Ribosomal protein L5 helps anchor peptidyl-tRNA to the P-site in Saccharomyces cerevisiae

Our previous demonstration that mutants of 5S rRNA called mof9 can specific ally alter efficiencies of programmed ribosomal frameshifting (PRF) suggest ed a role for this ubiquitous molecule in the maintenance of translational reading frame, though the repetitive nature of the 5S rDNA gene (>100 copie s/cell) inhibited more detailed analyses. However, given the known interact ions between 5S rRNA and ribosomal protein L5 (previously called L1 or YL3) encoded by an essential, single-copy gene, we monitored the effects of a s eries of well-defined rpl5 mutants on PRF and virus propagation. Consistent with the mof9 results, we find that the rpl5 mutants promoted increased fr ameshifting efficiencies in both the -1 and +1 directions, and conferred de fects in the ability of cells to propagate two endogenous viruses. Biochemi cal analyses demonstrated that mutant ribosomes had decreased affinities fo r peptidyl-tRNA. Pharmacological studies showed that sparsomycin, a peptidy ltransferase inhibitor that specifically increases the binding of peptidyl- tRNA with ribosomes, was antagonistic to the frameshifting defects of the m ost severe mutant, and the extent of sparsomycin resistance correlated with the severity of the frameshifting defects in all of the mutants. These res ults provide biochemical and physiological evidence that one function of L5 is to anchor peptidyl-tRNA to the P-site. A model is presented describing how decreased affinity of ribosomes for peptidyl-tRNA can affect both -1 an d +1 frameshifting, and for the effects of sparsomycin.