Difficulty: Intermediate | Prerequisites: Part 1 (Central Dogma, Gene Structure), basic enzyme kinetics.
This section covers how bacteria transcribe their genes, from the moment RNA polymerase finds a promoter to the release of the finished transcript. Bacterial transcription is simpler than its eukaryotic counterpart, which makes it an ideal system for understanding core principles. It is also medically relevant: antibiotics like rifampicin target the bacterial transcription machinery. If you understand how sigma factors direct promoter recognition, how elongation proceeds, and how termination works via Rho-dependent and Rho-independent pathways, you have the toolkit for the regulation topics (lac operon, trp operon) that come next.
Bacterial RNA polymerase is a multi-subunit enzyme that requires a sigma (σ) factor to recognise promoters at conserved -35 and -10 elements. Transcription proceeds through initiation (with abortive cycling), elongation (~60 nt/sec with proofreading), and termination (either Rho-dependent or intrinsic). Accessory proteins like NusA, NusG, and Gre factors fine-tune the process.
RNA polymerase (bacterial)
The enzyme that catalyses RNA synthesis from a DNA template, working in the 5' to 3' direction using ribonucleoside triphosphates (NTPs) as substrates. In simple terms, this is the molecular machine that reads DNA and builds an RNA copy.
Holoenzyme
The complete, transcription-competent form of bacterial RNA polymerase: the core enzyme (α₂ββ'ω) plus a sigma factor. Think of it as the "ready to go" version of the polymerase.
Core enzyme
Bacterial RNA polymerase without the sigma factor. It can synthesise RNA but cannot recognise promoters on its own.
Sigma (σ) factor
A dissociable subunit of bacterial RNA polymerase that confers promoter specificity. In simple terms, the sigma factor is the "address reader" that tells the polymerase where to start.
σ70
The primary (housekeeping) sigma factor in E. coli, responsible for transcription of most genes during normal growth.
Regulon
A group of genes controlled by the same sigma factor or regulatory protein, even if they are scattered across the genome. Think of it as a coordinated response programme.
-10 element (Pribnow box)
A conserved A/T-rich promoter sequence located roughly 10 base pairs upstream of the transcription start site, recognised by σ2 domain. In simple terms, this is the "melting zone" where the DNA strands first separate.
-35 element
A conserved promoter sequence located roughly 35 base pairs upstream of the start site, recognised by σ domain 4. It helps position the polymerase correctly.
UP element
An optional A/T-rich sequence upstream of the -35 box that enhances promoter strength by interacting with the α subunit of RNA polymerase.
Abortive initiation
The repeated synthesis and release of short RNA transcripts (fewer than ~8 nucleotides) before the polymerase successfully transitions to productive elongation. Think of it as the engine stalling a few times before it catches.
Promoter escape
The transition from initiation to elongation, during which the sigma factor dissociates from the core enzyme.
Two-metal ion mechanism
The catalytic strategy used by RNA polymerase (and many other polymerases), employing two Mg²⁺ ions to facilitate nucleotide addition.
Pyrophosphorolysis
A proofreading mechanism in which RNA polymerase reverses the synthesis reaction, removing a misincorporated nucleotide by re-adding pyrophosphate (PPi).
Backtracking
A proofreading event in which RNA polymerase slides backward along the template, extruding the 3' end of the nascent RNA, allowing cleavage and removal of the mismatched nucleotide.
Rho (ρ) factor
An ATP-dependent helicase that binds to rut (Rho utilisation) sites on nascent RNA and translocates along the transcript to catch up with a paused polymerase, causing transcription termination.
Rut site
A C-rich, G-poor sequence on the nascent RNA where Rho factor initially binds. In simple terms, it is the landing pad for the termination machinery.
Intrinsic termination (Rho-independent)
Transcription termination caused by a G-C rich hairpin structure in the RNA followed by a run of uridines, without the need for any protein factor. The hairpin pauses the polymerase; the weak rU-dA base pairs destabilise the RNA-DNA hybrid, causing release.
NusA
A bacterial accessory protein that regulates transcriptional pausing, elongation rate, and termination.
NusG
A bacterial accessory protein that couples transcription with translation and stimulates Rho-dependent termination.
