Molecular and Cellular Biology, July 2000, p. 4522-4531, Vol. 20, No. 13
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Laboratoire de Biologie Moléculaire Eucaryote du CNRS, 31062 Toulouse, France
Received 16 February 2000/Returned for modification 31 March 2000/Accepted 6 April 2000
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 the
cis-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
-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.
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