DISRUPTION OF U8 NUCLEOLAR SNRNA INHIBITS 5.8S AND 28S RIBOSOMAL-RNA PROCESSING IN THE XENOPUS-OOCYTE

The nucleoli of vertebrate cells contain several snRNPs, of which only one, U3, has been assigned a role in rRNA processing. We present the primary sequence of Xenopus U8, a fibrillarin-associated nucleolar snR NA, and examine its expression through oocyte development. Antisense d eoxyoligonucleotides were microinjected into Xenopus oocytes to deplet e the endogenous pool of U8 RNA. Analysis of the mature rRNAs and rRNA intermediates that accumulate in the U8-depleted oocytes indicate tha t the U8 snRNP is essential for correct maturation of the 5.8S and 28S rRNAs at both their 5' and 3' ends. U8 is therefore a nucleolar snRNA implicated in a nucleolytic rRNA processing step other than 18S matur ation. Evidence for a long-lived 5.8S rRNA intermediate (12S) in Xenop us is also presented.