Genetic Regulation

DNA contains genetic material that codes for the formation and regulation of protein synthesis within the cell. This can be described as a 2-step process:

1. mRNA production of DNA sequence (transcription)

   • mediated by RNA polymerase

2. Protein synthesis from mRNA (translation) - mediated by Ribosomes

Genetic expression in this context refers to the end product of a section of genetic material.

Genetics Background

Regulation of Transcription

Transcription factor proteins regulate gene transcription by increasing or decreasing its rate:

• Activators: Increase transcription rate.

• Repressors: Decrease transcription rate.

• Autoregulation: Occurs when a gene codes for its own transcription factor:

  • Positive Autoregulation: Gene codes for its own activator.

  • Negative Autoregulation: Gene codes for its own repressor.

Overall Balance Equations

Conservation equations track mRNA (R) and protein (P) concentrations over time based on synthesis and degradation rates:

• mRNA Balance: dR/dt​=v_{transcription​}−v_Rdeg​

• Protein Balance: dP/dt​=v_{translation​}−v_Pdeg​

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Focus: Transcription Flux

• Goal: Derive a constitutive equation for transcription rate (vtranscription​) using underlying gene regulatory states.

• Kinetic Approach: Uses equilibrium assumptions and detailed state models, analogous to enzyme kinetics, to determine overall transcription flux.

• Mechanism: RNA polymerase binds to the DNA sequence and reads it to synthesize mRNA.

Regulation of Transcription

4 Types of Regulation

Transcription factor (TF) activity is modified by control molecules (inducers/corepressors):

• Negative Inducible:

  • Baseline: Repressor TF binds DNA and blocks transcription.

  • With Control Molecule: Molecule binds repressor → repressor detaches → transcription allowed.

• Negative Repressible:

  • Baseline: Repressor cannot bind DNA on its own → transcription occurs.

  • With Control Molecule: Molecule binds repressor → repressor binds DNA → transcription prevented.

• Positive Inducible:

  • Baseline: Activator cannot bind DNA on its own.

  • With Control Molecule: Molecule binds activator → activator binds DNA → transcription allowed.

• Positive Repressible:

  • Baseline: Activator TF binds DNA and allows transcription.

  • With Control Molecule: Molecule binds activator → activator detaches → transcription prevented.

Types of regulation

Gene Regulatory States & Fluxes

• State Notation: Unique combinations of a gene, transcription factors (e.g., A, B), and control molecules are designated by state numbers (x0​,x1​,x2​,x3​) representing transitions between states with equilibrium constants (K1​,K2​,K3​,K4​).

• Normalized Concentrations (Occupancy Probabilities): Concentration of genetic material is normalized as state fractions (xs​) that sum to 1:

  x₀​+x₁​+x₂​+x₃​=1

Overall Flux Constitutive Equation

• Transcription Rate Equation: The overall constitutive equation sums the rate of each individual state multiplied by its state fraction:

  vtranscription​=x₀​v₀​+x₁​v₁​+x_2​v_2​+x₃​v₃​

• General Summation Form:

  v_{transcription}​=_s=0∑_N^s​−1​x_s​v_s​

  (where Ns​ is the total number of states, and the specific kinetic rate vs​ depends on whether the TFs act as activators or repressors).

Gene States

Lac Operon Regulation

• E. Coli Metabolic Switching:

  • Preference: Prefers glucose for metabolism.

  • Adaptation: Switches to lactose metabolism when glucose is unavailable by expressing specific enzymes.

• Key Concepts:

  • Genetic Switches: Distinct gene states that switch expression pathways (e.g., "off state" for glucose metabolism; "on state" upregulates lactose metabolism).

  • Operons: Cluster of genes controlled by a single promoter, allowing coordinated transcription of multiple proteins at once.

• Regulatory Mechanisms:

  • Negative Inducible Pathway (Lactose Control):

    • Inhibitor (TF) binds Lac operon to block transcription.

    • Lactose acts as an inducer (control molecule): binds inhibitor → inhibitor detaches → transcription allowed.

  • Positive Repressible Pathway (Glucose Control):

    • cAMP-CAP complex acts as an activator promoter to enable transcription.

    • Glucose acts as a repressor: high glucose reduces cAMP levels → prevents cAMP-CAP formation → transcription prevented.

