3 SMALL NUCLEOLAR RNAS THAT ARE INVOLVED IN RIBOSOMAL-RNA PRECURSOR PROCESSING

Three small nucleolar RNAs (snoRNAs), E1, E2 and E3, have been describ ed that have unique sequences and interact directly with unique segmen ts of pre-rRNA in vivo. In this report, injection of antisense oligode oxynucleotides into Xenopus laevis oocytes was used to target the spec ific degradation of these snoRNAs. Specific disruptions of pre-rRNA pr ocessing were then observed, which were reversed by injection of the c orresponding in vitro-synthesized snoRNA. Degradation of each of these three snoRNAs produced a unique rRNA maturation phenotype. El RNA dep letion shut down 18 rRNA formation, without overaccumulation of 208 pr e-rRNA. After E2 RNA degradation, production of 18S rRNA and 36S pre-r RNA stopped, and 388 pre-rRNA accumulated, without overaccumulation of 20S pre-rRNA. E3 RNA depletion induced the accumulation of 368 pre-rR NA. This suggests that each of these snoRNAs plays a different role in pre rRNA processing and indicates that El and E2 RNAs are essential f or 18S rRNA formation. The available data support the proposal that th ese snoRNAs are at least involved in pre-rRNA processing at the follow ing pre-rRNA cleavage sites: El at the 5' end and E2 at the 3' end of 18S rRNA, and E3 at or near the 5' end of 5.8S rRNA.