Fig. 4.
Summary of kinetics data and calculated selection efficiencies. Selection efficiency is defined as the ratio of d[state 3]cognate /dt to d[state 3]near-cognate /dt. Both the cognate and near-cognate ribosome complexes fluctuate rapidly and reversibly to the pseudo-GTPase-activated state (state 3′) before reaching successfully the bona fide GTPase-activated state (state 3). Preferential selection of the cognate ternary complex by the ribosome is achieved by the product of the higher frequency and the higher success rate of the attempts to reach the GTPase-activated state. Rate constants (units are s−1) are calculated by counting FRET transitions between states as described in the text. Missed events attributable to too short lifetimes were added (see SI Text). By assuming that the initial binding of the ternary complex to the ribosome (formation of state 1 in Fig. 3) is not rate-determining and making steady state approximations about state 2 and state 3′ in Fig. 3, the initial selection efficiencies are calculated to be 6.7 and 2.5 × 102 at 15 mM [Mg2+] and 5 mM [Mg2+], respectively.