Promoters and Cis-Acting Elements

Promoters and other cis-acting elements are DNA sequences, located on the same molecule as the gene they control, that determine where transcription starts and how it is regulated. They provide the binding platform for RNA polymerase and regulatory proteins, and their composition sets the strength, timing and tissue specificity of a gene's expression.

Definition

A promoter is the cis-acting DNA region that directs the initiation of transcription of a gene; cis-acting elements more broadly are DNA sequences on the same molecule whose presence and arrangement regulate transcription by serving as binding sites for the transcription machinery and regulatory proteins.

Scope

The topic covers the core promoter that positions the start site (including elements such as the TATA box and initiator), proximal and distal regulatory sequences (enhancers, silencers, insulators), and how chromatin features mark active promoters. It distinguishes cis-acting (same-molecule) sequences from the trans-acting proteins that read them, and is treated at a reference-educational level.

Core questions

• Which DNA sequences specify where and how strongly transcription begins?
• How do distal enhancers influence a promoter over long genomic distances?
• What distinguishes a cis-acting element from a trans-acting factor?

Key concepts

• Core promoter and transcription start site
• TATA box and initiator element
• Enhancers and silencers
• Insulators
• CpG-island promoters
• Cis versus trans regulation
• Enhancer RNAs

Mechanisms

The core promoter contains short sequence motifs, such as the TATA box and initiator, that position the basal transcription machinery and define the start site, while the precise mix of elements tunes promoter strength and responsiveness. Distal enhancers act largely independently of orientation and distance by looping into contact with their target promoters, and insulators delimit regulatory domains. In human cells most active promoters carry characteristic chromatin marks and many are embedded in CpG islands, and some enhancers are themselves transcribed into noncoding RNAs that accompany their regulatory activity.

Clinical relevance

Mutations and variants in promoters and enhancers can change gene dosage and contribute to inherited and acquired disease, and many disease-associated genetic variants fall in such regulatory regions rather than in coding sequence. This entry describes those sequences at a reference level and is not a basis for clinical interpretation of an individual's variants.

History

Classic promoter and enhancer studies in the 1980s defined the short core elements and the long-range action of enhancers. The Smale and Kadonaga 2003 review consolidated the RNA polymerase II core promoter, and genome-scale chromatin and transcription mapping later showed how promoters are marked and how enhancers, including transcribed ones, organise gene expression.

Key figures

• Stephen T. Smale
• James T. Kadonaga
• Joanna Wysocka
• Richard A. Young

Seminal works

• smale-kadonaga-2003
• long-2016

Frequently asked questions

What is the difference between a promoter and an enhancer?

A promoter is the sequence at the gene's start that positions the transcription machinery, while an enhancer is a regulatory sequence that can boost transcription from a distance and in either orientation by contacting the promoter.

What does cis-acting mean?

A cis-acting element is a DNA sequence that regulates a gene on the same DNA molecule, in contrast to trans-acting factors, which are diffusible proteins (or RNAs) that act on target sequences regardless of location.