U17^XS8, 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

The family of box ACA small nucleolar RNAs is defined by an evolutionarily conserved secondary structure and ubiquitous sequence elements essential for RNA accumulation.

“…4C). Apparently, these extended secondary structures are not confined to yeast ACA snoRNAs, as mammalian U19 snoRNA car ries an /H2-type element , and an evolutionarily conserved secondary structure proposed for vertebrate U17 snoRNA (Cecconi et al 1994(Cecconi et al , 1996 fea tures both IHl and IH2 elements.…”

“…Question marks in dicate that the numbering of these RNAs is tentative. Sequences of human (Kiss and Filipowicz 1993;Ruff et al 1993), Xenopus laevis (Cecconi et al 1994), and Fugu lubripes (Cecconi et al 1996) U17; human and mouse U19 ; and human E2 and E3 (Ruff et al 1993;Seraphin 1993) To test this assumption, we conducted a mutational analysis of the box H element of human U64 snoRNA. Human (S-globin pre-mRNAs carrying mutant U64 snoRNAs were expressed in simian COS cells.…”

Exonucleolytic processing of small nucleolar RNAs from pre-mRNA introns.

“…The U3, U8, and U13 RNAs, which are Pol II products, acquire a trimethylguanosine (m3G) cap characteristic of all Pol ll-synthesized U snRNAs, whereas the Pol III-transcribed 7-2/MRP RNA contains a pppN 5' end {for review, see Hernandez 1992}. In contrast to U3, U8, U13, and 7-2/MRP RNAs, most of the other known snoRNAs (U14---U22) are not transcribed from independent genes but are encoded within introns of genes coding for proteins (Liu and Max-well 1990;Leverette et al 1992;Fragapane et al 1993;Kiss and Filipowicz 1993;Prislei et al 1993;Tycowski et al 1993;Cecconi et al 1994;Nicoloso et al 1994;Qu et al 1994;Tycowski et al 1994). Because these snoRNAs are processed from pre-mRNA transcripts, their 5' ends are not capped but, rather, contain a monophosphate group Nag et al 1993;Tycowski et al 1993).…”

SnoRNA U21 is also intron‐encoded in Drosophila melanogaster but in a different host‐gene as compared to warm‐blooded vertebrates

“…Among the few apparent exceptions to this pattern, host-genes for U 14 in vertebrates and for U17 in mammals code for proteins involved in protein and RNA transport into the nucleus [31,32], which could still participate in the control in early stages of ribosome biogenesis. U17, which is encoded in a ribosomal protein gene, rpXS8, in amphibian Xenopus laevis [15] has been the first example of a change of host-gene for an intronic snoRNA. Besides U21, two other cases of intronic snoRNAs positioned within different hostgenes in different organisms have been reported to date [3] for U18 (located in rpL1 gene in vertebrates and in elongation factor EF-1 fl gene in S. cerevisiae) and U24.…”

“…Particularly, a growing family of snoRNAs contain long, phylogenetically conserved sequence complementarities to rRNAs, which must reflect the biological importance of their transient pairing with pre-rRNA in the nucleolus [4]. Moreover, most of the newly identified snoRNAs exhibit a unique biosynthetic pathway [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Not only they are encoded in introns of protein-coding genes, but they result from a novel form of intronic RNA processing of their host-gene transcript.…”