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Sequence organization of the spacer in the ribosomal genes of Xenopus clivii and Xenopus borealis.

We have studied in X. clivii and X. borealis cloned EcoRI fragments containing the spacer located between the 28S and 18S ribosomal genes. We report for these two species the nucleotide sequences at both ends of the NTS region with special emphasis on the sequences around the transcription initiation site of the 40S rRNA precursor. In X. clivii the location of the 5' end of the precursor was mapped. In both species the sequences around the 40S origin are duplicated in the NTS. Nucleotide sequence comparison has revealed a stretch of 13 identical bases around the transcription initiation site of X. laevis and X. clivii. The same sequence is also present at the presumptive transcription initiation site of X. borealis rDNA.