In vivo evidence for back and forth oscillations of the transcription elongation complex.

We have used a combination of DNA and RNA footprinting experiments to analyze the structural rearrangements experienced by a transcription elongation complex that was halted in vivo by a protein readblock. We show that the complex readblocked within an (ATC/TAG)(n) sequence is in a dynamic equilibrium between upstream- and downstream- translocated conformers. By increasing the strength of the putative RNA-DNA hybrid, the ternary complex is readily trapped in the downstream-translocated conformation, where the melted DNA region is limited to 8 bp. The shift of the equilibrium towards the downstream location is also achieved by introducing within the 5' end of the message an RNA sequence that can pair with a segment of the transcript in the vicinity of the halted ternary complex. Our results demonstrate that within certain template DNA sequences, the back and forth oscillations of the ternary complex actually occur in a multipolymerase system and inside the cell. Furthermore, the cis-acting effect of the upstream RNA sequence underscores an important phenomenon in gene regulation where a transcript may regulate its own elongation.