13. Regulation of Gene Expression in Eukaryotes

13. Regulation of Gene Expression in Eukaryotes

Introduction

Gene expression in eukaryotes is tightly regulated at multiple levels to ensure proper cellular function, development, and response to environmental signals. This regulation involves complex interactions between DNA sequences, regulatory proteins, and chromatin structure.

Cis-Acting Regulatory Sequences and Trans-Acting Regulatory Proteins

DNA-Protein Interactions in Eukaryotes

• Activator proteins bind regulatory DNA sequences to stimulate transcription.

• Repressor proteins bind other sequences to hinder transcription.

• Regulatory proteins in eukaryotes often form large complexes, with individual transcription factors regulating many target genes.

Overview of Gene Regulation Mechanisms

• Gene regulation occurs at several stages: transcriptional, post-transcriptional, translational, and post-translational.

• Transcriptional regulation is the primary control point, involving DNA-protein interactions at specific regulatory sequences.

Regulatory DNA Sequences

• Core promoter region: Contains the TATA box and is adjacent to the transcription start site; binds RNA polymerase II and general transcription factors (GTFs).

• Proximal elements: Located upstream of the core promoter; also regulate gene expression.

• Enhancer and silencer sequences: Can be located far upstream, downstream, or even within the gene; bind regulatory proteins to modulate transcription.

Cis- and Trans-Acting Elements

• Cis-acting regulatory sequences: Regulate transcription of genes on the same chromosome.

• Trans-acting regulatory proteins: Can bind to target sequences on any chromosome.

• At enhancers, multiple proteins aggregate to form enhanceosomes.

Enhanceosome Action

• Enhanceosomes induce DNA bending, forming loops that bring regulatory proteins into contact with the core promoter and transcription factors.

• The size of the loop depends on the distance between the enhancer and the gene it regulates.

Integration and Modularity of Regulatory Sequences

• Enhancers and silencers contain multiple binding sites (modules) for different transcription factors.

• This modularity allows integration of various signals to produce specific regulatory outcomes.

• Pioneer factors are the first to bind regulatory DNA, facilitating the binding of additional transcription factors.

Transcriptional Regulation by Enhancers and Silencers

• The same DNA sequence can act as an enhancer or silencer depending on the regulatory proteins present.

• Models of transcriptional regulation must account for the variable locations and tissue-specific activity of enhancers and silencers.

Example: Sonic hedgehog (SHH) Gene Regulation

• The SHH gene is regulated by enhancers located up to 1 million base pairs away.

• Tissue-specific expression is achieved by different combinations of regulatory proteins binding to distinct enhancers in brain and limb tissues.

Locus Control Regions (LCRs)

Definition and Function

• The human β-globin gene is part of a complex regulated by a locus control region (LCR).

• LCRs are specialized enhancers that regulate transcription of multiple related genes within a gene complex.

Structure of the β-Globin LCR

• The LCR contains four distinct sequences (HS1–HS4) that bind regulatory proteins and direct DNA looping.

• These loops bridge the LCR to the promoters of β-globin complex genes, ensuring correct developmental expression.

• Each gene in the complex encodes a polypeptide with unique oxygen-carrying properties.

Developmental Expression Table

Developmental Stage	Globin Genes Expressed
Embryonic	ε, Gγ, Aγ
Fetal	Gγ, Aγ
Adult	β

Mutations in Regulatory Sequences

• Mutations in α- or β-globin genes cause thalassemia, a hereditary anemia.

• Some cases are due to deletions or rearrangements affecting the LCR, not the globin genes themselves.

Enhancer-Sequence Conservation

• Some enhancer sequences are highly conserved across species, indicating evolutionary importance.

• Example: The β-interferon gene enhancer is conserved among mammals and regulates body plan development.

Yeast as a Model for Eukaryotic Transcription

Galactose Utilization Pathway in Saccharomyces cerevisiae

• Transcription of GAL1, GAL2, GAL7, and GAL10 is regulated by enhancer sequences called upstream activator sequences (UASG).

• The activator protein Gal4 binds UASG to stimulate transcription.

• In the absence of galactose, Gal80 binds Gal4, preventing activation.

• In the presence of galactose, Gal3 binds Gal80, releasing Gal4 to activate transcription.

Regulation Table

Condition	Gal4 Status	Transcription
No galactose	Bound by Gal80 (inactive)	No transcription
Galactose present	Released by Gal80 (active)	Transcription occurs

Repressor Proteins and Silencer Sequences

• Repressors can inhibit transcription by binding silencer sequences.

• Example: In the presence of glucose, Mig1 binds a silencer between UASG and the GAL1 promoter, recruiting Tup1 to form a repressor complex.

Key Terms

• Enhancer: DNA sequence that increases transcription when bound by activators.

• Silencer: DNA sequence that represses transcription when bound by repressors.

• Enhanceosome: Large protein complex formed at enhancers to facilitate transcription.

• Pioneer factor: The first transcription factor to bind a regulatory region, enabling further binding events.

• Locus control region (LCR): Specialized enhancer that regulates a gene cluster.

Example Application

• The regulation of the SHH gene and the β-globin gene complex illustrates how distant regulatory elements and complex protein-DNA interactions control tissue-specific and developmental gene expression in eukaryotes.