Nucleotide sequence of the transcription initiation region for rRNA in the sea urchin Lytechinus variegatus.

The 1191-bp sequence which includes the 5' end of 18S X rDNA and its adjacent spacer have been determined for a cloned fragment of sea urchin rDNA. The 5' end of 33S precursor rRNA (pre-rRNA) has been located by S1 nuclease mapping and primer extension. Pre-rRNA appears to be initiated at an A, 658 bp upstream from the 5' terminus of 18S rRNA. The first nucleotide of 18S rRNA was also analyzed by S1 nuclease mapping and found to correspond to T in the nontranscribed DNA strand. Comparisons of the transcription initiation region in rDNA and the 5' end of 18S X rDNA with the corresponding regions in other eukaryotes reveal no significant nucleotide sequence homology in the precursor portion while the 5' end of 18S X rDNA is highly conserved.