NOVEL INTRON-ENCODED SMALL NUCLEOLAR RNAS WITH LONG SEQUENCE COMPLEMENTARITIES TO MATURE RIBOSOMAL-RNAS INVOLVED IN RIBOSOME BIOGENESIS

Recently, several new snoRNAs encoded in introns of genes coding for r ibosomal, ribosome-associated, or nucleolar proteins have been discove red. We are presently studying four of these intronic snoRNAs. Three o f them, U20, U21, and U24, are closely related to each other on a stru ctural basis. They are included in genes encoding nucleolin and riboso mal proteins L5 and L7a, respectively, in warm-blooded vertebrates. Th ese three metabolically stable snoRNAs interact with nucleolar protein fibrillarin. In addition, they display common features that make them strikingly related to snoRNA U14. U14 contains two tracts of compleme ntarity to 18S rRNA, which are required for the production of 18S rRNA . U20 displays a 21 nucleotide (nt) long complementarity to 18S rRNA. U21 contains a 13 nt complementarity to an invariant sequence in eukar yotic 28S rRNA. U24 has two separate 12 nt long complementarities to a highly conserved tract of 28S rRNA. Phylogenetic evidences support th e fundamental importance of the pairings of these three snoRNAs to pre -rRNA, which could be involved in a control of pre-rRNA folding during preribosome assembly. By transfection of mouse cells, we have also an alyzed the processing of U20 and found that the -cis acting signals fo r its processing from intronic RNA are restricted to the mature snoRNA sequence. Finally, we have documented changes of host genes for these three intronic snoRNAs during the evolution of eukaryotes.