U86, a novel snoRNA with an unprecedented gene organization in yeast

The Xenopus laevis Nop56 gene (XNOP56), coding for a snoRNP-specific factor, belongs to the 5'-TOP gene family. XNOP56, as many 5'-TOP genes, contains an intron-encoded snoRNA. This previously unidentified RNA, named U86, was found as a highly conserved species in yeast and human. While in human it is also encoded in an intron of the hNop56 gene, in yeast it has an unprecedented gene organization: it is encoded inside an open-reading frame. Both in X. laevis and yeast, the synthesis of U86 snoRNA appears to be alternative to that of the cotranscribed mRNA. Despite the overall homology, the three U86 snoRNAs do not show strong conservation of the sequence upstream from the box D and none of them displays significant sequence complementarity to rRNA or snRNA sequences, suggesting a role different from that of methylation.