  • Dual Requirement: Lac operon transcription requires both the absence of glucose AND the presence of lactose.

  • Positive Autoregulation: Lac operon proteins transport more lactose into the cell, which further drives its own activation.

Genetic switches

• Definition: A section of genetic material containing a cluster of genes under the control of a single promoter.

• Key Function: Allows the transcription of multiple related proteins to be regulated and controlled at a single genomic site simultaneously.

Operons

Lac Operon Regulation

• Negative Inducible Pathway (Lactose Control):

  • Repressor/Inhibitor: Binds to the Lac operon in the default state to block transcription.

  • Lactose (Inducer): Binds to the repressor → repressor detaches → transcription is initiated/increased.

• Positive Repressible Pathway (Glucose Control):

  • cAMP-CAP Complex: Binds to the Lac operon as an activator/promoter to enable transcription.

  • Glucose (Repressor): Reduces cAMP availability → prevents cAMP-CAP complex formation → prevents transcription.

• Dual Requirement:

  • Full Lac operon transcription requires both the absence of glucose (to allow cAMP-CAP activation) AND the presence of lactose (to remove repressor inhibition).

• Positive Autoregulation:

  • Translational products of the Lac operon transport more lactose into the cell, which further drives its own activation.

Lac Operon Regulation

Lac Operon Regulation & Glucose Dependence

• Dual Control Requirement: High-level transcription of the Lac operon requires both the absence of glucose and the presence of lactose.

• Negative Inducible Regulation (Lactose Sensing):

  • Off State: A repressor protein remains bound to the Lac operon, preventing transcription.

  • On State: When present, lactose binds to the repressor, inducing a conformational change that causes it to detach from the operon and permit transcription.

  • Positive Autoregulation: Products of the Lac operon import additional lactose into the cell, further stimulating transcription.

• Positive Repressible Regulation (Glucose Sensing):

  • CAP Activator: Catabolite Activator Protein (CAP) must form a complex with cAMP (cAMP-CAP) to bind the Lac operon and act as a promoter.

  • Glucose Suppression: Glucose lowers intracellular cAMP levels, preventing cAMP-CAP complex formation.

  • Result: Without the cAMP-CAP complex bound to the operon, transcription remains suppressed even if lactose is available.

Lac Operon Regulatory Network

• Definition & Core Function: Microarrays are a mature transcriptomics technology featuring a grid of compartments containing cDNA (complementary DNA). They allow simultaneous measurement of thousands of mRNA expression levels.

• Mechanism:

  • mRNA strands hybridize (preferentially bind) to their corresponding cDNA probes as a form of reverse transcription (mRNA→DNA).

  • Fluorescent tags attached to the samples allow expression levels to be quantified by measuring the fluorescence intensity of the cDNA-mRNA complex.

  • mRNA product can be amplified using PCR or qPCR prior to measurement.

Microarrays

• Perturbation (Comparative): Evaluates the effect of different treatments or tissue responses across different sample types at the same time point.

  • Competitive Hybridization: Treatment and control samples are tagged with different fluorescent colors and run competitively on the same chip to determine expression direction relative to control.

• Time Series: Measures continuous, sequential changes in gene expression within a sample over time.

• Combined: Integrates both approaches (e.g., comparing time-series profiles across different treatments).

Data and Experiment types

• Core Concept: Groups genes based on the similarity of their expression responses to an intervention, identifying genes that are likely functionally related.

• Similarity Measurement:

  • Evaluated numerically by calculating the "distance" between the expression profiles of two genes.

  • Identical expression profiles yield a distance of zero (d=0).

  • Shorter distances indicate stronger co-regulation or functional relationships.

• Common Algorithms:

  • k-means Clustering: Partition-based grouping method.

  • Hierarchical Clustering: Tree-based (dendrogram) grouping method.

Clustering

• Core Concept: Evaluates regulatory networks by modifying (perturbing) the steady-state transcription rate of a single gene and tracking how other genes respond.

• Output Formats: Results are mapped to either a network diagram or a regulatory strength matrix.

• Regulatory Strength Matrix Structure:

  • Rows (Dependent Variables): Represent the gene expression levels of target genes responding to the change.

  • Columns (Independent Variables): Represent the specific gene that was perturbed (via an increase or decrease in transcription).

Control Analysis