RNA polymerase molecules initiating transcription at tandem promoters can collide and cause premature transcription termination.

Using purified E. coli RNA polymerase we have studied the transcription in vitro of a series of DNA fragments carrying two tandemly arranged promoters, where the corresponding transcription start points were separated by 263, 138, 83 and 78 base pairs. In the case where the transcription start points are 83 base pairs apart, there is an interaction between RNA polymerase molecules transcribing from the two promoters. This interaction results in premature termination of the upstream transcript at a precise site. We propose that this is the result of RNA polymerase transcribing from the upstream promoter bumping into polymerase at the downstream promoter. The interaction between the two polymerase molecules is crucially dependent on the distance between the two promoters.