Yeast Rrp9p is an evolutionarily conserved U3 snoRNP protein essential forearly pre-rRNA processing cleavages and requires box C for its association

Pre-rRNA processing in eukaryotic cells requires participation of several s noRNPs, These include the highly conserved and abundant Us snoRNP, which is essential far synthesis of 18S rRNA. Here we report the characterization o f Rrp9p, a novel yeast Us protein, identified via its homology to the human U3-55k protein, Epitope-tagged Rrp9p specifically precipitates Us snoRNA, but Rrp9p is not required for the stable accumulation of this snoRNA, Genet ic depletion of Rrp9p inhibits the early cleavages of the primary pre-rRNA transcript at A(0), A(1), and A(2) and, consequently, production of 18S, bu t not 25S and 5.8S, rRNA, The hU3-55k protein can partially complement a ye ast rrp9 null mutant, indicating that the function of this protein has been conserved. Immunoprecipitation of extracts from cells that coexpress epito pe-tagged Rrp9p and various mutant forms of Us snoRNA limits the region req uired for association of Rrp9p to the Us-specific box B/C motif, Box C is e ssential, whereas box B plays a supportive role.