Researchers observe spontaneous 'ratcheting' of single ribosome...(ded expertise on the ribosome. FRET ...)

FRET makes use of fluorescent molecules whose signals vary in intensity depending on their proximity to one another. By labeling each of the two subunits of a single ribosomal molecule with these fluorescent markers, the researchers were able to watch the subunits move in relation to one another.

When Ha and postdoctoral fellow Peter Cornish observed the signal from the labeled ribosomes, they saw a spontaneous back-and-forth rotation between the subunits even in the absence of the elongation factor, EF-G.

"Other researchers proposed that this rotation is induced by EF-G that you have to have EF-G to cause this rotation," Ha said. "But we showed that no, that's not the case. Actually the ribosome can rock back and forth spontaneously, and can do it quite rapidly."

The researchers were able to view this motion even in the absence of tRNA. The ribosomal subunits were spontaneously switching back and forth between the classical (that is, non-rotated) state and a hybrid (rotated) state.

When they added a single tRNA with an amino acid permanently attached to it, the ribosome became "essentially stuck in the classical, non-rotated state," Cornish said. "And as soon as we removed that, it started to move spontaneously."

To better understand the role of EF-G, the researchers added a modified EF-G molecule that could not deliver its normal energy payload to the ribosome. The modified EF-G bound to the ribosome only in the rotated, hybrid state.

These findings led the researchers to propose that EF-G has a critical role in the process of protein translation: It stabilizes the rotated position of the ribosomal subunits relative to one another.

This allows the tRNA molecules to add amino acids to the growing protein and to exit, making room for the next tRNA specified in the messenger RNA code.

The researchers believe that EF-G acts as a linchpin, temporarily holdi
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