DIFFERENT FORMS OF U15 SNORNA ARE ENCODED IN THE INTRONS OF THE RIBOSOMAL-PROTEIN S1 GENE OF XENOPUS-LAEVIS

Recent cloning and sequencing of one of the two Xenopus gene copies (S 1b) coding for the ribosomal protein S1 has revealed that its introns III, V and VI carry a region of about 150 nt that shares an identity o f 60%. We show here the presence in Xenopus oocytes and cultured cells of a 143 - 147 nt long RNA species encoded by these three repeated se quences on the same strand as the S1 mRNA and by at least one repeat p resent in the S1 a copy of the r-protein gene. We identify these RNAs as forms of the small nucleolar RNA U15 (U15 snoRNA) because of their sequence homology with an already described human U15 RNA encoded in t he first intron of the human r-protein S3 gene, which is homologous to Xenopus S1. Comparison of the various Xenopus and human U15 RNA forms shows a very high conservation in some regions, but considerable dive rgence in others. In particular the most conserved sequences include t wo box C and two box D motifs, typical of most snoRNAs interacting wit h the nucleolar protein fibrillarin. Adjacent to the two D boxes there are two sequences, 9 and 10 nt in length, which are perfectly complem entary to an evolutionary conserved sequence of the 28S rRNA. Modeling the possible secondary structure of Xenopus and human U15 RNAs reveal s that, in spite of the noticeable sequence diversity, a high structur al conservation in some cases may be maintained by compensatory mutati ons. We show also that the different Xenopus U15 RNA forms are express ed at comparable levels, localized in the nucleoli and produced by pro cessing of the intronic sequences, as recently described for other sno RNAs.