SYNTHETIC LETHALITY WITH FIBRILLARIN IDENTIFIES NOP77P, A NUCLEOLAR PROTEIN REQUIRED FOR PRE-RIBOSOMAL-RNA PROCESSING AND MODIFICATION

The nucleolar protein fibrillarin (encoded by the NOP1 gene in yeast), is required for many post-transcriptional steps in yeast ribosome syn thesis. A screen for mutations showing synthetic lethality with a temp erature sensitive nop1-5 allele led to the identification of the NOP77 gene. NOP77 is essential for viability and encodes a nucleolar protei n with a predicted molecular weight of 77 kDa. Depletion of NOP77p imp airs both the processing and methylation of the pre-rRNA. The processi ng defect is greatest for the pathway leading to 25S rRNA synthesis, a nd is distinctly different from that observed for mutations in other n ucleolar components. NOP77p contains three canonical RNA recognition m otifs (RRMs), suggesting that it is an RNA binding protein. The NOP77 allele which complements the synthetic lethal nop1 strains has an alan ine at position 308, predicted to lie in helix alpha 1 of RRM3, wherea s the non-complementing nop77-1 allele contains a proline at the corre sponding position. We propose that NOP77p mediates specific interactio ns between NOP1p and the pre-rRNA.