U14 SMALL NUCLEOLAR RNA MAKES MULTIPLE CONTACTS WITH THE PRE-RIBOSOMAL-RNA

The small nucleolar RNA (snoRNA) U14 has two regions of extended prima ry sequence complementarity to the 18S rRNA. The 3' region (domain B) shows the consensus structure for the methylation guide class of snoRN As, whereas base-pairing between the 5' region (domain A) and the 18S rRNA sequence is required for the formation of functional ribosomes. B etween domains A and B lies another essential region (domain Y). Here we report that yeast U14 can be cross-linked in vivo to the pre-rRNA; cross-linking is detected exclusively with the 35S primary transcript. Many nucleotides in U14 that lie outside of domains A and B are cross -linked to the pre-rRNA; in particular the essential domain Y region i s cross-linked at several sites. U14 is, therefore, in far more extens ive contact with the pre-rRNA than predicted from simple base-pairing models. Moreover, U14 can be cross-linked to other small RNA species. The functional interactions made by U14 during ribosome synthesis are likely to be very complex.