Transcriptional Control of Eukaryotic Gene Expression

Chapter 8: Transcriptional Control of Eukaryotic Gene Expression

Differentiating Between Simple and Complex Organisms

• Single-cell yeast vs. multicellular organisms
  • Yeast, similar to bacteria, possesses genes that react to environmental conditions.
  • Environmental factors include sugars (nutrition), temperature, and oxygen tension.
  • In multicellular organisms, gene expression is less influenced by immediate environmental conditions.
  • Developmental programs play a significant role in regulating gene expression in multicellular organisms.

Gene Expression Regulation

• Transcription Rates in Eukaryotes
  • In higher eukaryotes, most genes are managed by regulating transcription rates.
  • The rate of transcription initiation (measured in starts per minute) is the critical component of controlling gene expression.

Chromatin and Transcription Control

• Impact of Chromatin Structure
  • The structure of chromatin, which includes histone proteins and nucleosomes, significantly influences transcriptional control in eukaryotes.

Regulation of Protein Concentration

• Interrelationship of Transcription, Translation, Degradation
  • Protein concentrations are primarily managed through gene transcription rates.
  • The primary regulator of transcription rates is the rate of transcription initiation.
  • Classic experiments, such as the run-on experiment, quantify transcription rates.

Transcription Rate Statistics

• Transcription, Translation, and Degradation Rates
  • Approximately 73% of transcription is attributed to the process of transcription itself.
  • Correspondingly:
  • mRNA translation constitutes about 8%.
  • Protein degradation also accounts for about 8%.
  • mRNA degradation is approximately 11%.

Run-On Experiment

• Methodology of Measurement
  • Techniques utilize radioactive nucleotides (such as $^{32}P$) to label nascent RNA chains.
  • The process involves hybridization to excess gene-specific DNA, separating hybrids, and counting radioactivity in total RNA versus the RNA of gene A.

Tissue-Specific Gene Regulation

• Detailed Applications of Run-On Experiments
  • Examination of gene regulation is conducted using liver, kidney, and brain cells labeled with $^{32}P$.
  • Hybridization occurs with different cDNAs to ascertain which genes are actively transcribed in cells.
  • Findings reveal gene 1 as highly active in liver cells, but non-expressed in kidney and brain cells.

Eukaryotic Cis-Regulatory Elements

• Types and Locations
  • Cis-regulatory elements can be located both upstream or downstream from transcription start sites.
  • These elements influence the regulation of transcription at various levels.

Alternative Promoters: Case Study on Mouse Pax6 Gene

• Functional Implications
  • Different cellular tissues utilize alternative promoters to drive transcription of the Pax6 gene, influencing gene output in various organ systems.

Comparative Genomics

• Identifying Conserved Cis-Regulatory Elements
  • Genomic sequence comparison helps detect conserved sequences that regulate gene expression across species.
  • For example, the SALL1 gene plays a critical role in limb development, with conserved regulatory elements found within 500 kb downstream of SALL1 across species (mouse, chicken, frog, fish).

Eukaryotic RNA Polymerases

• Types and Functions

Table 8-1: Classes of RNA Transcribed

• RNA Polymerase I
  • Transcribes pre-rRNA, which constitutes ribosomal RNA components essential for protein synthesis.
  • Sensitivity to a toxin (a-amanitin) is low.
• RNA Polymerase II
  • Responsible for transcribing mRNA, snRNAs, siRNAs, and miRNAs, reflecting its very high sensitivity to a-amanitin.
• RNA Polymerase III
  • Transcribes tRNAs and other small RNAs, showing intermediate sensitivity to a-amanitin.

Evolutionary Conservation

• Comparative Anatomy of RNA Polymerases
  • Comparison between bacterial and eukaryotic polymerases shows evolutionary links, with specific subunit conservation across species.

Mechanism of Transcription Initiation

• Closed vs. Open Pre-Initiation Complex (PIC)
  • Model representation of RNA polymerase activity shows changes from the closed PIC (where all components are bound but inactive) to open PIC (where transcription can commence).

Carboxy Terminal Domain (CTD) of RNA Pol II

• Role in Transcription
  • CTD consists of a repeated motif of seven amino acids (e.g., $\text{Tyr - Ser - Pro - Thr - Ser - Pro - Ser}_{26-52}$).
  • Specific residues (Ser-5 and Ser-2) are phosphorylated during transcription initiation and elongation respectively.
  • Phosphorylation is crucial for commencing transcription as it allows assembly of the transcription initiation complex.

Chromatin Immunoprecipitation (ChIP) Assay

• Identifying Protein-DNA Interactions
  • Technique allows identification of specific interactions between transcription factors (e.g., RNA polymerase II and its associated components) with chromatin, which assists in mapping active transcription events across the genome.

General Transcription Factors (GTFs) and Transcription

Formation of Pre-Initiation Complex

• Role of GTFs:
  • GTFs are essential proteins that provide a precise platform for RNA polymerase II (RNA Pol II) to accurately initiate transcription at the promoter.
  • They are required at all Pol II promoters and help position the polymerase at the start site, melt the DNA template, and regulate Pol II.
• Sequential Assembly:
  • The assembly of the pre-initiation complex (PIC) at the promoter is a highly ordered process:
  1. TFIID binding: The first step typically involves the TATA Binding Protein (TBP), a subunit of the TFIID complex, binding to the TATA box (if present) or other initiator elements. TBP causes a sharp bend in the DNA, initiating local DNA unwinding.
  2. TFIIA and TFIIB binding: TFIIA stabilizes TFIID binding, while TFIIB helps bridge TFIID to RNA Pol II and determines the start site of transcription.
  3. RNA Pol II and TFIIF recruitment: TFIIF associates with RNA Pol II and helps deliver it to the promoter, where it then binds to the TFIID-TFIIA-TFIIB complex.
  4. TFIIE and TFIIH recruitment: TFIIE recruits TFIIH, which is a key multi-subunit complex. TFIIH possesses helicase activity (to unwind DNA at the promoter, creating the transcription bubble for the open PIC) and kinase activity (to phosphorylate the CTD of RNA Pol II, transitioning it from initiation to elongation).
• This sequential binding ultimately 'opens' the chromatin local to the promoter and recruits RNA Pol II in a conformation ready for transcription initiation.

Effect of Chromatin Structure on Transcription

• Nucleosome Modification
  • Nucleosomes can be modified through acetylation, methylation, or phosphorylation of histones, influencing access to the DNA template for transcription.

Repressor Proteins and Mechanisms

• Regulatory Functions of Repressors
  • Repressors can prevent transcription by several mechanisms, including competition for binding sites, direct interaction with activation domains, or recruitment of co-repressors like histone deacetylases.

Enhancer and Promoter Interactions

• Long-Range Regulation in Eukaryotes
  • Enhancers can act at long distances from their corresponding promoters, often looping to contact transcription initiation complexes.

Epigenetics in Gene Regulation

• Stable Gene Expression Changes
  • Epigenetic changes, such as DNA methylation and histone modifications, influence gene expression without altering the DNA sequence itself.

Summary of Transcription Control

• Integrated Role of Transcription Factors
  • Multiple transcription factors interact cooperatively to modulate the precise expression of genes across various cellular and developmental contexts.
• Future Directions
  • Continued research into transcriptional dynamics remains essential for understanding gene regulation across organisms, particularly in the context of development, disease, and evolutionary biology.