The exonuclease ERI-1 negatively regulates RNA interference (RNAi) in Caenorhabiditis elegans and Schizosaccharomyces pombe, and is required for production of some C. elegans endogenous small-interfering RNAs. We show that ERI-1 performs 3′ end processing of the 5.8S ribosomal RNA (rRNA) in both C. elegans and S. pombe. In C. elegans, two protein isoforms of ERI-1 are localized to the cytoplasm, and each has distinct functions in rRNA processing and negative regulation of RNAi.The C. elegans eri-1 gene encodes a 3′ to 5′ exonuclease of the DEDDh superfamily of RNase T exonucleases that was identified as a negative regulator of RNA interference (RNAi) 1 . Mutations in eri-1 cause an enhanced RNA interference (Eri) phenotype by which double stranded RNAs (dsRNAs) that are ineffective in silencing target mRNAs in wildtype animals trigger robust silencing in the Eri mutant. In the fission yeast S. pombe loss of Eri1 causes increased levels of small interfering RNAs (siRNAs) corresponding to centromeric repeats and a concomitant increase in RNAi-dependent heterochromatin formation at these genomic loci 2 . Analysis of ERI-1 in C. elegans, human and fission yeast has shown that it can degrade the 3′ end of siRNAs and histone mRNAs in vitro [1][2][3][4] , but in vivo substrates for this conserved enzyme are poorly understood.In the course of the analysis of RNAs isolated from the eri-1 null mutant, we observed that the 5.8S rRNA in an eri-1 worm is longer than wild-type 5.8S rRNA (Fig. 1a). This length difference is present in all detectable 5.8S rRNA, suggesting that eri-1 mutants have an rRNA processing defect in most, if not all, cells. The mature 5.8S, 18S, and 25-28S rRNAs in eukaryotes are generated from a 35S-47S precursor RNA via a series of processing steps mediated by multiple nucleases 5 . The activity of ERI-1 as a 3′ to 5′ exonuclease [1][2][3][4] suggests that the longer 5.8S rRNA is due to an extension of the 3′ end. RNase H cleavage and 3′ end cloning on the 5.8S rRNA of wild-type and mutant C. elegans indicated that all eri-1 5.8S rRNA is at least 1 nucleotide longer than the wild-type 5.8S rRNA specifically at the 3′ end, with a substantial fraction of eri-1 5.8S rRNA containing 2 to 4 additional nucleotides ( Supplementary Fig. 1 online). The 5.8S processing defect was observed in two independently derived eri-1 alleles, including another null allele (mg388, data not shown), and is rescued by an eri-1 transgene (see below), proving that loss of eri-1 causes the 5.8S processing defect. The 5.8S processing defect was not observed in other enhanced RNAi mutants with pleiotropies in common with eri-11 ,6 (data not shown). To characterize whether ERI-1 function in rRNA trimming is an ancient feature of this orthology group, we examined the 5.8S rRNA in the S. pombe erilΔ mutant and observed a length defect similar to the C. elegans eri-1 mutant (Fig. 1b). RNase H and sequencing analysis revealed a 3′ extended 5.8S rRNA species in erilΔ containing from 2 to 8 additional 3′ nucleotides ( Supplement...