  Determining the mechanism by which transfer RNAs (tRNAs) rapidly and
precisely transit through the ribosomal A, P and E sites during translation
remains a major goal in the study of protein synthesis. Here, we report the
real-time dynamics of the L1 stalk, a structural element of the large ribosomal
subunit that is implicated in directing tRNA movements during translation.
Within pre-translocation ribosomal complexes, the L1 stalk exists in a dynamic
equilibrium between open and closed conformations. Binding of elongation factor
G (EF-G) shifts this equilibrium towards the closed conformation through one of
at least two distinct kinetic mechanisms, where the identity of the P-site tRNA
dictates the kinetic route that is taken. Within post-translocation complexes,
L1 stalk dynamics are dependent on the presence and identity of the E-site
tRNA. Collectively, our data demonstrate that EF-G and the L1 stalk
allosterically collaborate to direct tRNA translocation from the P to the E
sites, and suggest a model for the release of E-site tRNA.
