PROCESSING OF THE PRECURSORS TO SMALL NUCLEOLAR RNAS AND RIBOSOMAL-RNAS REQUIRES COMMON COMPONENTS

The genes encoding the small nucleolar RNA (snoRNA) species snR190 and U14 are located close together in the genome of Saccharomyces cerevis iae. Here we report that these two snoRNAs are synthesized by processi ng of a larger common transcript. In strains mutant for; two 5'-->3' e xonucleases, Xrn1p and Rat1p, families of 5'-extended forms of snR190 and U14 accumulate; these have 5' extensions of up to 42 and 55 nucleo tides, respectively. We conclude that the 5' ends of both snR190 and U 14 are generated by exonuclease digestion from upstream processing sit es. In contrast to snR190 and U14, the snoRNAs U18 and U24 are excised from the introns of pre-mRNAs which encode proteins in their exonic s equences. Analysis of RNA extracted from a dbr1-Delta strain, which la cks intron lariat-debranching activity, shows that U24 can be synthesi zed only from the debranched lariat. In contrast, a substantial level of U18 can be synthesized in the absence of debranching activity. The 5' ends of these snoRNAs are also generated by Xrn1p and Rat1p. The sa me exonucleases are responsible for the degradation of several excised fragments of the pre-rRNA spacer regions, ih addition to generating t he 5' end of the 5.8S rRNA. Processing of the pre-rRNA and both intron ic and polycistronic snoRNAs therefore involves common components.