Rrp6p, the Yeast Homologue of the Human PM-Scl 100-kDa Autoantigen, Is Essential for Efficient 5.8 S rRNA 3′ End Formation

The eukaryotic 25 S, 18 S, and 5.8 S rRNAs are synthesized as a single transcript with two internal transcribed spacers (ITS1 and ITS2), which are removed by endo- and exoribonucleolytic steps to produce mature rRNA. Genetic selection for suppressors of a polyadenylation defect yielded two cold-sensitive alleles of a gene that we named RRP6 ( r ibosomal R NA p rocessing). Molecular cloning of RRP6 revealed its homology to a 100-kDa human, nucleolar PM-Scl autoantigen and to Escherichia coli RNase D, a 3′–5′ exoribonuclease. Recessive mutations in rrp6 result in the accumulation of a novel 5.8 S rRNA processing intermediate, called 5.8 S*, which has normal 5′ ends, but retains ∼30 nucleotides of ITS2. Pulse-chase analysis of 5.8 S rRNA processing in an rrp6- strain revealed a precursor-product relationship between 5.8 S* and 5.8 S rRNAs, suggesting that Rrp6p plays a role in the removal of the last 30 nucleotides of ITS2 from 5.8 S precursors. A portion of 5.8 S* rRNA assembles into 60 S ribosomes which form polyribosomes, suggesting that they function in protein synthesis. These findings indicate that Rrp6p plays a role in 5.8 S rRNA 3′ end formation, and they identify a functional intermediate in the rRNA processing pathway.