Homo sapiens U17a/E1 snoRNA.
This H/ACA box snoRNA was cloned from human cells by Ruff et al. (1993) and Kiss and Filipowicz (1993). Two isoforms of human U17 are encoded within the first (U17a) and second (U17b) introns of the non-coding U17HG gene (Pelczar and Filipowicz, 1998). Contrary to other H/ACA box snoRNAs, the 3'-terminal stem-loop (nts 135-207) of U17 is sufficient for assembly with the four core proteins of H/ACA snoRNPs, hGAR1, dyskerin, hNHP2 and hNOP10 (Dragon, Pogacic and Filipowicz, 2000). U17 is an evolutionarily conserved snoRNA that is also present in fission (S. pombe) and budding (S. cerevisiae) yeasts and the unicellular protozoan T. thermophila (Atzorn et al. 2004). Upon depletion of E1 RNA by anti-sens oligonucleotides in frog oocytes, 20S pre-rRNA was made, but 18S rRNA did not accumulate, suggesting that E1 snoRNA is involved in rRNA processing at the 5' end of 18S RNA (site 1) (Mishra and Eliceiri, 1997). Similarly, the yeast S. cerevisiae equivalent of U17, called snR30, is required for the nucleolytic processing of 18S rRNA. Consistent with this, psoralen cross-linking experiments found mammalian U17 to interact with primary rRNA transcripts (Rimoldi et al. 1993). Most likely, U17 functions as an RNA chaperone that safeguards the correct folding of 18S rRNA during pre-rRNA processing.