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 1) is mediated by the 9 bp sequence GACTTGCNC and RNA 3'-ends are formed similar to 15-20 n t upstream of this terminator element. Here I show that this 9 bp elem ent, also called the 'T3 box', is a pause signal for the elongating tr anscription complex. The two major transcripts in the paused complex h ave 3'-ends mapping to 15 and 21 nt upstream of the T3 box, remain bou nd to the template in 0.25% Sarkosyl, are subject to pyrophosphorolysi s and can be chased into longer transcripts. Mutations that reduce ove rall termination also affect pausing, indicating that pausing is a lim iting step in the termination process. Oligonucleotide competition exp eriments, furthermore, suggest that pausing requires a DNA binding fac tor. The data support a model in which the first step leading to trans cription termination by pol 1 In Xenopus is pausing of the elongation complex upstream of the T3 box.