INTRON-ENCODED, ANTISENSE SMALL NUCLEOLAR RNAS - THE CHARACTERIZATIONOF 9 NOVEL SPECIES POINTS TO THEIR DIRECT ROLE AS GUIDES FOR THE 2'-O-RIBOSE METHYLATION OF RIBOSOMAL-RNAS

A growing number of small nucleolar RNAs (snoRNAs) are intron-encoded, contain the characteristic box C (UGAUGA) and box D (CUGA) motifs and exhibit long complementarities to conserved sequences in mature rRNAs . We have identified nine additional members of this family, U32 to U4 0, ALI but one are encoded in introns of ribosomal protein genes in ve rtebrates: U32 to U35 in rpL13a, U36 in rpL7a and U38 to U40 in rpS8. By contrast, U37 is encoded in elongation factor 2 gene. Interestingly , U32 and U36 each contain two complementarities (one to 18 S and the ether to 28 S rRNA). U32 to U40 are fibrillarin-associated, devoid of a 5'-trimethyl-cap and display an exclusively nucleolar localization. They are all metabolically stable and roughly as abundant as previousl y reported members of this family. Characterization of their homologs in distant species shows that their 10 to 14 nt long rRNA complementar ities are conserved. A clue on the function of this snoRNA family is p rovided by the comparative analysis of the largely expanded collection of their conserved duplexes with rRNA. Not only does each duplex span at least one site of 2'-O-ribose methylation in the rRNA but the modi fication site is always at the same position in the duplex, paired to the fifth nucleotide upstream from a box D motif in the snoRNA. Consis tent with the notion that each snoRNA of this family guides one partic ular methylation along the rRNA sequence, we have detected several Fai rs of snoRNAs with overlapping complementarities to rRNA tracts with v icinal sites of ribose methylations. In each case, the two overlapping complementarities are shifted from each other by a distance equal to the spacing between the methylated sites which are thus found at the s ame position within each of the mutually exclusive duplexes. Finally, we have also identified, within three previously known snoRNAs, novel antisense elements able to form a canonical duplex around ribose-methy lated sites in rRNA, which further supports the conclusion that the du plex structure provides the 2'-O-methyltransferase with the appropriat e site-specificity on the substrate. (C) 1996 Academic Press Limited