Regulation of pre-mRNA splicing in Xenopus oocytes by targeted 2′-O-methylation

The 29-OH group of the branch point adenosine is a key moiety to initiate pre-mRNA splicing. We use RNA-guided RNA modification to target the pre-mRNA branch point adenosine for 29-O-methylation, with the aim of blocking pre-mRNA splicing in vertebrate cells. We show that, under certain conditions, injection of a branch point-specific artificial box C/D RNA into Xenopus oocytes effectively 29-O-methylates adenovirus pre-mRNA at the target nucleotide. However, 29-O-methylation at the authentic branch point activates a host of cryptic branch points, thus allowing splicing to continue. These cryptic sites are mapped, and mutated. Upon injection, pre-mRNA free of cryptic branch points fails to splice when the branch point-specific box C/D RNA is present. However, 29-O-methylation at the branch point does not prevent pre-mRNA from being assembled into pre-catalytic spliceosome-like complexes prior to the first chemical step of splicing. Our results demonstrate that RNAguided pre-mRNA modification can occur in the nucleoplasm of vertebrate cells, thus offering a powerful tool for molecular biology research.