High dosage of the small nucleolar RNA snR10 specifically suppresses defects of a yeast rrp5 mutant

We have previously described a yeast strain in which cleavage at site A2 during processing of rRNA is absent and is functionally replaced by cleavage at site A3. This strain expresses a variant of the essential RRP5 gene that results in the synthesis of two noncontiguous segments of the protein. We have used the slow-growth phenotype of this strain to screen for revertants. The gene for the small nucleolar RNA snR10 was isolated as a multicopy suppressor of this "bipartite" RRP5 allele. Suppression by snR10 efficiently rescues the slow-growth (sg) and temperature-sensitive (ts) phenotypes of the mutant strain and is specific for this small nucleolar RNA. Deletion derivatives of snR10 were constructed and tested for the ability to suppress the sg and ts phenotypes of the RRP5 mutant, as well as for complementation of the cold sensitivity of a delta snr10 strain. The results indicate that the suppression effect is more sensitive to snR10 mutations than is complementation. The high dosage of wild-type snR10 does not restore cleavage at A2, but improves the rate of pre-rRNA processing and significantly increases the level of active ribosomes in the suppressed strain. These effects probably account for the suppression of the sg and ts phenotypes of the rrp5 mutant strain.