TRANSCRIPTIONAL PAUSING, ARREST, AND READTHROUGH AT THE ADENOVIRUS MAJOR LATE ATTENUATION SITE

RNA polymerase II (pol II) transcription complexes initiated from the adenovirus major late promoter can become blocked both in vitro and in vivo at a specific site within the first intron of the transcription unit. In vitro, polymerases that fail to read through the major late a ttenuation site remain stably bound to the template in a ternary compl ex that is indefinitely blocked from continuing elongation, a phenomen on referred to as ''arrest.'' Elongation factor SII has been shown bot h to promote readthrough of this and other arrest sites and to stimula te a previously unknown 3' to 5, exonuclease activity of pol II. We ha ve proposed that the two activities are related and that SII promotes readthrough by means of the enhancement of the exonuclease activity. I n the experiments reported here, we have tested several features of th at model. In particular, we have examined the hypothesis that SII stim ulates readthrough by allowing the polymerase to undergo multiple cycl es of removal and resynthesis of RNA bases preceding the attenuation s ite. In addition, we present experimental support for the proposal tha t the length of time polymerase pauses at the attenuation site is impo rtant to the efficiency of arrest. The results of these experiments ar e discussed in the context of the model.