22 Transcriptional Regulation in Eukaryotes

Introduction to Transcriptional Regulation

  • Lecture Date: 09/12/2025

  • Purpose: To discuss transcriptional regulation, specifically in eukaryotes, highlighting differences from prokaryotic systems.

  • Overview:

    • Focus on the connection between chromatin structure and gene expression regulation.

    • Emphasis on the complexity of transcriptional regulation in eukaryotes compared to prokaryotes.

Learning Outcomes

  • Students should familiarize themselves with transcriptional regulation for examinations and quizzes.

  • Key learning outcomes include:

    • Understanding chromatin structure's role in gene expression regulation.

    • Analyzing differences in gene expression levels across different cell types.

    • Identifying regulatory elements and their roles in transcriptional regulation.

Gene Expression Variability

  • Genes are expressed at different levels in various cell types.

  • Example 1: Cell Type Comparison

    • Cell Type 1:

    • Gene A: Highly transcribed; results in many mRNA and protein molecules.

    • Gene B: Low transcription level, resulting in fewer proteins.

    • Gene C: Not transcribed at all.

    • Cell Type 2:

    • Gene A: Downregulated (not transcribed).

    • Gene B: Same expression level as in Cell Type 1.

    • Gene C: Strongly expressed.

Regulatory Elements and Transcription Factors

  • Gene expression depends on regulatory elements; short DNA sequences where transcription factors bind.

  • Cis-acting elements: These elements are part of the gene's promoter.

  • Trans-acting factors: Proteins that interact with these cis-acting elements (e.g., transcription factors).

Types of Genes

  • Housekeeping Genes:

    • Function: Encode proteins necessary for basic cellular functions (e.g., DNA polymerase, RNA polymerase).

    • Characteristics: Continuously expressed and can vary in expression levels.

  • Facultative Genes:

    • Function: Can be induced (turned on) or repressed (turned off).

    • Characteristics: Genes express differently under varying conditions (e.g., Gene C expression varies).

Prokaryotic Vs. Eukaryotic Gene Structure

  • Prokaryotic Genes:

    • Simplicity: Lack introns.

    • Structure: Terminator sequence (stops RNA synthesis) and start site (beginning of mRNA synthesis).

    • Single RNA polymerase oversees transcription of all gene types throughout prokaryotic cells.

  • Eukaryotic Genes:

    • Complexity: Contain introns and exons (coding and non-coding regions).

    • RNA Polymerases: Eukaryotic cells have three distinct RNA polymerases.

    • RNA Polymerase I: Transcribes ribosomal RNA (rRNA).

    • RNA Polymerase II: Transcribes protein-coding genes (mRNAs).

    • RNA Polymerase III: Transcribes tRNA and other small RNA genes.

Eukaryotic Gene Promoter Structure

  • Promoter Composition: Contains TATA box and initiator sequences at the core promoter, influenced by various transcription factors.

  • Regulatory Elements:

    • Proximal Promoter Elements: Near the core promoter, affecting cell-type-specific expression.

    • Enhancers: Increase transcription when specific transcription factors bind.

    • Silencers: Decrease transcription levels.

    • Insulators: Prevent influence of regulatory elements on distant genes.

Experimental Identification of Regulatory Elements

  • Eve Gene in Drosophila melanogaster:

    • Investigation of expression patterns using regulatory segments to control reporter genes (e.g., LUXZ gene).

    • Resulting experiments highlighted regions of expression and regulatory controls.

Transcription Factor Dynamics

  • General Transcription Factors:

    • Required to form the pre-initiation complex with RNA polymerase II.

    • Example: TFIID interacts with TATA-binding protein (TBP).

  • Transcription Factor Assembly:

    • Various transcription factors interact to assist RNA polymerase positioning and initiation.

Mechanisms of Transcriptional Regulation

  • Mediator Complex:

    • Key in mediating interactions between transcription factors and RNA polymerase.

    • Helps in stabilizing or destabilizing transcription initiation.

  • Chromatin Remodeling:

    • Recruitment of proteins by transcription factors to alter chromatin structure, enhancing accessibility for RNA polymerase.

Epigenetic Modifications and Methylation

  • Methylation of DNA:

    • Typically occurs on cytosines in CG dinucleotides, controlled by DNA methyltransferases (DNMT).

    • Impacts gene expression through inhibition or changes in chromatin structure.

    • Methylation stability can be inherited through generations.

Impact of Methylation on Health and Disease

  • Epigenetic changes can influence gene expression relevant to diseases, including cancer.

  • Study of Twins:

    • Identical twins can develop different methylation patterns due to environmental influences.

  • Developmental Origins of Health and Disease (DOHaD):

    • Epigenetic mechanisms significantly impact lifelong health outcomes.

  • Potential Biomarkers:

    • Methylation patterns may serve as indicators for early cancer detection, influencing treatment strategies.

Conclusion

  • Lecture's goal: Understanding transcriptional regulation dynamics, epigenetic influence on gene expression, and implications for health and disease.