Structural organization of rat ribosomal genes restriction endonuclease analysis of genomic and cloned ribosomal DNAs.

Structural organization of the rat ribosomal repeating unit was studied using hybridization of blotted restriction fragments of total rat DNA with alpha-32P-labeled cDNA probe synthesized on the 18S and 28S rRNAs. A detailed restriction endonuclease map was constructed, the 18S and 28S rRNA genes mapped and the sizes of the rat ribosomal repeating units determined. Considerable site heterogeneity of rat rDNA was revealed in both nontranscribed and external transcribed spacers. Recombinant phages containing the whole set of transcribed regions and a considerable part of a nontranscribed spacer of the rDNA were selected from the rat gene library. The restriction maps of the cloned rDNA fragments are in good agreement with the map constructed by Southern's technique, add to this map and support the existence of site heterogeneity in ribosomal repeating units. Proximal to the 3'-end of the 28S rRNA gene an internally repetitive region was found, each repeating unit being equal to approx. 150 bp. The site for transcription initiation was mapped 4.0-4.5 kb upstream from the 5'-end of the 18S rRNA gene. Frequently reiterated interspersed sequences were found in the nontranscribed spacer at approx. 2-3 kb distance from both ends of the transcribed region.