A 12-base-pair sequence is an essential element of the ribosomal gene terminator in Xenopus laevis.

rRNA transcription in Xenopus laevis terminates near a 7-base-pair (bp) conserved sequence (T3 box) located 200 bp upstream of the site of transcription initiation for the adjacent gene promoter. We present evidence here that a 12-bp element containing the T3 box is an essential part of the terminator. Using an oocyte injection assay, we found that the 12-bp element (but not the T3 box alone) severely reduced the amount of RNA detectable at sites downstream from itself and that the T3 box within the 12-bp element was required to specify the formation of correct 3' ends. This requirement for the 12-bp element was also seen in pulse-label experiments by using a homogenate of oocyte nuclei, but the present data did not allow us to determine the exact mechanism by which the 12-bp element acts. Removal of the T3 region from its normal location allowed a significant amount of readthrough transcripts to accumulate, indicating that additional sequences may be required for complete terminator function.