U3 snoRNA is transcribed from two intron-containing genes in yeast, snR17A and snR17B. Although the assembly of the U3 snoRNP has not been precisely determined, at least some of the core box C/D proteins are known to bind pre-U3 co-transcriptionally, thereby affecting splicing and 3-end processing of this snoRNA. We identified the interaction between the box C/D assembly factor Nop17p and Cwc24p, a novel yeast RING finger protein that had been previously isolated in a complex with the splicing

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... actor Cef1p. Here we show that, consistent with the protein interaction data, Cwc24p localizes to the cell nucleus, and its depletion leads to the accumulation of both U3 pre-snoRNAs. U3 snoRNA is involved in the early cleavages of 35 S pre-rRNA, and the defective splicing of pre-U3 detected in cells depleted of Cwc24p causes the accumulation of the 35 S precursor rRNA. These results led us to the conclusion that Cwc24p is involved in pre-U3 snoRNA splicing, indirectly affecting pre-rRNA processing. In eukaryotes, three of the rRNAs (18, 5.8, and 25 S in the yeast Saccharomyces cerevisiae) are transcribed by RNA pol 3 I as a single precursor, which undergoes various processing steps that include nucleotide modifications and endo-and exonucleolytic cleavages. In yeast, at least 150 factors are involved in 35 S pre-rRNA processing, including different proteins and snoRNPs. Processing and modification of the pre-rRNA seems to occur simultaneously with the assembly of ribosomal proteins, and large transient ribonucleoprotein (RNP) particles are formed in the nucleolus (1-3). Different sets of proteins were found in each complex. Among the proteins identified in the 90 S preribosomal complex are many of the small subunit ribosomal proteins, the core box C/D proteins (Nop1p, Nop56p, and Nop58p), U3 snoRNP-specific proteins, and other factors that might also be involved in the early steps of pre-rRNA processing (2). Proteins present in the pre-60 S complex are mainly involved in processing of 5.8 S and 25 S and in the formation of the large ribosome subunit (4, 5).