The Xenopus 9 bp ribosomal terminator (T3 box) is a pause signal for the RNA polymerase I elongation complex.

In Xenopus, termination by RNA polymerase I (pol I) is mediated by the 9 bp sequence GACTTGCNC and RNA 3'-ends are formed -15-20 nt upstream of this terminator element. Here I show that this 9 bp element, also called the 'T3 box', is a pause signal for the elongating transcription complex. The two major transcripts in the paused complex have 3'-ends mapping to 15 and 21 nt upstream of the T3 box, remain bound to the template in 0.25% Sarkosyl, are subject to pyrophosphorolysis and can be chased into longer transcripts. Mutations that reduce overall termination also affect pausing, indicating that pausing is a limiting step in the termination process. Oligonucleotide competition experiments, furthermore, suggest that pausing requires a DNA binding factor. The data support a model in which the first step leading to transcription termination by pol I in Xenopus is pausing of the elongation complex upstream of the T3 box.