SMALL NUCLEOLAR RNAS ENCODED BY INTRONS OF THE HUMAN CELL-CYCLE REGULATORY GENE RCC1

Eukaryotic cells contain a large number of U small nuclear RNAs (U-snR NAs) involved in various RNA processing reactions in the nucleoplasm a nd nucleolus. Most of the U-snRNAs have 5'-terminal caps added to the end of the primary transcript. Here we describe two variants of a snRN A, called U17, identified in human HeLa cells. U17 RNA may be involved in ribosome biogenesis since it is found in the nucleolus and sedimen ts with 40S structures possibly representing nascent ribosomal subunit s. U17 RNAs contain no cap but have a monophosphate at the 5'-terminus indicating that they are processed from longer precursors. The U17 RN As are encoded within introns 1 and 2 of the single copy gene RCC1 whi ch codes for an important cell cycle regulatory protein. In HeLa cell S-100 extract, U17 RNA is faithfully excised from a longer RNA transcr ipt derived from the intron yielding 5'-monophosphorylated RNA. These data suggest that U17 RNAs are not independently transcribed but are p rocessed out of the RCC1 pre-mRNA or out of the spliced introns.