Maintaining a conserved methylation in plant and insect U2 snRNA through compensatory mutation by nucleotide insertion

Open Access
Publisher Website
Google Scholar
Abstract
The extensive post‐transcriptional modification of U2 snRNA is required for spliceosome assembly and pre‐mRNA splicing in vertebrates. However, the rare modification of U2 snRNA in yeast implies a different mechanism for regulating spliceosome biogenesis in single‐celled eukaryotes. To understand the evolutionary pattern of U2 snRNA methylation, we determined for the first time, the 2'‐O‐methylations of U2 snRNA in Oryza sativa, Arabidopsis thaliana and Drosophila melanogaster, and revealed two methylations which are conserved in a crucial region of U2 snRNA in plants. Interestingly, one of the methylations, U2‐Cm29 is also methylated in D. melanogaster, but not in vertebrates. According to the methylation of U2‐C29, computational analysis of databases identified three canonical box C/D snoRNAs, named OsmgU2‐29, AtmgU2‐29 and DmmgU2‐28, as small methylation guides of U2 snRNA from O. sativa, A. thaliana and D. melanogaster, respectively. Although very divergent in their sequence, the three snoRNAs exhibit in common an 11 nucleotide‐long sequence complementarity to corresponding U2 snRNA, implying a functional constraint on the modification during evolution. Interestingly, a nucleotide is found to be inserted both in U2 snRNA and DmmgU2‐28 and maintains a perfect match of duplex specifying the methylation of C28 in Drosophila U2 snRNA. This is the first time a new model is being provided for compensatory mutations between a small guide RNA and its target by nucleotide insertion, instead of the known nucleotide substitution. In contrast to small Cajal body‐specific RNAs (scaRNAs), the snoRNAs are similar to the reported singlet guide RNAs and are known to localize in nucleolus. IUBMB Life, 57: 693‐699, 2005