U24, A NOVEL INTRON-ENCODED SMALL NUCLEOLAR RNA WITH 2 12 NT LONG, PHYLOGENETICALLY CONSERVED COMPLEMENTARITIES TO 28S RIBOSOMAL-RNA

Following computer searches of sequence banks, we have positively iden tified a novel intronic snoRNA, U24, encoded in the ribosomal protein L7a gene in humans and chicken. Like previously reported intronic snoR NAs, U24 is devoid of a 5'-trimethyl-cap. U24 is immunoprecipitated by an antifibrillarin antibody and displays an exclusively nucleolar loc alization by fluorescence microscopy after in situ hybridization with antisense oligonucleotides. In vertebrates, U24 is a 76 nt long conser ved RNA which is metabolically stable, present at similar to 14 000 mo lecules per human HeLa cell. U24 exhibits a 5'-3' terminal stem-box C- box D structure, typical for several snoRNAs, and contains two 12 nt l ong conserved sequences complementary to 28S rRNA. It is, therefore, s trikingly related to U14, U20 and U21 snoRNAs which also possess long sequences complementary to conserved sequences of mature 18S or 28S rR NAs. In 28S rRNA the two tracts complementary to U24 are adjacent to e ach other, they involve several methylated nucleotides and are surpris ingly close, within the rRNA secondary structure, to complementarities to snoRNAs U18 and U21. Identification of the yeast Saccharomyces cer evisiae U24 gene directly confirms the outstanding conservation of the complementarity to 28S rRNA during evolution, suggesting a key role o f U24 pairing to pre-rRNA during ribosome biogenesis, possibly in the control of pre-rRNA folding. Yeast S.cerevisiae U24 is also intron-enc oded but not in the same host-gene as in humans or chicken.