RRP6P, THE YEAST HOMOLOG OF THE HUMAN PM-SCL 100-KDA AUTOANTIGEN, IS ESSENTIAL FOR EFFICIENT 5.8 S-RIBOSOMAL-RNA 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), whi ch are removed by endo-and exoribonucleolytic steps to produce mature rRNA. Genetic selection for suppressors of a polyadenylation defect yi elded two cold-sensitive alleles of a gene that we named RRP6 (ribosom al RNA processing), Molecular cloning of RRP6 revealed its homology to a 100-kDa human, nucleolar PM-Sd autoantigen and to Escherichia coli RNase D, a 3'-5' exoribonuclease, Recessive mutations in rrp6 result i n the accumulation of a novel 5.8 S rRNA processing intermediate, call ed 5.8 S, which has normal 5' ends, but retains similar to 30 nucleot ides 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 l ast 30 nucleotides of ITS2 from 5.8 S precursors. A portion of 5.8 S rRNA assembles into 60 S ribosomes which form polyribosomes, suggestin g that they function in protein synthesis. These findings indicate tha t Rrp6p plays a role in 5.8 S rRNA 3' end formation, and they identify a functional intermediate in the rRNA processing pathway.