Elements modulating the block of transcription elongation at the adenovirus 2 attenuation site

We have previously reported that a block of transcription elongation is functioning in vivo and in vitro within the leader sequences of SV40 and the adenovirus 2 major late transcription units and in the regulation of transcription of the P4 promoter of minute virus of mice. In the present study using the HeLa whole cell extract-Sarkosyl system with adenovirus 2 major late promoter as a template we have analyzed several basic parameters that can contribute to our understanding of the mechanism that regulates the elongation block at the adenovirus 2 attenuation site. We show that the elongation block is augmented at elevated temperatures (40–45 °C). The elongation block can be reversed by the addition of 0.2 M NaCl to the transcription reaction and the reversibility is temperature-dependent. Furthermore, while at 30–35 °C the elongation block is reversible with dilution of the Sarkosyl, at 40–45 °C it is only partially reversible. These results may indicate that a factor(s) is involved in the regulation of the elongation block and/or that the conformation of the transcription complex is temperature dependent. Finally, we show that the extent of the elongation block is dependent on the consecutive T residues at the attenuation site and we discuss the involvement of RNA secondary structure in eliciting the elongation block.