Transcriptional Regulation

The first and most economical point at which a cell controls a gene is the decision to transcribe it, made by regulatory proteins that read signals and bind specific DNA sequences to switch transcription on or off.

Definition

Transcriptional regulation is the control of whether and how often a gene is transcribed into RNA, achieved largely by regulatory proteins that bind specific DNA sequences and modulate the activity of RNA polymerase.

Scope

This topic covers the bacterial operon as the classic model of inducible and repressible transcription, the roles of repressors, activators, and inducers, the promoters and enhancers that recruit the transcription machinery in eukaryotes, sequence-specific transcription factors and their domains, and combinatorial control. It treats regulation at the level of transcription initiation; regulation by RNA after transcription and by chromatin state are covered in the adjacent topics.

Core questions

• How does the lac operon turn gene expression on and off in response to nutrients?
• What is the difference between an activator and a repressor?
• How do promoters and enhancers recruit and position the transcription machinery in eukaryotes?
• How does combinatorial binding of transcription factors generate precise patterns of expression?

Key concepts

• The operon: inducible and repressible control
• Repressors, activators, and inducers
• Promoters and enhancers
• Sequence-specific transcription factors
• Combinatorial control of expression

Mechanisms

Regulatory proteins recognize specific DNA sequences and either block or promote the binding and progression of RNA polymerase; in bacteria a single repressor can control an operon of co-transcribed genes in response to a small-molecule signal, while in eukaryotes many transcription factors act combinatorially through distant enhancers that loop to the promoter to set the rate of transcription.

Clinical relevance

Transcriptional control determines cell identity, and its disruption by mutated transcription factors or hijacked enhancers underlies many cancers and developmental disorders, while engineered transcriptional switches are tools in research and emerging therapies.

History

Jacob and Monod's 1961 study of the lac operon in Escherichia coli introduced the idea that genes are switched by regulatory proteins responding to the environment; Ptashne's later dissection of phage repressors detailed how such proteins work, and the study of eukaryotic enhancers extended the principle to complex developmental control.

Key figures

• François Jacob
• Jacques Monod
• Mark Ptashne

Seminal works

• jacobMonod1961

Frequently asked questions

What is an operon?

An operon is a cluster of genes in bacteria transcribed together from a single promoter and controlled as a unit, allowing the cell to switch a whole set of related genes on or off at once in response to a signal.

How do enhancers act on genes from a distance?

Enhancers are DNA sequences that bind transcription factors and influence a gene's promoter even when far away in sequence; the DNA loops so that the enhancer-bound proteins contact the transcription machinery at the promoter.