SACCHAROMYCES-CEREVISIAE U14 SMALL NUCLEAR-RNA HAS LITTLE SECONDARY STRUCTURE AND APPEARS TO BE PRODUCED BY POSTTRANSCRIPTIONAL PROCESSING

Yeast U14 small nuclear (sn) RNA is required for normal processing of RNA. The sequence and folding properties of U14 were analyzed in the p resent study, with the aim of defining the structures of natural U14 s ubspecies and characterizing the folding properties of free U14 RNA. N atural U14 was determined to consist of four subspecies of 125-128 nuc leotides, none containing a 5'-cap structure. Length heterogeneity occ urs at both ends and is presumed to reflect post-transcriptional proce ssing of U14 precursors. Results from nuclease and chemical probing re vealed that U14 has surprisingly little secondary structure overall. T hree essential sequence elements conserved among all U14 RNAs occur in regions that are largely single-stranded, i.e. box C, box D, and a 13 -nucleotide segment complementary to 18 S rRNA, a non-essential 14-nuc leotide sequence complementary to 18 S rRNA is also unpaired. Two non- conserved segments required for activity are part of a stably folded 3 2-base domain that is unique to yeast U14. Finally, a 5'-, 3'-stem sho wn earlier to be required for U14 accumulation appears to exist only i n precursors to U14 and not in protein-free mature RNA. The implicatio ns of these results are discussed in terms of U14 synthesis and functi on.