Orchestrated positioning of post-transcriptional modifications at the branch point recognition region of U2 snRNA.

The branch point recognition region of spliceosomal snRNA U2 is heavily modified post-transcriptionally in most eukaryotic species. We focused on this region to learn how nearby positions may interfere with each other when targeted for modification. Using an in vivo yeast cell system, we tested the modification activity of several guide RNAs from human, mouse, the frog , the fruit fly , and the worm We experimentally verified predictions for vertebrate U2 modification guide RNAs SCARNA4 and SCARNA15, and identified a ortholog of SCARNA15. We observed crosstalk between sites in the heavily modified regions, such that modification at one site may inhibit modification at nearby sites. This is true for the branch point recognition region of U2 snRNA, the 5' loop of U5 snRNA, and certain regions of rRNAs, when tested either in yeast or in HeLa cells. The position preceding a uridine targeted for isomerization by a box H/ACA guide RNA is the most sensitive for noncanonical base-pairing and modification (either pseudouridylation or 2'--methylation). Based on these findings, we propose that modification must occur stepwise starting with the most vulnerable positions and ending with the most inhibiting modifications. We discuss possible strategies that cells use to reach complete modification in heavily modified regions.