A Translocation-Associated Ribosomal Conformation Change Detected by Hydrogen Exchange and Sedimentation Velocity

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Abstract
Translocation in ribosomes consists of transposition of peptidyl-tRNA from the aminoacyl to the peptidyl site and, probably concomitantly, the movement of ribosomes on mRNA. Does a conformational change in the ribosome provide the motive force for this process? Hydrogen exchange and sedimentation velocity experiments indicate that the Escherichia coli ribosome does undergo a conformational change associated with translocation. When pretranslocational ribosomes carrying acetyldiphenylalanyl-tRNA in the aminoacyl site were incubated with G factor and GTP, translocation occurred, with a concomitant increase in hydrogen exchange rate and a decrease in sedimentation constant. These changes did not occur when GTP was replaced by a nonhydrolyzable analogue, GDP-CH(2)-P, and they were blocked by the antibiotics fusidic acid and thiostrepton. When posttranslocational ribosomes were cycled back to the pretranslocational state by T factor, GTP, and phenylalanyl-tRNA, the sedimentation constant reverted to the original value. Whether or not this conformation change drives translocation requires further study.