Nucleotide sequence through the 18S-28S intergene region of a vertebrate ribosomal transcription unit

We have determined the nucleotide sequence of part of a cloned ribosomal transcription unit from Xenopus laevis extending from the 3' region of the 18S gene through the 18S-28S intergene region into the start of the 28S gene. The 18S 3' region possess two tracts of high homology with the corresponding segments of other eukaryotic 18S genes (yeast and Bombyx mori) separated by a tract of low homology which in X. laevis is rich in G plus C. The first internal transcribed spacer, between the 18S and 5.8S genes, is 557 nucleotides long, very rich in G plus C (84%) and shows no sequence homology with the corresponding yeast sequence. The 5.8S rRNA sequence is revised slightly in the light of the DNA sequence. The second internal transcribed spacer, between the 5.8S and 28S genes, is 262 nucleotides long and is even richer in G plus C (88%) than the first internal spacer. 28S rRNA starts with the sequence pUCAG. This is encoded at the first of three closely linked TCAG sites in rDNA.