Determinants of enhancer and promoter activities of regulatory elements

The proper activities of enhancers and gene promoters are essential for coordinated transcription within a cell. Although diverse methodologies have been developed to identify enhancers and promoters, most have tacitly assumed that these elements are distinct. However, studies have unexpectedly shown that regulatory elements may have both enhancer and promoter functions. Here we review these results, focusing on the factors that determine the promoter and/or enhancer activity of regulatory elements. We discuss emerging models that define regulatory elements by accessible DNA and their non-mutually-exclusive abilities to drive transcription initiation (promoter activity) and/or to enhance transcription at other such regions (enhancer activity). Activating transcriptional regulatory elements have traditionally been categorized into promoters, which define transcription start sites, and enhancers, which act distally to stimulate transcription. In this Review, Andersson and Sandelin discuss the latest findings from methodologies for profiling and testing transcriptional regulatory elements at scale. They explain how the data support an updated, nuanced model that accounts for the numerous overlapping molecular properties of promoters and enhancers.