Eukaryotic Transcriptional Control

How eukaryotic cells set the transcription of each gene through combinations of regulatory factors, enhancers, coactivators, and signalling — the basis of differential gene expression.

Definition

Eukaryotic transcriptional control is the regulation of how much a gene is transcribed by RNA polymerase, achieved through combinations of sequence-specific transcription factors and coregulators that act on promoters and enhancers and connect to the basal transcription apparatus.

Scope

This topic covers the regulation of transcription in eukaryotes: sequence-specific activators and repressors, enhancers and silencers acting at a distance, coactivator and corepressor complexes, the Mediator bridge to the basal machinery, and the integration of signalling pathways at promoters. It emphasises the combinatorial logic that produces cell-type-specific expression; chromatin-level control is treated in a companion topic.

Core questions

• How do distant enhancers communicate with a gene's promoter?
• How do combinations of factors specify cell-type-specific expression?
• What role do coactivators, corepressors, and Mediator play?
• How do signalling pathways change transcription in response to the environment?

Key theories

Combinatorial control

A gene's transcription is determined by the specific set of activators and repressors present, so a modest number of factors, used in different combinations, can specify the many distinct expression programmes of a multicellular organism.

Enhancer–promoter communication

Enhancers bound by activators act over long distances by looping the DNA and recruiting coactivators and the Mediator complex to the promoter, raising transcription without being adjacent to the start site.

Mechanisms

Sequence-specific transcription factors bind enhancers and promoter-proximal elements through DNA-binding domains and recruit coactivators or corepressors via separate effector domains. Coactivators include chromatin-modifying complexes and the Mediator complex, which bridges enhancer-bound factors to RNA polymerase and the general transcription factors at the core promoter, often through DNA looping. Signalling pathways modify transcription factors or trigger their nuclear entry, allowing extracellular cues to be converted into changes in transcription, while repressors and silencers counterbalance activation.

Clinical relevance

Aberrant transcription-factor activity and enhancer mutations drive many cancers and developmental disorders, and transcriptional coregulators are active drug targets; provided as significance, not clinical guidance.

History

The cloning of eukaryotic transcription factors and the discovery of enhancers in the late twentieth century established combinatorial control, and identification of coactivators and the Mediator complex showed how distant regulators reach the polymerase, shaping the modern view of eukaryotic transcription.

Key figures

• Robert Tjian
• Roger Kornberg
• Mark Ptashne

Seminal works

• alberts2014
• lodish2016

Frequently asked questions

How can an enhancer control a gene from far away?

The DNA loops so that factors bound at the enhancer come into contact with the promoter and its machinery, transmitting the regulatory signal across the intervening distance.

Why is eukaryotic regulation called combinatorial?

Because the output of a gene depends on the particular combination of activators and repressors present, letting a limited set of factors generate many distinct expression patterns.