L27-tRNA interaction revealed by mutagenesis and pH titration.

The movement of peptidyl tRNA into the P-site after ribosome translocation reduces the ribosome dynamics in the post-translocation complex, which "locks" the ribosome to less conformational fluctuations. Here, we used single molecule FRET method to reveal that ribosomes bearing L27 with N-terminal truncations are less competent to "lock" the tRNA fluctuations after translocation. We found that: (1) truncation of the first three N-terminal residues of L27 increases peptidyl tRNA fluctuation; and (2) increasing the solution pH increases peptidyl tRNA fluctuation in WT and some of the ribosome mutants. We propose that one role of L27 at the catalytic center is to stabilize peptidyl tRNA in the post-translocation complex.