Questions: Prokaryotic Transcription Initiation: Sigma Factors and Promoters

Question 1

A researcher engineers an E. coli strain where sigma⁷⁰ is covalently attached to the core RNA polymerase and cannot dissociate. Which of the following best predicts the consequences for gene regulation?

• A. All transcription stops immediately because sigma must cycle off for elongation to proceed
• B. Only stress-response genes are affected since sigma⁷⁰ controls only housekeeping gene transcription
• C. Stress responses fail because alternative sigma factors cannot displace sigma⁷⁰ from the core polymerase, and initiation becomes slower because sigma cannot recycle to new promoters
• D. Gene expression is unaffected since sigma only assists with promoter finding but is not required for regulation

Answer: (C)

Sigma factor dissociation after initiation serves two critical functions: it allows alternative sigma factors to compete for the core polymerase (enabling regulatory reprogramming during stress), and it allows the released sigma to associate with a new core polymerase and initiate again at another promoter (recycling). If sigma⁷⁰ cannot dissociate, the core polymerase is permanently committed to sigma⁷⁰ promoters — stress responses controlled by σ³², σˢ, or σ⁵⁴ cannot be activated because those sigma factors cannot access the core enzyme. The cell loses the ability to globally reprogram its transcriptional output in response to environmental change, which is the primary function of the interchangeable sigma factor system.

Question 2

Why is the AT-rich consensus sequence (TATAAT) of the -10 element functionally important for transcription initiation, rather than being an arbitrary recognition code?

• A. A-T base pairs are specifically recognized by sigma factor's DNA-binding domain but are invisible to non-specific DNA-binding proteins, providing selectivity
• B. A-T base pairs have only two hydrogen bonds compared to three for G-C pairs, making the -10 region easier to melt apart — which is required to form the open complex and expose the template strand
• C. The TATAAT sequence encodes the start codon region that the ribosome will later recognize on the mRNA
• D. AT-rich regions attract RNA polymerase through electrostatic interactions more effectively than GC-rich regions

Answer: (B)

The functional importance of the AT-rich -10 element is thermodynamic, not just recognition-based. After sigma factor positions the holoenzyme at the promoter (closed complex), the DNA must locally unwind to form the open complex — a bubble of single-stranded DNA that exposes the template strand for synthesis. A-T base pairs are weaker (2 hydrogen bonds vs. 3 for G-C) and require less energy to separate. Placing the most easily melted sequence at the -10 element — precisely where strand separation must begin — is not coincidental. It reduces the energy barrier for open complex formation. This principle is conserved across bacteria: -10 elements universally tend to be AT-rich regardless of the exact consensus.

Question 3

The core RNA polymerase in E. coli can bind DNA and synthesize RNA efficiently, but requires a sigma factor specifically to recognize and bind promoter sequences at the -10 and -35 elements.

• T. True
• F. False

Answer: True

This is the central division of labor in prokaryotic transcription initiation. The core enzyme (α₂ββ'ω) has all the catalytic machinery needed to polymerize RNA and can bind DNA nonspecifically, but it lacks the structural domains that make specific contacts with promoter elements. Sigma factor supplies these: it makes direct contacts with the -35 element and the -10 element in the major groove of DNA, positioning the holoenzyme precisely at the transcription start site. This modularity is the key to bacterial gene regulation — swapping sigma factors changes which promoters are recognized without altering the core catalytic machinery.

Question 4

Each of E. coli's different sigma factors (σ⁷⁰, σ³², σ⁵⁴) associates with a dedicated RNA polymerase molecule, so the cell maintains separate pools of holoenzyme for different gene classes.

• T. True
• F. False

Answer: False

All sigma factors compete for the same limited pool of core RNA polymerase. There is one core enzyme; sigma factors are interchangeable subunits. When cellular conditions change (heat shock, nitrogen starvation, stationary phase), the concentration or activity of specific alternative sigma factors changes, and they outcompete sigma⁷⁰ for core polymerase binding. Because all sigma factors share the same pool of core enzyme, upregulating one sigma factor effectively reprograms the entire transcriptional output — a simple but powerful form of global regulation that would be impossible if each sigma factor had its own dedicated polymerase.

Question 5

Why do sigma factors dissociate from the RNA polymerase after initiation rather than remaining attached throughout elongation? What two functional advantages does this dissociation provide?

Model answer: Sigma dissociates because it is not needed for elongation — the core polymerase can synthesize RNA processively once it has cleared the promoter — and its retention would actively interfere with elongation by keeping the polymerase tethered to the promoter sequence. Two functional advantages follow: (1) Recycling — released sigma factor can immediately associate with another core polymerase and initiate at a new promoter, so the cell needs far fewer sigma molecules than core polymerase molecules (sigma acts catalytically with respect to initiation events); (2) Competitive regulation — once sigma dissociates, the free core polymerase can be captured by a different sigma factor, enabling rapid global reprogramming of transcription in response to stress without requiring new synthesis of core enzyme.

The recycling advantage is often underappreciated. During active growth, a few thousand sigma⁷⁰ molecules can service many more core polymerase molecules because each initiation event releases the sigma factor. If sigma remained attached, every elongating polymerase would permanently sequester one sigma molecule, requiring the cell to produce far more sigma protein. The regulatory advantage is equally important: the competition between sigma factors for the core enzyme means that changing the cellular abundance of a single alternative sigma factor can redirect a substantial fraction of total transcriptional capacity to a new gene program, which is energetically efficient and fast.