Our previous demonstration that mutants of 5S rRNA called
mof9 can specifically alter efficiencies of programmed
ribosomal frameshifting (PRF) suggested a role for this ubiquitous
molecule in the maintenance of translational reading frame,
though the repetitive nature of the 5S rDNA gene (>100
copies/cell) inhibited more detailed analyses. However, given
the known interactions 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 series of
well-defined rpl5 mutants on PRF and virus propagation.
Consistent with the mof9 results, we find that the
rpl5 mutants promoted increased frameshifting efficiencies
in both the −1 and +1 directions, and conferred defects
in the ability of cells to propagate two endogenous viruses.
Biochemical analyses demonstrated that mutant ribosomes had
decreased affinities for peptidyl-tRNA. Pharmacological studies
showed that sparsomycin, a peptidyltransferase inhibitor that
specifically increases the binding of peptidyl-tRNA with ribosomes,
was antagonistic to the frameshifting defects of the most severe
mutant, and the extent of sparsomycin resistance correlated
with the severity of the frameshifting defects in all of the
mutants. These results 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
and +1 frameshifting, and for the effects of sparsomycin.