Ribosomal RNA processing undergoes surveillance by the mRNA guard protein Npl3

Ribosome production is an essential, but highly energy-demanding and complex cellular process. High-fidelity ribosome assembly is critical to ensure integrity of the proteome, and during their biogenesis, the pre-ribosomal subunits undergo surveillance so that defective complexes are removed. Currently, knowledge on the mechanisms of pre-ribosome quality control lag behind understanding of other RNA surveillance pathways. Interestingly, a family of "guard proteins" has been shown to monitor mRNA maturation in S. cerevisiae and act as maturation switches by either licensing ongoing biogenesis or, in case assembly defects are detected, recruiting components of the RNA degradation machinery. Here, we reveal association of the mRNA guard protein Npl3 with RNA polymerase I, the ribosomal DNA (rDNA) locus and early pre-ribosomal particles. Consistent with this, our data show that Npl3 is required for recruitment of a subset of ribosome assembly factors to the nascent pre-rRNA transcript, and we demonstrate a role for Npl3 in the turn-over of aberrant precursor ribosomal RNA (pre-rRNA). Aberrant pre-rRNAs polyadenylated by Trf5 are bound by Npl3, which in turn interacts with Air1 to recruit the RNA exosome for degradation. Thus, Npl3 is a multifunctional RNA surveillance factor, recognizing different types of aberrant pre-RNAs and promoting recruitment of the RNA degradation machinery to clear these transcripts.