U17(XS8), A SMALL NUCLEOLAR RNA WITH A 12 NT COMPLEMENTARITY TO 18S RIBOSOMAL-RNA AND CODED BY A SEQUENCE REPEATED IN THE 6 INTRONS OF XENOPUS-LAEVIS RIBOSOMAL PROTEIN-S8 GENE

U17XS8 RNA is a 220 nt small RNA coded by a sequence repeated in each of the six introns of the gene for ribosomal protein S8 of Xenopus lae vis. It is mainly localized in the nucleolus, as shown by in situ hybr idization, and it is assembled in a ribonucleo-protein particle (RNP) sedimenting at about 12S, slightly faster than U3 RNP, and with a dens ity of 1.45 g/ml. DNA and RNA microinjections in Xenopus oocytes have shown that U17XS8 RNA is not the product of an independent transcripti on unit, but is produced by processing ot intron sequences of r-protei n S8 transcript, as has been recently shown for other small nucleolar RNAs encoded in the introns of other genes. Its accumulation during Xe nopus development, oogenesis and embryogenesis, increases in parallel to that of r-protein S8 mRNA. Another interesting feature is the prese nce in the U17XS8 RNA of a 12 nt sequence complementary to 18S rRNA. T he results presented suggest a possible role of this RNA in some step( s) of ribosome assembling in the nucleolus. Some relevant differences between Xenopus U17X88 RNA and the corresponding human U17 RNA, recent ly described, have been observed.