7.1 Gene Regulation in Prokaryotes & Eukaryotes

Prokaryotic Gene Regulation: The Operon Model

In prokaryotes, genes encoding enzymes of a single metabolic pathway are frequently clustered together into a single transcriptional unit termed an operon. This organization allows the cell to coordinate the expression of functionally related proteins rapidly in response to environmental fluctuations. The classical Jacob-Monod model defines the structural components of an operon:

1. Regulator Gene: Located upstream or downstream, it transcribes mRNA for a repressor (or activator) protein. The regulator gene possesses its own promoter and is expressed constitutively.
2. Promoter Region: The specific DNA sequence recognized and bound by bacterial RNA polymerase (specifically the $\sigma$ subunit) to initiate transcription.
3. Operator Site: A non-coding DNA sequence positioned between the promoter and structural genes that serves as the binding site for the repressor protein. When bound, the repressor physically blocks RNA polymerase from translocating along the DNA template.
4. Structural Genes: A polycistronic gene cluster transcribed into a single continuous mRNA molecule containing coding sequences for multiple distinct proteins.

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The lac Operon: Negative Inducible & Positive Control

The Escherichia coli lac operon controls the transport and catabolism of lactose. It contains three structural genes:

• lacZ: Encodes $\beta$-galactosidase, an enzyme that hydrolyzes lactose into glucose and galactose, and converts a small fraction of lactose into allolactose.
• lacY: Encodes lactose permease, a membrane transport protein that facilitates lactose entry into the cell.
• lacA: Encodes $\beta$-galactoside transacetylase, an enzyme that transfers an acetyl group to $\beta$-galactosides.

  Regulator        Promoter   Operator   lacZ       lacY       lacA
+------------+   +----------+----------+----------+----------+----------+
|    lacI    |   |  CAP/P   |    O     |  beta-Gal| Permease | Transac  |
+------------+   +----------+----------+----------+----------+----------+

1. Negative Inducible Control (Repressor Dynamic)

Under basal conditions without lactose, the constitutively expressed LacI repressor binds tightly to the operator ($O$), blocking RNA polymerase. When lactose enters the cell, a small amount is isomerized into allolactose (the true inducer). Allolactose binds allosterically to the LacI repressor, inducing a conformational change that dramatically reduces its affinity for the operator. The repressor dissociates, removing the physical block to transcription.

2. Positive Control (Catabolite Repression via CAP/cAMP)

Prokaryotes preferentially metabolize glucose over lactose because glucose enters glycolysis directly. When glucose levels are high, the enzyme adenylate cyclase is inactivated, resulting in extremely low intracellular concentrations of cyclic AMP (cAMP).

When glucose is depleted, adenylate cyclase becomes active, causing intracellular cAMP concentrations to surge. cAMP binds to the Catabolite Activator Protein (CAP), forming a active cAMP-CAP complex. This complex binds to a specific promoter site upstream of RNA polymerase, inducing a 90° bend in the DNA that dramatically enhances RNA polymerase binding affinity and transcription initiation rate.

AAMC MCAT Core Rule: Removal of the repressor by allolactose is necessary but insufficient for maximal lac operon transcription. High-level expression demands both the removal of the repressor (lactose present) and the activation of CAP by elevated cAMP (glucose absent).

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The trp Operon: Negative Repressible Control & Attenuation

The E. coli trp operon encodes five structural genes (trpE, trpD, trpC, trpB, trpA) required for the biosynthesis of the essential amino acid tryptophan. Unlike the lac operon, it is a repressible system that is normally active and turned off when its metabolic product is abundant.

1. Negative Repressible Control

The regulator gene trpR transcribes an inactive apo-repressor. When intracellular tryptophan levels are high, tryptophan functions as a corepressor, binding allosterically to TrpR to form an active repressor complex. This complex binds the operator, blocking transcription.

2. Transcriptional Attenuation (Fine-Tuning Control)

Because prokaryotes lack a nuclear membrane, transcription and translation occur simultaneously (coupled transcription-translation). The trp operon features a leader sequence (trpL) located between the operator and trpE that contains four distinct mRNA regulatory domains (regions 1, 2, 3, and 4) and two tandem tryptophan codons within region 1.

