A systematic dissection of determinants and consequences of snoRNA-guided pseudouridylation of human mRNA

RNA can be extensively modified post-transcriptionally with >170 covalent modifications, expanding its functional and structural repertoire. Pseudouridine (Ψ), the most abundant modified nucleoside in rRNA and tRNA, has recently been found within mRNA molecules. Due to technical challenges, it remained unclear whether pseudouridylation of mRNA can be snoRNA-guided, which has important implications for understanding the physiological target spectrum of snoRNAs and for their potential therapeutic exploitation in genetic diseases. Here, using a massively parallel reporter based strategy we simultaneously interrogate Ψ levels across hundreds of synthetic constructs with predesigned complementarity against endogenous snoRNAs. Our results demonstrate that snoRNA-mediated pseudouridylation can occur on mRNA targets. However, this is typically achieved at low efficiencies, and is constrained by the localization of mRNA, by the expression levels of snoRNAs and by the length of the snoRNA:mRNA complementarity stretches. We exploited these insights for the design of snoRNAs targeting pseudouridylation at premature termination codons, which was previously suggested to suppress translational termination. However, in contrast to previous studies, in this and follow-up experiments we observe no evidence for readthrough of pseudouridylated stop codons. Our study enhances our understanding of the scope, ‘design rules’ and constraints of snoRNA-mediated pseudouridylation, and challenges a key functional outcome associated with this modification.