Prokaryotic Gene Regulation and The Operon

Learning Objectives for Prokaryotic Gene Regulation

• Describe the two general gene regulation pathways found in prokaryotes.

• Describe the structural composition of an operon.

• Differentiate between regulatory genes and structural genes through description and contrast.

• Detailed examination and comparison of the $lac$ and $trp$ operons.

• Understand the fundamental functional differences between inducible operons and repressible operons.

• Define key regulatory terms: Inducer, repressor, operator, and corepressor.

General Principles of Gene Expression Control

• Levels of Regulation: Gene expression can be regulated at multiple stages within the cell (based on Figure 8-3, Essential Cell Biology, 2/e):     1. Transcriptional Control: Regulation occurring at the DNA level where RNA transcripts are produced. This is the most common form of regulation.     2. RNA Processing Control: Control occurring during the maturation of the RNA transcript (in eukaryotic models, this involves the nucleus).     3. Transport Control: Regulating the movement of mRNA from the nucleus to the cytosol through nuclear pores.     4. Translation Control: Regulating the rate at which mRNA is translated into protein in the cytosol.     5. Protein Activity Control: Modifying or regulating the function of a protein once it has been synthesized to make it active or inactive.

• Modes of Gene Expression:     * Constitutive ("House-keeping") Expression: Genes that are always expressed because the proteins they encode are required for life under all environmental conditions.         * The amount of constitutive protein produced is determined by promoter strength.         * No additional regulatory factors are required for this type of expression.     * Induced Expression: The gene is expressed specifically in response to an environmental signal.     * Repressed Expression: Gene expression is shut off when a specific environmental signal is present.

Structural Components of the Operon

• Operator ($o$): A segment of DNA that acts as a functional "off/on" switch, controlling the entire operon.

• Promoter ($p$): A specific sequence of nucleotides where RNA polymerase binds to initiate the process of transcription.

• Regulatory Gene ($r$): A nucleotide sequence that produces a repressor protein capable of interacting with the operator to inhibit transcription.

• Structural Genes ($a, b, c, ext{etc.}$): Sequences of DNA that code for specific polypeptides or enzymes.

Tryptophan Biosynthesis and the Trp Operon

• Metabolic Control Levels: Regulation of tryptophan occurs at two distinct levels:     1. Alteration of the physical catalytic activity of existing enzymes.     2. Regulation of the expression of the genes that produce those enzymes.

• The Tryptophan Biosynthetic Pathway:     * The pathway begins with chorismate biosynthesis.     * $chorismate + L ext{-glutamine} ightarrow anthranilate + L ext{-glutamate} + pyruvate$ (catalyzed by anthranilate synthase).     * $anthranilate + ext{5-phosphoribosyl 1-pyrophosphate} ightarrow phosphoribosylanthranilate + diphosphate$ (catalyzed by phosphoribosyl transferase).     * $phosphoribosylanthranilate ightarrow ext{1-(o-carboxyphenylamino)-1'-deoxyribulose-5'-phosphate}$ (catalyzed by phosphoribosylanthranilate isomerase).     * $ext{1-(o-carboxyphenylamino)-1'-deoxyribulose-5'-phosphate} + H_2O ightarrow ext{indole-3-glycerol-phosphate} + CO_2$ (catalyzed by indole-3-glycerol phosphate synthase).     * $ext{indole-3-glycerol-phosphate} ightarrow indole + ext{D-glyceraldehyde-3-phosphate}$ (catalyzed by indoleglycerol phosphate aldolase).     * $indole + L ext{-serine} ightarrow L ext{-tryptophan} + H_2O$ (catalyzed by tryptophan synthase, $\beta ext{ subunit dimer}$).

• The Trp Operon Structure in E. coli:     * Consists of five structural genes: $trpE, trpD, trpC, trpB, ext{and } trpA$.     * These genes are transcribed together into a single mRNA molecule and translated into enzymes for tryptophan biosynthesis.

• Mechanism of the Repressible Operon:     * The $trp$ operon is a repressible operon; its enzymes are considered repressible enzymes because synthesis is inhibited by the metabolite (tryptophan).     * Low Tryptophan Levels: The $trp$ repressor is inactive. RNA polymerase binds to the promoter and transcribes the structural genes. The operon is "ON."     * High Tryptophan Levels: Tryptophan acts as a corepressor, binding to the $trp$ repressor to activate it. The active repressor binds to the operator, blocking RNA polymerase. Transcription is halted. The operon is "OFF."

Lactose Metabolism and the Lac Operon

• Lactose Hydrolysis: The enzyme $\beta ext{-Galactosidase}$ catalyzes the hydrolysis of lactose (a disaccharide) into glucose and galactose through the addition of $H_2O$.

• The Lac Operon Structure:     * Contains genes for lactose metabolism: $lacZ$ ($\beta ext{-Galactosidase}$), $lacY$ (Permease), and $lacA$ (Transacetylase).     * Regulatory components include the $CAP$ site, Promoter ($P$), and Operator ($O$).

• Mechanism of the Inducible Operon:     * An inducible operon is usually in an "off" state.     * Inactivation of the repressor by an inducer turns on transcription.     * No Lactose Present: The $lac$ repressor (produced by the $I$ gene) binds tightly to the operator, physically blocking RNA polymerase. Transcription is blocked.     * Lactose Present: Lactose is converted into its isomer, allolactose (the inducer). Allolactose binds to the $lac$ repressor, changing its shape so it can no longer bind to the operator. RNA polymerase is free to transcribe the operon. The operon is "ON."

Dual Control of the Lac Operon: Glucose and cAMP

• The Role of Glucose and CAP: The $lac$ operon is sensitive to glucose levels via the Catabolite Activator Protein (CAP).

• Low Glucose Conditions:     * When glucose is low, the cell produces cyclic AMP ($cAMP$).     * $cAMP$ attaches to $CAP$, activating it to bind to the $CAP$ site on the DNA.     * Activated $CAP$ facilitates the binding of RNA polymerase to the promoter.     * Result: High levels of transcription.

• High Glucose Conditions:     * When glucose is high, $cAMP$ levels are low.     * $CAP$ cannot bind to the DNA without $cAMP$.     * RNA polymerase has a lower affinity for the promoter without $CAP$ assistance.     * Result: Low levels of transcription, even if lactose is present.