Xenopus U3 snoRNA GAC-Box A' and Box A sequences play distinct functional roles in rRNA processing.
Mutations in the 5' portion of Xenopus U3 snoRNA were tested for function in oocytes. The results revealed a new cleavage site (A0) in the 3' region of vertebrate external transcribed spacer sequences. In addition, U3 mutagenesis uncoupled cleavage at sites 1 and 2, flanking the 5' and 3' ends of 18S rRNA, and generated novel intermediates: 19S and 18.5S pre-rRNAs. Furthermore, specific nucleotides in Xenopus U3 snoRNA that are required for cleavages in pre-rRNA were identified: box A is essential for site A0 cleavage, the GAC-box A' region is necessary for site 1 cleavage, and the 3' end of box A' and flanking nucleotides are required for site 2 cleavage. Differences between metazoan and yeast U3 snoRNA-mediated rRNA processing are enumerated. The data support a model where metazoan U3 snoRNA acts as a bridge to draw together the 5' and 3' ends of the 18S rRNA coding region within pre-rRNA to coordinate their cleavage.
Borovjagin
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Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912, USA.
Gerbi
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SA
D000818:Animals; D001483:Base Sequence; D005786:Gene Expression Regulation; D008969:Molecular Sequence Data; D012323:RNA Processing, Post-Transcriptional; D012335:RNA, Ribosomal / Q000235:genetics* / Q000378:metabolism; D020537:RNA, Small Nucleolar / Q000235:genetics* / Q000378:metabolism; D017423:Sequence Analysis, RNA; D014982:Xenopus laevis / Q000235:genetics* / Q000378:metabolism
D012335:RNA, Ribosomal; D020537:RNA, Small Nucleolar; C117378:RNA, U3 small nucleolar