U20, A NOVEL SMALL NUCLEOLAR RNA, IS ENCODED IN AN INTRON OF THE NUCLEOLIN GENE IN MAMMALS

We have found that intron 11 of the nucleolin gene in humans and roden ts encodes a previously unidentified small nucleolar RNA, termed U20. The single-copy U20 sequence is located on the same DNA strand as the nucleolin mRNA. U20 RNA, which does not possess a trimethyl cap, appea rs to result from intronic RNA processing and not from transcription o f an independent gene. In mammals, U20 RNA is an 80-nucleotide-long, m etabolically stable species, present at about 7 x 10(3) molecules per exponentially growing HeLa cell. It has a nucleolar localization, as i ndicated by fluorescence microscopy following in situ hybridization wi th digoxigenin-labeled oligonucleotides. U20 RNA contains the box C an d hox D sequence motifs, hallmarks of most small nucleolar RNAs report ed to date, and is immunoprecipitated by antifibrillarin antibodies. I t also exhibits a 5'-3' terminal stem bracketing the box C-bos D motif s like U14, U15, U16! or Y RNA, A U20 homolog of similar size has been detected in all vertebrate classes by Northern (RNA) hybridization,vi th mammalian oligonucleotide probes. U20 RNA contains an extended regi on (21 nucleotides) of perfect complementarity with a phylogenetically conserved sequence in 188 rRNA. This complementarity is strongly pres erved among distant vertebrates, suggesting that U20 RNA may be involv ed in the formation of the small ribosomal subunit like nucleolin, the product of its host gene.