Gre factors (GreA, GreB)
Bacterial proteins that stimulate the intrinsic hydrolytic (cleavage) activity of RNA polymerase, rescuing backtracked complexes. Think of them as the "rescue crew" for stalled polymerase.
Rifampicin
An antibiotic that inhibits bacterial RNA polymerase by blocking the RNA exit channel, preventing initiation. In simple terms, it jams the machine before it can get going.
Bacterial RNA polymerase is a large, multi-subunit complex.
Core enzyme composition: α₂ββ'ω.
The core enzyme carries out RNA synthesis but needs a sigma factor to find promoters.
Holoenzyme = core enzyme + σ factor.
RNA synthesis proceeds 5' to 3', using NTPs (ATP, GTP, CTP, UTP) as building blocks.
Bacterial promoters contain two key conserved elements:
-10 element (Pribnow box): consensus TATAAT; A/T-rich for easy strand separation (melting). Recognised by σ region 2.
-35 element: consensus TTGACA; recognised by σ domain 4. Sets the spacing and orientation for polymerase binding.
UP elements are optional sequences upstream of the -35 box that boost transcription by providing an additional contact surface for the α subunit.
The spacing between the -10 and -35 elements (~17 bp) is critical; altered spacing weakens or abolishes promoter function.
Different sigma factors recognise different promoter motifs, enabling the cell to switch on entire sets of genes (regulons) in response to stress, starvation, or developmental signals.
σ70: housekeeping genes in E. coli.
Alternative sigma factors (e.g., σ32 for heat shock, σ54 for nitrogen metabolism) redirect RNA polymerase to specialised promoters.
Step 1 – Holoenzyme formation: σ factor binds the core enzyme.
Step 2 – Promoter recognition: holoenzyme scans DNA; σ factor recognises the -35 and -10 elements.
Step 3 – DNA melting (open complex formation): the σ2 domain facilitates local unwinding of DNA at the -10 element, creating the transcription bubble (~12–14 bp).
Step 4 – Abortive initiation: the polymerase repeatedly synthesises and releases short transcripts (<8 nt). This is normal and can happen several times before successful elongation.
Step 5 – Promoter escape: once a transcript of ~8–10 nt is made, the σ factor dissociates, and the core enzyme transitions to processive elongation.
Regulation at initiation:
Activator proteins bind upstream or near the promoter to enhance RNA polymerase recruitment (e.g., CAP protein at the lac promoter).
Repressor proteins bind at or near the promoter to block polymerase access.
Nucleoid-associated proteins (e.g., H-NS, IHF) bend or compact DNA, influencing promoter accessibility.
Rifampicin blocks initiation by physically obstructing the RNA exit channel.
Once the polymerase escapes the promoter, it enters processive elongation.
Elongation rate: approximately 60 nucleotides per second in bacteria.
Catalytic mechanism: the two-metal ion mechanism, using two Mg²⁺ ions to position the incoming NTP and catalyse phosphodiester bond formation.
Fidelity comparison:
DNA replication: ~10⁻⁹ to 10⁻¹⁰ errors per nucleotide (extremely accurate).
Transcription: ~10⁻⁴ to 10⁻⁶ errors per nucleotide (lower fidelity, tolerated because RNA is transient and does not alter the heritable genome).
Proofreading mechanisms:
Pyrophosphorolysis: the reverse of the synthesis reaction, removing the last-added nucleotide.
Backtracking: the polymerase reverses along the template, extruding the 3' end of the RNA, which is then cleaved. Gre factors stimulate this cleavage.
Pausing: RNA polymerase pauses at specific sequences or when it encounters obstacles (e.g., DNA-bound proteins). Pausing is regulated by NusA and NusG and can influence both elongation efficiency and termination.
Two main mechanisms in bacteria:
Rho-independent (intrinsic) termination:
A G-C rich palindromic sequence in the RNA folds into a stable hairpin structure.
Immediately downstream of the hairpin lies a run of uridines (in the RNA) / adenines (in the template DNA).
The hairpin causes the polymerase to pause.
The weak rU-dA base pairs in the RNA-DNA hybrid destabilise the complex, and the transcript dissociates.
Rho-dependent termination:
Rho factor (a hexameric ring-shaped ATPase/helicase) binds to rut sites on the nascent RNA.
Rho uses ATP hydrolysis to translocate along the RNA in the 5' to 3' direction.
