Processing of Intron-Encoded Box C/D Small Nucleolar RNAs Lacking a 5′,3′-Terminal Stem Structure

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Abstract
The C and D box-containing (box C/D) small nucleolar RNAs (snoRNAs) function in the nucleolytic processing and 2′-O-methylation of precursor rRNA. In vertebrates, most box C/D snoRNAs are processed from debranched pre-mRNA introns by exonucleolytic activities. Elements directing accurate snoRNA excision are located within the snoRNA itself; they comprise the conserved C and D boxes and an adjoining 5′,3′-terminal stem. Although the terminal stem has been demonstrated to be essential for snoRNA accumulation, many snoRNAs lack a terminal helix. To identify thecis-acting elements supporting the accumulation of intron-encoded box C/D snoRNAs devoid of a terminal stem, we have investigated the in vivo processing of the human U46 snoRNA and an artificial snoRNA from the human β-globin pre-mRNA. We demonstrate that internal and/or external stem structures located within the snoRNA or in the intronic flanking sequences support the accumulation of mammalian box C/D snoRNAs lacking a canonical terminal stem. In the intronic precursor RNA, transiently formed external and/or stable internal base-pairing interactions fold the C and D boxes together and therefore facilitate the binding of snoRNP proteins. Since the external intronic stems are degraded during snoRNA processing, we propose that the C and D boxes alone can provide metabolic stability for the mature snoRNA.