Small nucleolar RNAs encoded by introns of the human cell cycle regulatory gene RCC1.

Abstract

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

U17XS8, a small nucleolar RNA with a 12 nt complementarity to 18S rRNA and coded by a sequence repeated in the six introns ofXenopus laevisribosomal protein S8 gene

“…It is well known that a whole genome duplication occurred in Xenopus laevis about 30 million years ago (21), resulting in duplication followed by some limited divergence of all genes. That also r-protein genes are present in two copies, and in particular the gene for r-protein S8, has been previously demonstrated (22,23 U17a RNA (9,10). Comparison of these, strongly indicates a structure common to the seven of them ( Figure 4B shows the structures for U17Xs8 f and human U17a).…”

“…These examples are numerous enough to permit the conclusion that this gene organization is not coincidental but must have an important functional relevance. More intriguing is the case of U17 RNA, very recently described in HeLa cells (9,10). In fact this RNA, which is 69.3% homologous to Xenopus U17XS8 RNA, is encoded only in two introns present in the non coding region of the human gene for RCC 1, a protein involved in cell cycle regulation (9), while Ul7Xs8 RNA is encoded only in the introns of r-protein S8 genes.…”

“…We show here that the sequences repeated in the introns of S8a gene, on the contrary, code for a small nucleolar RNA (snoRNA), which we named Ul7Xs8 RNA, presumably involved in ribosome production. Similar genome organizations, with snoRNAs encoded in the introns of other genes, are being found with increasing frequency: U14 RNA (6), U15 RNA (7), U16 RNA (8), U17 RNA (9) and U20 RNA (Caizergues-Ferrer, personal communication). We have adopted the name 'U 17' for the Xenopus S8a intron encoded snoRNA after consideration of the strong homology with the U17 RNA described in HeLa cells (9), also called El RNA by Ruff et al (10); however, because of some relevant differences in structure and properties, we identify the Xenopus RNA with a specifying superscript.…”

“…Similar genome organizations, with snoRNAs encoded in the introns of other genes, are being found with increasing frequency: U14 RNA (6), U15 RNA (7), U16 RNA (8), U17 RNA (9) and U20 RNA (Caizergues-Ferrer, personal communication). We have adopted the name 'U 17' for the Xenopus S8a intron encoded snoRNA after consideration of the strong homology with the U17 RNA described in HeLa cells (9), also called El RNA by Ruff et al (10); however, because of some relevant differences in structure and properties, we identify the Xenopus RNA with a specifying superscript. 1 Itg of linear plasmid templates as described (15) using about 50 ,uCi of [(x-32p] CTP in a total CTP concentration of 10 mM, and gel purified.…”

“…The name U 17 has been adopted in consideration of the sequence homology between our RNA and human U17 RNA (9), also called El RNA (10); however, because of the some differences in their structure and gene organization (see below), we added the superscript 'XS8' to identify the one encoded in the Xenopus S8a gene. The actual existence in the cell of this RNA species, at first observed only by Northern blot hybridization, has been proved by RNase mapping experiments.…”