Figure 1.

Enhancers versus promoters. Enhancers juxtapose with their specific target promoters via chromatin looping. Enhancers and promoters both recruit transcription factors and coactivators such as CBP (CREB-binding protein), as well as RNA polymerase II. Whereas enhancers initiate transcription of unspliced, nonpolyadenylated eRNAs, promoters initiate transcription of spliced, polyadenylated mRNAs or long, noncoding RNAs. eRNA transcription typically occurs bidirectionally. In addition to transcribing mRNAs, promoters also drive upstream antisense transcription of transcripts called uaRNAs, which exhibit properties similar to eRNAs. The differences between enhancers and promoters may arise entirely or in part due to the presence of the 5′ splice signal at the end of the first exon of mRNAs. For simplicity, nucleosomes are depicted as gray circles and DNA as a black line, but in reality, DNA is looped around nucleosomes.