A transcriptional terminator between enhancer and promoter does not affect remote transcriptional control

Enhancers stimulate transcription of RNA polymerase II-transcribed genes in an orientation-independent manner and over long distances. This stimulation is known to be associated with an increased polymerase density over the linked gene. However, many aspects of the exact mechanism of remote gene control remain to be elucidated. Based on some reports on RNA polymerase I transcription, we wanted to test whether RNA polymerase II enters at the enhancer and from there proceeds towards the promoter while synthesizing unstable transcripts ("scanning/readthrough transcription" model). For this, we have inserted a complete terminator region from the mouse beta-globinmaj gene between the SV40 enhancer and the rabbit beta-globin promoter. In contrast to what the model predicts, insertion of the terminator had no affect on remote enhancer action. Furthermore, we have determined the RNA polymerase density over the spacer DNA between enhancer and promoter, and over the reporter gene, by means of the so-called run-on transcription assay. We find very low transcription of the spacer, but high transcription of the globin reporter gene. Thus, our data are not consistent with a scanning/readthrough transcription mechanism where RNA polymerase II would move from the enhancer to the promoter while transcribing the intervening spacer DNA. These and other findings are compatible with a model where enhancer and promoter are brought into close proximity, perhaps with concomitant looping out of the intervening DNA.