Release of Peptide Promoted by the GGQ Motif of Class 1 Release Factors Regulates the GTPase Activity of RF3

E. coli mutants of RF1 and RF2, in which the universal GGQ motif is changed to GAQ, are slow in peptide release from ribosomes. Other kinetic properties are unchanged, suggesting that the GGQ motif is in contact with the peptidyl-transferase center. Deacylated tRNA terminates protein synthesis codon specifically, indicating that the CCA end of tRNA and the GGQ motif operate similarly. Addition of a mutant factor to a pretermination ribosomal complex stimulates exchange of RF3-bound GDP with free GDP, but binding of GTP to RF3 and GTP hydrolysis requires peptide chain release. Therefore, the sequence of steps during termination of translation is regulated by removal of the polypeptide, an event that might trigger a conformational change in the ribosome.