The L1 stalk is a mobile domain of the large ribosomal subunit E site that interacts with the elbow of deacylated tRNA during protein synthesis. Here, by using single-molecule FRET, we follow the real-time dynamics of the L1 stalk and observe its movement relative to the body of the large subunit between at least 3 distinct conformational states: open, half-closed, and fully closed. Pretranslocation ribosomes undergo spontaneous fluctuations between the open and fully closed states. In contrast, posttranslocation ribosomes containing peptidyl-tRNA and deacylated tRNA in the classical P/P and E/E states, respectively, are fixed in the half-closed conformation. In ribosomes with a vacant E site, the L1 stalk is observed either in the fully closed or fully open conformation. Several lines of evidence show that the L1 stalk can move independently of intersubunit rotation. Our findings support a model in which the mobility of the L1 stalk facilitates binding, movement, and release of deacylated tRNA by remodeling the structure of the 50S subunit E site between 3 distinct conformations, corresponding to the E/E vacant, P/E hybrid, and classical states.
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