RNA Synthesis and Regulation in Bacteria

Core Concepts of Transcription

• Definition of Transcription: Transcription is the biological process of producing RNA from a DNA template.
• Major Types of RNA produced:     * Messenger RNA (mRNA): Responsible for encoding proteins.     * Ribosomal RNA (rRNA): The most abundant type of RNA in the cell.     * Transfer RNA (tRNA): A critical component for the translation process.

The Biochemical Reaction of RNA Synthesis

• Catalyst: The addition of RNA nucleotides is catalyzed by RNA polymerase (RNAP).
• Directionality: Synthesis occurs exclusively in the $5' \rightarrow 3'$ direction.
• Mechanism Characteristics:     * The reaction resembles the addition of nucleotides by DNA polymerase.     * It involves a nucleophilic attack by the $3'\text{-OH}$ group of the growing chain on the innermost phosphate (alpha phosphate) of the incoming ribonucleoside triphosphate.     * The reaction is driven forward by the hydrolysis of pyrophosphate ($PP_i$).
• Reaction Requirements:     * DNA Template: Necessary to dictate the sequence.     * NTPs: Uses ribonucleoside triphosphates (ATP, UTP, GTP, CTP) rather than dNTPs.     * Divalent Metal Ions: Requires $Mg^{2+}$ or $Mn^{2+}$.     * Primer Independence: Unlike DNA polymerase, RNA polymerase does not require a primer to initiate synthesis.

Conventions and Nomenclature in Gene Structure

• Start Site: The site of the first nucleotide incorporated into the RNA is designated as $+1$.
• Coding (Sense) Strand: The DNA strand that is not used as a template; its sequence matches the RNA transcript (with U replacing T).
• Template (Antisense) Strand: The DNA strand that is used as the template for synthesis; it is complementary to the RNA transcript.

RNA Polymerase in Escherichia coli

• Nature of the Enzyme: E. coli RNA polymerase (RNAP) is a multi-subunit enzyme that transcribes all types of RNA (mRNA, tRNA, rRNA).
• Subunit Composition of the Core Enzyme: $\alpha_2 \beta \beta' \omega$. This form is present during the elongation phase.
• Subunit Composition of the Holoenzyme: $\alpha_2 \beta \beta' \omega$, plus the sigma ($\sigma$) factor. This form is required for the initiation phase.
• Subunit Identification:     * $\alpha$ (orange)     * $\beta$ (beige)     * $\beta'$ (blue)     * $\omega$ (red)

The Three Stages of Transcription

• Stage 1: Initiation:     * Involves promoter recognition and engagement by the RNAP holoenzyme.     * Features a transition from a "closed complex" to an "open complex," a process taking approximately $0.5$ to $1$ second.
• Stage 2: Elongation:     * Characterized by the loss of the sigma ($\sigma$) factor.     * The core enzyme carries out the synthesis.
• Stage 3: Termination:     * Intrinsic Termination: Dependent on the RNA sequence itself.     * Protein-Dependent Termination: Mediated by the Rho ($\rho$) protein.

Detailed Mechanics of Transcription Initiation

• Promoters: DNA sequences that direct RNAP to the proper start site. Common sequences located upstream of the start site include:     * -10 Site: A conserved sequence located 10 base pairs upstream of the start.     * -35 Sequence: A conserved sequence located 35 base pairs upstream of the start.
• Initiation Efficiency: Depends on the extent of the match to consensus sequences. The initiation rate ranges from approximately $0.001\,s^{-1}$ to $0.5\,s^{-1}$.
• Holoenzyme Binding: The holoenzyme ($\alpha_2 \beta \beta' \omega \sigma$) binds the promoter through specific interactions:     * $\sigma$ interacts with the $-10$ and $-35$ sites.     * $\alpha$ subunits interact with the UP element.
• Role of Sigma ($\sigma$):     * Decreases affinity for generic (non-specific) DNA sequences by a factor of $10,000$.     * Increases binding strength to $-10$ and $-35$ regions.     * Released when the nascent RNA reaches approximately $10$ nucleotides in length.     * Various sigma factors exist to allow for the selection of different promoter sets.
• DNA Scrunching:     * A process critical for promoter escape.     * Creates energetic stress of approximately $1\,kcal/mol/nt$, helping to offset the energy lost when the $\sigma$ factor is released.     * Rebinding of $\sigma$ may also play a role in distal pausing.
• The UP Element:     * Located at the $-40$ to $-60$ position relative to the start site.     * Further strengthens RNAP binding by interacting with the C-terminal domain of the $\alpha$ subunits. It is not present in all promoters.

