LAUSR

Box C/D snoRNAs constitute a class of abundant noncoding RNAs that associate with common core proteins to form catalytic snoRNPs. Most of these operate in trans to assist the maturation of rRNAs by guiding and catalyzing the 2'-O-methylation of specific nucleotides. Here, we report that the human intron-hosted box C/D snoRNA snoRD86 acts in cis as a sensor and master switch controlling levels of the limiting snoRNP core protein NOP56, which is important for proper ribosome biogenesis. Our results support a model in which snoRD86 adopts different RNP conformations that dictate the usage of nearby alternative splice donors in the NOP56 pre-mRNA. Excess snoRNP core proteins prevent further production of NOP56 and instead trigger the generation of a cytoplasmic snoRD86-containing NOP56-derived lncRNA via the nonsense-mediated decay pathway. Our findings reveal a feedback mechanism based on RNA structure that controls the precise coordination between box C/D snoRNP core proteins and global snoRNA levels.