A posttermination ribosomal complex is the guanine nucleotide exchange factor for peptide release factor RF3

The mechanism by which peptide release factor RF3 recycles RF1 and RF2 has been clarified and incorporated in a complete scheme for translation termin ation. Free RF3 is in vivo stably bound to GDP, and ribosomes in complex wi th RF1 or RF2 act as guanine nucleotide exchange factors (GEF). Hydrolysis of peptldyl-tRNA by RF1 or RF2 allows GTP binding to RF3 on the ribosome. T his induces an RF3 conformation with high affinity for ribosomes and leads to rapid dissociation of RF1 or RF2. Dissociation of RF3 from the ribosome requires GTP hydrolysis. Our data suggest that RF3 and its eukaryotic count erpart, eRF3, have mechanistic principles in common.