Ribosomal RNAs in translation termination: Facts and hypotheses

It is now well established that ribosomal RNAs (rRNAs) play an active role in every aspect of translation. This review focuses on recent evidence for the involvement of rRNAs from both subunits of the ribosome in translation termination. This evidence comprises data obtained with rRNA mutants both i n vivo and in vitro. In particular, mutations in specific regions of rRNAs caused readthrough of nonsense codons in vivo. Consistent with their in viv o characteristics, the mutations decreased the productive association of th e ribosome with release factor 2 (RF2) and the efficiency of catalysis of p eptidyl-tRNA hydrolysis in the presence of RF2 in realistic in vitro termin ation systems. It is now evident that genetic selections for termination-de fective mutants in vivo and their characterization in realistic in vitro te rmination assays will rapidly advance our understanding of the mechanism of termination.