U21, A NOVEL SMALL NUCLEOLAR RNA WITH A 13-NT COMPLEMENTARITY TO 28S RIBOSOMAL-RNA, IS ENCODED IN AN INTRON OF RIBOSOMAL-PROTEIN-L5 GENE INCHICKEN AND MAMMALS

Following a search of sequence data bases for intronic sequences exhib iting structural features typical of snoRNAs, we have positively ident ified by Northern assays and sequence analysis another intron-encoded snoRNA, termed U21. U21 RNA is a 93 nt. long, metabolically stable RNA , present at about 104 molecules per HeLa cell. It is encoded in intro n 5 of the ribosomal protein L5 gene, both in chicken and in the two m ammals studied so far, human and mouse. U21 RNA is devoid of a 5'-trim ethyl-cap and is likely to result from processing of intronic RNA. The nucleolar localization of U21 has been established by fluorescence mi croscopy after in situ hybridization with digoxigenin-labeled oligonuc leotide probes. Like most other snoRNAs U21 contains the box C and box D motifs and is precipitated by anti-fibrillarin antibodies. By the p resence of a typical 5' - 3' terminal stem, U21 appears more particula rly related to U14, U15, U16 and U20 intron-encoded snoRNAs. Remarkabl y, U21 contains a long stretch (13 nt.) of complementarity to a highly conserved sequence in 28S rRNA. Sequence comparisons between chicken and mammals, together with Northern hybridizations with antisense olig onucleotides on cellular RNAs from more distant vertebrates, point to the preferential preservation of this segment of U21 sequence during e volution. Accordingly, this complementarity, which overlaps the comple mentarity of 28S rRNA to another snoRNA, U18, could reflect an importa nt role of U21 snoRNA in the biogenesis of large ribosomal subunit.