Abstract
Ribosome biogenesis requires multiple nuclease activities to process pre-rRNA transcripts into mature rRNA species and eliminate defective products of transcription and processing. We find that in mammalian cells, the 5' exonuclease Xrn2 plays a major role in both maturation of rRNA and degradation of a variety of discarded pre-rRNA species. Precursors of 5.8S and 28S rRNAs containing 5' extensions accumulate in mouse cells after siRNA-mediated knockdown of Xrn2, indicating similarity in the 5'-end maturation mechanisms between mammals and yeast. Strikingly, degradation of many aberrant pre-rRNA species, attributed mainly to 3' exonucleases in yeast studies, occurs 5' to 3' in mammalian cells and is mediated by Xrn2. Furthermore, depletion of Xrn2 reveals pre-rRNAs derived by cleavage events that deviate from the main processing pathway. We propose that probing of pre-rRNA maturation intermediates by exonucleases serves the dual function of generating mature rRNAs and suppressing suboptimal processing paths during ribosome assembly.