Comprehensive Study Notes on Transcription and Transcriptional Regulation

Fundamental Principles of Transcription

  • Conceptual Overview: Transcription is the process by which instructions contained within DNA are converted into usable orders (RNA) within the cell. This process addresses how cellular information is read and regulated.

  • RNA Polymerase Characteristics:

    • Primer Independence: Unlike DNA Polymerases, RNA-Polymerases do NOT require a primer to initiate synthesis. (Note: Primase is a specialized RNA-Polymerase).

    • Directionality: RNA synthesis occurs exclusively in the 5′→3′5' \rightarrow 3' direction, which is the same orientation as DNA synthesis.

Prokaryotic Transcription

  • The Four Stages:

    1. Binding: Localizing RNA Polymerase to the specific DNA sequence.

    2. Initiation: The start of RNA synthesis.

    3. Elongation: The growth of the RNA chain.

    4. Termination: The cessation of transcription and release of the RNA product.

  • Binding and the Promoter:

    • Definition: A promoter is a specific DNA sequence to which RNA Polymerase binds to initiate transcription of a gene.

    • Functions:

      • Directs RNA Polymerase to the correct template.

      • Dictates which of the two DNA strands serves as the template.

      • Positions the enzyme precisely over the initiation site.

    • Sequence Architecture: Typical prokaryotic promoters contain two conserved sequences located Upstream of the Transcription Start Site (TSS) (labeled as +1+1):

      • -35 Sequence: Located approximately 3535 base pairs upstream.

      • -10 Sequence (TATAAT): Also known as the Pribnow box.

      • Spacer: A sequence of 16−1816-18 base pairs (bp) typically separates the −35-35 and −10-10 regions.

  • The Prokaryotic RNA Polymerase:

    • Core Enzyme: Comprised of subunits α,α,β,β′, and ω\alpha, \alpha, \beta, \beta', \text{ and } \omega.

    • Holo-enzyme: Formed when the core enzyme complexes with the σ\sigma (sigma) factor. The σ\sigma factor is essential for identifying and binding specifically to the −35-35 and −10-10 promoter sequences.

  • Initiation Phases:

    • Closed-promoter Complex: The holo-enzyme is bound to the DNA, but the strands are still double-helical.

    • Open-promoter Complex: Unwinding of the DNA occurs around the initiation site (approximately 10−1510-15 bp) to allow the template to be read.

  • Elongation:

    • Once the RNA chain reaches approximately 8−98-9 base pairs in length, the σ\sigma factor is released.

    • The core enzyme continues the elongation of the RNA chain until it reaches a termination signal.

  • Termination Mechanisms:

    • Intrinsic Termination (Rho-independent):

      • The DNA contains a Symmetrical Inverted Repeat of a GC-rich sequence followed by four or more A (adenine) residues.

      • As this is transcribed, the RNA forms a stable stem-loop structure due to the GC-rich repeats.

      • The association of the RNA with the template is weakened by the subsequent poly-U tail (transcribed from the A residues), leading to dissociation.

    • Rho-dependent Termination:

      • Requires the Rho (Rho factor), an ATP-dependent unwinding enzyme.

      • Rho moves along the newly formed RNA towards the 3′3' end and actively unwinds the RNA from the DNA template.

Eukaryotic Transcription: General Differences and RNA Polymerases

  • Key Differences from Prokaryotes:

    • Transcription Factors: RNA Polymerase alone is insufficient. It requires General Transcription Factors (for basic machinery across all promoters) and Specific Transcription Factors (to regulate individual genes).

    • Three RNA Polymerases: While prokaryotes use one enzyme for all RNAs, eukaryotes use three specialized versions.

    • Template Complexity: Prokaryotic DNA is circular and not organized in nucleosomes. Eukaryotic DNA is linear and highly organized into nucleosomes (chromatin).

  • The Three Eukaryotic RNA Polymerases:

    • RNA Polymerase I: Synthesizes larger ribosomal RNAs (28S,18S, and 5.8S28S, 18S, \text{ and } 5.8S).

