SnoRNA U21 is also intron‐encoded in Drosophila melanogaster but in a different host‐gene as compared to warm‐blooded vertebrates

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Abstract
U21 is an intron‐encoded snoRNA in vertebrates which contains a 13‐nt tract of complementarity to an invariant sequence in eukaryotic 28S rRNA. Here, we report the characterization of its Drosophila melanogaster homolog which is the first case of an intron‐encoded snoRNA in an invertebrate metazoan. In D. melanogaster, U21 is encoded within the ARF‐1 (ADP ribosylation factor 1) gene, whereas in chicken and mammals it is found in the ribosomal protein L5 gene. In D. melanogaster, like in vertebrates, U21 is devoid of a 5′ trimethylguanosine cap, thus, likely resulting from processing of intronic RNA. The only portion of U21 sequence preserved between D. melanogaster and vertebrates, in addition to the hallmark box C and box D motifs, corresponds precisely to the 13‐nt complementary to rRNA, pointing to an important role of the pairing of U21 to pre‐rRNA.