Figure 4.

smFRET studies of translocation in the bacterial ribosome (Fei et al., 2009). (a) The dominant pathway for translocation is believed to have three steps: A reversible rotation of the two subunits (purple and tan), followed by the binding of EF-G (green) which stabilizes the rotated GS2 state long enough for a GTP-driven transition to the post-translocation (POST) complex to take place. (b) smFRET signals show a shift of the equilibrium from the GS1 state (magenta, E_FRET ≃ 0.55) towards the GS2 state (cyan, E_FRET ≃ 0.35) in the presence of EF-G. (c) Histograms of observables x_{n, t}, split by state, showing a continuous shift of the equilibrium towards GS2 as the concentration of EF-G is increased. Out of 4 states used for inference, only two dominant states are significantly populated. (d) Summed posterior distributions on the relative free energy q(ΔG_{n, k}), showing a weak bi-modal distribution in the free energy of the GS2 state whose lower component becomes more populated with increasing EF-G concentration.