    • RNA Polymerase II: Synthesizes all protein-coding RNAs (mRNA), microRNAs (miRNAs), and long non-coding RNAs (lncRNAs).

    • RNA Polymerase III: Synthesizes the smallest ribosomal RNA (5S5S), transfer RNAs (tRNA), and small RNAs involved in splicing and protein transport.

  • Functional Rationale for Three Polymerases:

    1. Allows for tighter, more complex control of specific genes while maintaining continuous high-level transcription of essential genes (e.g., rRNAs).

    2. Enables near-complete silencing of potentially damaging genes while other genes remain active.

    3. Supports the simultaneous transcription of thousands of different genes at varying levels.

Transcription Mechanism for RNA Polymerase II

  • The General Transcription Factors (GTFs): Mnemonic: "Do their Best For the Polymerase and Enhance Huge Phosphorylation."

    1. TFIID: The first factor to bind. It is a multi-protein complex containing:

      • TBP (TATA-binding protein): Specifically binds the TATA box and induces a sharp bend in the DNA to mark the promoter.

      • TAFs (TBP-associated factors): Involved in recognizing other promoter elements.

    2. TFIIB: Serves as a bridge to RNA Polymerase II; recognizes the BRE (TFIIB-recognition element).

    3. TFIIF: Transports RNA Polymerase II to the complex.

    4. TFIIE and TFIIH: Bind last to complete the Preinitiation Complex.

      • TFIIH possesses two critical activities: Helicase activity (via subunits XPB and XPD for unwinding DNA) and Protein Kinase activity.

  • Phosphorylation of the CTD (C-Terminal Domain):

    • RNA Polymerase II has a unique C-terminal domain (CTD).

    • Initiation: TFIIH phosphorylates Serine-5 of the CTD to trigger the start of transcription.

    • Elongation: For productive elongation to occur, Serine-2 of the CTD must be phosphorylated by P-TEFb (Positive Transcription Elongation Factor b).

  • Transcriptional Pausing and Elongation Control:

    • NELF (Negative Elongation Factor) and DSIF (DRB Sensitivity Inducing Factor) can cause the polymerase to pause shortly after initiation.

    • Productive Elongation: P-TEFb phosphorylates NELF (causing it to dissociate), DSIF, and the CTD at Serine-2, allowing the polymerase to proceed.

  • Mediator Complex: A large protein complex that interacts with both General Transcription Factors and Specific Transcription Factors to integrate regulatory signals.

Promoter Architectures

  • RNA Polymerase I Promoter: Consists of a Core Promoter (−45 to +20-45 \text{ to } +20) and an Upstream Control Element (UCE) (−180 to −107-180 \text{ to } -107). Factors like UBF (Upstream Binding Factor) and SL1 (Selectivity Factor, containing TBP) are required.

  • RNA Polymerase II Promoter: Contains motifs such as the TATA box (−25-25), Initiator (Inr) sequence (+1+1), and DPE (Downstream Promoter Element).

  • RNA Polymerase III Promoter: Often located downstream of the transcription start site (internal to the gene). Different types exist for 5S5S rRNA (requiring TFIIIA, TFIIIB, TFIIIC), tRNAs (requiring TFIIIB and TFIIIC), and U6 snRNA (requiring SNAP and TFIIIB).

Transcriptional Regulation: Sequence Elements

  • Regulation Overview: Control ensures genes are transcribed in the right tissue, at the right time, and at the right level. Regulation primarily affects the level of transcripts through five mechanisms:

    1. Sequence elements regulating binding/initiation.

    2. DNA condensation (packaging).

    3. Elongation regulation.

    4. Processing of the transcript.

    5. Transcript stability.

  • Cis-Acting DNA Sequences:

    1. Core Promoter: Directs the basal level of transcription.

    2. Cis-acting sequences (Proximal control elements): Located near the promoter (100−500100-500 bp away). Bind regulatory TFs and induce intermediate transcription levels.