Detailed Mechanics of Elongation

• Transition: RNAP moves from the closed complex to the open complex to begin.
• Transcription Bubble: A region of $14\text{--}17$ nucleotides where the DNA is actively unwound. Within this bubble, an RNA-DNA hybrid of $8\text{--}9$ nucleotides is formed.
• Elongation Statistics:     * Rate: Approximately $50\,nt/sec$. This is significantly slower than DNA replication (at $1000\,nt/sec$).     * Fidelity: Relatively error-prone, making just over one mistake per $10,000$ nucleotides.
• Topological Challenges:     * Movement along a helical template requires rotation. Without free rotation, DNA becomes strained.     * Twin Supercoiled Domain Model: RNA production results in a net underwinding (negative supercoiling) behind/upstream of the RNAP and overwinding (positive supercoiling) ahead/downstream of the RNAP.     * Topoisomerases may be required to relieve this torsional strain.

Detailed Mechanics of Termination

• Intrinsic Termination:     * No additional proteins are required.     * The RNA product contains an inverted repeat rich in GC base pairs followed by a string of multiple Uracils (Ts in DNA).     * When the GC region is transcribed, it forms a stable stem-loop structure.     * The stem-loop causes RNAP to pause.     * The weak $\text{rU-dA}$ base pairs between the transcript and the template facilitate dissociation.
• Protein-Dependent Termination (Rho-dependent):     * Relies on the Rho ($\rho$) hexamer protein.     * Rho binds to a specific $70\text{--}90$ nucleotide site on the growing RNA.     * Utilizes translocase activity, fueled by ATP, to advance toward the RNAP.     * It is described as a "race" between Rho and the transcribing RNAP.     * Rho contains an interior channel and likely loops out RNA during its advance.     * Collision with RNAP breaks the RNA-DNA hybrid, terminating transcription. This helps reduce energy waste after accidental initiation where no ribosomes are present.

Clinical Insights: Antibiotics Targeting Transcription

• Rifampicin (Rifampin): An antibiotic specific to bacteria.
• Actinomycin D:     * Effective in both prokaryotes and eukaryotes.     * Binds DNA tightly, preventing the opening of the complex to initiate transcription.     * Blocks the channel intended for the RNA-DNA hybrid, preventing transcription beyond two to three nucleotides.     * Also utilized in chemotherapy.

Bacterial Transcription Regulation: Operons

• Definition: Operons allow for the coordinated expression of related genes.
• General Features:     * Usually contain two or more structural (protein-coding) genes.     * Structural genes are transcribed into a single, polycistronic mRNA.     * Contain regulatory elements recognized by one or more regulatory-gene products.

The lac Operon

• Function: Encodes proteins necessary for utilizing lactose as a carbon source when glucose is absent.
• Structural Genes (in a single transcript):     * LacZ: Encodes $\beta$-galactosidase, which hydrolyzes lactose into galactose and glucose.     * LacY: Encodes permease, which enables the uptake of lactose into the cell.     * LacA: Encodes transacetylase; its role is less clear, potentially involved in detoxification.
• Control Elements (Cis-acting):     * Promoter     * Operator     * Catabolite Activating Protein (CAP) binding site, also known as CRP.
• Control Proteins (Trans-acting):     * RNAP     * Lac Repressor: Encoded by the "i" gene ($LacI$).     * Catabolite Activating Protein (CAP/CRP).
• Negative Control (Repression):     * The lac repressor blocks initiation when bound to the operator.     * In the absence of lactose, the repressor is DNA-bound by default.
• Induction by Lactose:     * Lactose enters the cell and is converted into allolactose (the inducer).     * Allolactose binds to the lac repressor, causing a conformational change that prevents it from binding DNA, thereby lifting repression.
• Activation by CAP:     * Lifting repression results only in basal (low level) expression.     * High-level transcription (activation) requires CAP.     * CAP binds to DNA only when cAMP levels are high.     * cAMP levels are high exclusively under conditions of low or no glucose.     * The CAP-cAMP complex binds near the promoter and contacts RNAP to increase its affinity and transition to the open state.
• Why CAP is required: The lac promoter is suboptimal; its match to the consensus sequence is marginal and it lacks a UP element.
• Summary of lac Regulation State:     * $+\text{Glucose}, -\text{Lactose}$: Repressor bound. No transcription.     * $+\text{Glucose}, +\text{Lactose}$: Repressor-inducer complex does not bind; CAP not bound. Low level transcription.     * $-\text{Glucose}, -\text{Lactose}$: Repressor bound; CAP-cAMP bound. No transcription.     * $-\text{Glucose}, +\text{Lactose}$: Repressor-inducer complex release; CAP-cAMP bound. High level transcription.

Bacterial Transcription Regulation: Attenuation

• The trp Operon: Encodes five proteins required for tryptophan synthesis.
• Mechanism of Attenuation: Regulates transcription based on tryptophan levels via co-transcriptional translation.     * High Tryptophan: The ribosome moves quickly through trp codons in the leader sequence, allowing for the formation of a terminator stem-loop in the RNA, which terminates transcription.     * Low Tryptophan: The ribosome stalls at the trp codons due to lack of aminoacyl-tRNA. This stalling results in an alternate RNA secondary structure (anti-terminator) that does not include the termination stem-loop, allowing transcription of the full operon to continue.