  Region 1 (Trp codons) --- Region 2 --- Region 3 --- Region 4 (Poly-U tail)

• High Tryptophan Conditions: The ribosome translates region 1 rapidly without pausing at the Trp codons, covering region 2. Region 3 pairs exclusively with region 4, forming a 3:4 attenuation hairpin structure (rho-independent terminator). Followed by a poly-U tract, this stem-loop forces RNA polymerase to detach before transcribing the structural genes.
• Low Tryptophan Conditions: The ribosome stalls at the tandem Trp codons in region 1 due to charged $tRNA^{Trp}$ deficiency. This leaves region 2 exposed to pair with region 3, forming a 2:3 antiterminator hairpin structure. Because region 3 is occupied, the 3:4 terminator cannot form, allowing RNA polymerase to transcribe the entire operon.

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Eukaryotic Gene Regulation: Promoters, Enhancers & Transcription Factors

Eukaryotic gene expression is governed by complex interactions between cis-acting regulatory elements (DNA sequences on the same chromosome as the gene) and trans-acting factors (diffusible proteins synthesized elsewhere).

Core Promoter & Proximal Elements

• Core Promoter: Located immediately upstream of the transcription start site (+1). Contains the TATA box (consensus sequence $5'-TATAAA-3'$ located at position -25 to -30), which is bound by the TATA-binding protein (TBP) component of the basal transcription factor TFIID.
• Proximal Promoter Elements: Located within 200 base pairs upstream of +1, such as the CAAT box (-75) and GC box (-90), which recruit specific transcription factors to stabilize the pre-initiation complex.

Enhancers, Silencers & DNA Looping

• Enhancers: Cis-acting DNA sequences that can be located thousands of base pairs upstream, downstream, or within introns of the target gene. Activator proteins bind enhancers and interact with the basal transcription machinery at the promoter via DNA looping mediated by the Mediator complex and cohesin rings.
• Silencers: Cis-acting DNA elements that bind repressor proteins to suppress transcription via chromatin condensation or basal factor inhibition.

Transcription Factor Structural Motifs

Specific transcription factors possess a DNA-binding domain (DBD) that recognizes unique sequence motifs in the major groove of B-DNA, paired with a transactivation domain. Primary motifs include:

• Zinc Finger: Coordinates a $Zn^{2+}$ ion using cysteine and histidine residues to form a finger-like projection that inserts into the DNA major groove.
• Leucine Zipper: Contains amphipathic alpha-helices with leucine residues at every seventh position, forming a hydrophobic dimerization interface.
• Helix-Turn-Helix & Helix-Loop-Helix (bHLH): Basic amino acids interact directly with negative DNA phosphate backbones.

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Epigenetic Regulation & Chromatin Remodeling

Epigenetic modifications alter chromatin structure and gene expression without modifying the underlying DNA nucleotide sequence.

  Heterochromatin (Condensed, Silent)  <=======>  Euchromatin (Open, Active)
  - Low acetylation, High methylation               - High acetylation (HATs)
  - Histones tightly bind DNA                       - Neutralized Lysines

1. Histone Modifications

DNA wraps around histone octamers ($H2A, H2B, H3, H4$) to form nucleosomes. Basic, positively charged lysine residues on histone N-terminal tails interact electrostatically with the negatively charged DNA phosphate backbone.

• Histone Acetyltransferases (HATs): Transfer acetyl groups from Acetyl-CoA to lysine residues. Acetylation neutralizes the positive charge on lysine, weakening histone-DNA interactions. Chromatin unwinds into euchromatin, allowing transcription factor binding.
• Histone Deacetylases (HDACs): Remove acetyl groups, restoring positive charges on histones. Chromatin condenses into transcriptionally inert heterochromatin.
• Histone Methylation: Histone Methyltransferases (HMTs) add methyl groups to lysine or arginine residues. Effects depend on the specific site: methylation of H3K4 is associated with active transcription, whereas H3K27 and H3K9 methylation recruits repressors to silence chromatin.

2. DNA Methylation & Imprinting

• DNA Methyltransferases (DNMTs): Transfer methyl groups to cytosine bases in CpG islands (cytosine-poly-G rich promoter regions), producing 5-methylcytosine.
• Mechanism: Methylation directly blocks transcription factor binding and recruits HDACs, leading to long-term transcriptional silencing. DNA methylation plays a central role in genomic imprinting (parent-of-origin specific gene silencing) and X-chromosome inactivation (mediated by the non-coding Xist RNA).