The Arrangement of 18‐S and 28‐S Ribosomal Ribonucleic Acids within the 40‐S Precursor Molecule of Xenopus laevis

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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 of 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.