    3. Enhancers:

      • Can be located very far (100,000100,000 bp) upstream or downstream.

      • Independent of orientation.

      • Bind Activators.

      • Mechanism: DNA Looping brings the enhancer close to the promoter. Facilitated by Cohesin and Mediator.

      • Endeososome: A complex of activators bound to the enhancer region.

      • TADs (Topologically Associating Domains): Functional loops of DNA that organize enhancers and promoters. Insulators and CTCF proteins help define boundaries to prevent enhancers from activating the wrong genes.

    4. Silencers: Remote elements that bind Repressors to inhibit transcription.

  • Mapping Regulatory Elements Experimentally:

    • Step 1: Insert the target sequence into a plasmid carrying a Reporter Gene (e.g., GFP - Green Fluorescent Protein).

    • Step 2: Perform progressive deletions of the control region.

    • Logic: If deleting a segment drops activity from (+++)(+++) to (+)(+), that segment likely contains an enhancer or activator-binding site. If activity increases, it may contain a silencer.

  • Case Study: The ZRS Enhancer:

    • The ZRS enhancer regulates the Shh (Sonic Hedgehog) gene for limb development.

    • In snakes, specific mutations in the ZRS enhancer prevent it from activating Shh, leading to the loss of limbs.

    • Experiment: Replacing a mouse ZRS with a cobra ZRS via CRISPR/Cas9 results in a "serpentized" mouse with truncated limbs.

Specific Transcription Factors and Modularity

  • Structural Modularity: Activators typically have two distinct domains:

    1. DNA-binding domain: Recognizes specific sequences (e.g., Zinc fingers, Helix-turn-helix).

    2. Activation domain: Interacts with Mediator, GTFs, or chromatin remodelers.

  • Liquid-Liquid Phase Transitions: Some transcription factors contain Intrinsically Disordered Regions (IDRs). These can form concentrated liquid droplets (condensates) in the nucleus to facilitate coordinated gene expression.

  • Regulation of Transcription Factor Activity:

    1. Cellular Levels: TFs are often unstable and rapidly degraded.

    2. Cellular Localization: Functionality depends on being in the nucleus. Example: The Glucocorticoid Receptor only enters the nucleus when the ligand (Dex) is present.

    3. Post-translational Modifications: Phosphorylation or ubiquitination (e.g., p53, ERK).

Regulation of DNA Condensation (Epigenetics)

  • Mechanism 1: Histone Post-translational Modifications:

    • Acetylation: Catalyzed by HATS (Histone Acetyl Transferases). It adds acetyl groups to lysine residues, neutralizing their positive charge, thereby weakening the attraction to DNA and opening the chromatin (Euchromatin). Reversed by HDACs (Histone De-Acetylases).

    • Methylation: Catalyzed by H-MT (Histone Methyl Transferases). Does not affect charge but creates binding sites for other proteins.

      • H3K4me3: Associated with active chromatin.

      • H3K9me3 / H3K27me3: Associated with inactive chromatin (Heterochromatin).

      • Reversed by Histone De-Methylases.

  • Mechanism 2: DNA Methylation:

    • Cytosine residues in CpG islands can be converted to 5-Methylcytosine by D-MT (DNA Methyl Transferases).

    • Generally leads to gene silencing. Important in Genomic Imprinting (e.g., H19 gene where maternal/paternal alleles behave differently).

    • Reversibility: Requires TET enzymes and base-excision repair.

  • Mechanism 3: Nucleosome Remodeling:

    • Remodeling factors (e.g., SWI/SNF) utilize energy to change nucleosome positions via Sliding (moving the DNA along the histone core) or Eviction (removing the nucleosome entirely to expose DNA).

  • Mechanism 4: Non-coding RNAs:

    • miRNAs/lncRNAs can recruit RITS (RNA-induced transcriptional silencing) complexes or PRC2 (Polycomb Repressive Complex 2) to deposit repressive marks like H3K27me3H3K27me3.