SNORA57 guide activities on yeast 18S rRNA. (A) Postulated base-pairing of SNORA57 with human (red) and yeast (blue) 18S rRNA; the 5′ terminal pseudouridylation pocket is shown on the left and the 3′-terminal pseudouridylation pocket on the right. To make a yeast optimized version of SNORA57 an indicated U-to-C mutation was introduced in the ASE of the 5′ terminal pseudouridylation pocket. (B) Mapping pseudouridines in human 18S [968–1086 nt] (HeLa, top gray trace) and the equivalent region of yeast 18S rRNA (BY4741, black, red, and blue traces). Normally, yeast 18S rRNA has only Ψ999 in this region (black trace). Expression of vertebrate SNORA57 did not change the modification pattern of yeast 18S rRNA (red trace, arrowheads indicate predicted target positions for SNORA57). Yeast optimized SNORA57 with the U-to-C mutation could induce pseudouridylation of yeast 18S rRNA at two positions, 947 and 989 (blue trace, stars).
