Characterization of three sites of RNA 3' end formation in the Xenopus ribosomal gene spacer.

We have studied three sites of 3' end formation on the Xenopus laevis ribosomal genes, designated T1, T2, and T3. Site T1 coincides with the unique HindIII site at the 3' end of the 28S sequence and is shown to be a site of rapid RNA processing. Transcription continues another 235 bp past T1 to site T2, which marks a boundary between relatively stable and highly unstable transcripts. However, T2 does not cause polymerase release. In nuclear run-off experiments transcription is detected across the entire spacer until site T3, 215 bp upstream of the gene promoter. T3 shares some sequence homology with T2 but appears to cause polymerase release and is presently the best candidate for a true terminator of transcription on these genes.