title: The arrangement of 18-S and 28-S ribosomal ribonucleic acids within the 40-S precursor molecule of Xenopus laevis.
abstract: The arrangement of 18-S rRNA and 28-S rRNA within their 40-S common precursor molecule (pre-rRNA) of Xenopus laevis was investigated by electron microscopic analysis of secondary structure of nascent pre-rRNA chains of oocytes, and by 5'-end analysis of 18-S rRNA and 28-S rRNA hybridized to the EcoRI fragment of rDNA cloned as plasmid pCD42. Secondary structure mapping of phenol-extracted RNA from nucleolar cores revealed complete pre-rRNA chains or molecules at various stages of processing and pre-rRNA molecules apparently lacking one end. In this latter group, which was regarded as representing nascent chains, more than 90% of the molecules had no 28-S rRNA REGION. This shows that the 28-S rRNA sequence is transcribed after the 18-S rRNA region and hence must be located nearer to the 3' end of the pre-rRNA molecule. For 5' end-group determination [3H]uridine-labelled 18-S rRNA and 28-S rRNA were hybridized, as fragments of about 200 nucleotides, to the plasmid pCD42 containing coding sequences for four-fifths of the 18-S rRNA sequence, the external transcribed spacer, the non-transcribed spacer and a tenth of the 28-S rRNA sequence. The RNA was recovered from the hybrids and analyzed for uridine 3',5'-bisphosphate (pUp) after alkaline hydrolysis. The pUp content of the hybridized 18-S rRNA fragments was 20-fold higher than in those of 28-S rRNA, THUS DEMONSTRATING THAT THE 5' END OF THE 18-S rRNA is located next to the external spacer region. From these results it is concluded that the 18-S rRNA is located close to the 5' end of the 40-S pre-rRNA molecule.
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism* / Q000648:ultrastructure; D005260:Female; D008854:Microscopy, Electron; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009693:Nucleic Acid Hybridization; D009865:Oocytes / Q000378:metabolism* / Q000648:ultrastructure; D010063:Ovum / Q000378:metabolism*; D010957:Plasmids; D012335:RNA, Ribosomal / Q000096:biosynthesis; D014981:Xenopus
chemical_list: D012335:RNA, Ribosomal