Ribosome release factor RF4 and termination factor RF3 are involved in dissociation of peptidyl-tRNA from the ribosome

Peptidyl‐tRNA dissociation from ribosomes is an energetically costly but apparently inevitable process that accompanies normal protein synthesis. The drop‐off products of these events are hydrolysed by peptidyl‐tRNA hydrolase. Mutant selections have been made to identify genes involved in the drop‐off of peptidyl‐tRNA, using a thermosensitive peptidyl‐tRNA hydrolase mutant in Escherichia coli. Transposon insertions upstream of the frr gene, which encodes RF4 (ribosome release or recycling factor), restored growth to this mutant. The insertions impaired expression of the frr gene. Mutations inactivating prfC, encoding RF3 (release factor 3), displayed a similar phenotype. Conversely, production of RF4 from a plasmid increased the thermosensitivity of the peptidyl‐tRNA hydrolase mutant. In vitro measurements of peptidyl‐tRNA release from ribosomes paused at stop signals or sense codons confirmed that RF3 and RF4 were able to stimulate peptidyl‐tRNA release from ribosomes, and showed that this action of RF4 required the presence of translocation factor EF2, known to be needed for the function of RF4 in ribosome recycling. When present together, the three factors were able to stimulate release up to 12‐fold. It is suggested that RF4 may displace peptidyl‐tRNA from the ribosome in a manner related to its proposed function in removing deacylated tRNA during ribosome recycling.