Nucleotide sequence of the region between the 18S rRNA sequence and the 28S rRNA sequence of rat ribosomal DNA

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Abstract
The DNA sequence of the intragenic region of the rat 45S ribosomal RNA precursor was determined. This sequence contains 2282 nucleotides and extends from the conserved EcoE I site near the 3′ terminus of 18S rRNA to 69 nucleotides downstream of the 5′ terminus of 28S rRNA. The sequences corresponding to 18S and 5.8S rRNA were identified by comparison with previously published data. The 5′ terminus of rat 28S rRNA was identified by SI nuclease protection and reverse transcriptase elongation assays. The internal transcribed spacers were found to be 1066 and 765 nucleotides long and had little homology with those of Xenopus and yeast. Regions of sequence homology between rat and Xenopus were found at the junctions of the internal transcribed spacers with 18S, 5.8S and 28S rRNA. These homologies suggest that these sequences may function as recognition sites for the processing of the ribosomal precursor RNA.