RNA exonuclease that acts as a negative regulator of RNA interference (RNAi). Acts by degrading the 3'-overhangs of double-stranded short interfering RNAs (siRNAs). Represses the accumulation of heterochromatic siRNAs leading to negative regulation of the RNAi-mediated heterochromoatin assembly. Also involved in rRNA biogenesis, trimming the 5.8S ribosomal RNA (rRNA) from a slightly longer pre-5.8S RNA in the cytoplasm. Binds 2 magnesium ions per subunit.