SNORNA U21 IS ALSO INTRON-ENCODED IN DROSOPHILA-MELANOGASTER BUT IN ADIFFERENT HOST-GENE AS COMPARED TO WARM-BLOODED VERTEBRATES

U21 is an intron-encoded snoRNA in vertebrates which contains a 13-nt tract of complementarity to an invariant sequence in eukaryotic 28S rR NA, Here, we report the characterization of its Drosophila melanogaste r homolog which is the first case of an intron-encoded snoRNA in an in vertebrate 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 LS gene, In D. melanogaster, like i n vertebrates, U21 is devoid of a 5' trimethylguanosine cap, thus, lik ely resulting from processing of intronic RNA, The only portion of U21 sequence preserved between D. melanogaster and vertebrates, in additi on to the hallmark box C and box D motifs, corresponds precisely to th e 13-nt complementary to rRNA, pointing to an important role of the pa iring of U21 to pre-rRNA.