THE XENOPUS INTRON-ENCODED U17 SNORNA IS PRODUCED BY EXONUCLEOLYTIC PROCESSING OF ITS PRECURSOR IN OOCYTES

U17 is a small nucleolar RNA encoded in the introns of the Xenopus laevis gene for ribosomal protein S7 (formerly S8, see Note). To study the mechanisms involved in its in vivo processing from S7 transcripts, various in vitro synthesized RNAs embedding a U17 sequence have been microinjected into the germinal vesicle of Xenopus oocytes and their processing analysed. In particular, the Xenopus U17 gene copies a and f and a U17 gene copy from the pufferfish Fugu rubripes have been used, Information about the nature of the processing activities involved in U17 RNA maturation have been sought by injecting transcripts protected from exonucleolytic attack at their 5'-end by capping and/or lengthened at their 3'-end by polyadenylation. The results obtained indicate that U17 RNA processing is a splicing-independent event and that it is mostly or entirely due to exonucleolytic degradation at both the 5'- and 3'-ends of the precursor molecules. Moreover, it is concluded that the enzymes involved are of the processive type. It is suggested that the apparatus for U17 RNA processing is that responsible for the degradation of all excised and debranched introns. Protection from exonucleolytic attack due to the tight structure and/or to the binding of specific proteins, would be the mechanism by which U17 RNA is produced.