 1B), are shown as functions of time (x-axis, log₁₀ display). Inserts: spontaneous termination without RF2. From these curves average reaction times for peptide release were estimated and their inverses, the generalized rate constants for peptide release, k_rel, are displayed in (C) for UAA and in (D) for Um⁶AA codons. The maximal rates (k_cat) and efficiencies (k_cat/K_m) for these termination reactions are summarized in Table 2.