When Rho catches up to a paused polymerase, it unwinds the RNA-DNA hybrid, causing release.
NusG stimulates Rho's activity and helps couple transcription to translation; if ribosomes fall behind (e.g., because of a premature stop codon), Rho can access the exposed RNA and trigger termination.
NusA: promotes pausing and can enhance both intrinsic termination and antitermination (context-dependent).
NusG: has dual roles; it enhances Rho-dependent termination and couples transcription with translation by bridging the ribosome and the polymerase.
Gre factors: rescue backtracked polymerase complexes by stimulating the intrinsic endonuclease activity, allowing the polymerase to resume elongation.
DNA-binding proteins (activators, repressors): recognise specific operator or enhancer sequences to modulate transcription initiation and, in some cases, termination.
Elongation rate: ~60 nt/sec (bacteria).
Transcription error rate: ~10⁻⁴ to 10⁻⁶ per nucleotide.
DNA replication error rate: ~10⁻⁹ to 10⁻¹⁰ per nucleotide.
Abortive transcripts: <8 nt.
Prokaryotic mRNA half-life: ~10 minutes (from Part 1, for context).
Rifampicin is a front-line antibiotic for treating tuberculosis. It works by jamming bacterial RNA polymerase at the initiation stage, and because eukaryotic RNA polymerases are structurally different, human cells are largely unaffected. Understanding Rho-dependent termination also matters in biotechnology: synthetic biology often relies on inserting terminators to control gene circuits, and choosing between intrinsic and Rho-dependent terminators affects circuit behaviour.
Students often confuse the sigma factor with a transcription factor. Sigma is a subunit of the polymerase itself (part of the holoenzyme), not a separate DNA-binding regulatory protein.
Abortive initiation is sometimes mistaken for a sign that something has gone wrong. It is a normal part of the initiation process that all bacterial promoters undergo.
The -10 and -35 elements are not the start site. The -10 element is centred 10 bp upstream of the +1 transcription start site; the -35 element is 35 bp upstream. The numbers refer to position relative to +1.
Rho-independent termination requires no protein at all, just the RNA secondary structure and the poly-U tract. Students sometimes think "termination factor" is always needed.
⚠️ Be able to draw or describe the steps of bacterial transcription initiation: holoenzyme formation → promoter recognition → open complex → abortive initiation → promoter escape.
⚠️ Know the consensus sequences for the -10 and -35 elements and which σ domain recognises each.
⚠️ Expect a comparison question on Rho-dependent vs Rho-independent termination. Know the structural requirements for each (hairpin + poly-U vs rut site + Rho translocation).
⚠️ Rifampicin's mechanism of action (blocks initiation) is a common exam question, often contrasted with other transcription inhibitors.
⚠️ Understand why transcription errors are tolerated (RNA is transient, no heritable impact) while replication errors are not.
True or false: The core enzyme of bacterial RNA polymerase can recognise promoters without a sigma factor.
Fill in the blank: The -10 element is also called the _______ box.
True or false: Rho-independent termination requires an RNA hairpin followed by a poly-U stretch.
Fill in the blank: Bacterial RNA polymerase adds nucleotides at a rate of approximately _______ per second.
True or false: Rifampicin inhibits bacterial transcription by blocking elongation.
Answers: 1. False (sigma is required for promoter recognition). 2. Pribnow. 3. True. 4. 60 nucleotides. 5. False (it blocks initiation).
Q: Describe the role of the sigma factor in bacterial transcription. What happens to it after initiation?
A: The sigma factor is a dissociable subunit of RNA polymerase that confers promoter specificity. It recognises and binds to the -35 and -10 promoter elements, facilitates DNA melting at the -10 region, and guides the polymerase to the correct transcription start site. After promoter escape (once a transcript of ~8–10 nt is produced), the sigma factor dissociates from the core enzyme, which then proceeds with elongation on its own.
Q: Compare Rho-dependent and Rho-independent termination in bacteria.
A: Rho-independent (intrinsic) termination relies on an RNA hairpin formed by a G-C rich palindrome, followed by a run of uridines. The hairpin pauses the polymerase, and the weak rU-dA hybrid destabilises the complex, causing dissociation. Rho-dependent termination requires the Rho protein, which binds rut sites on the nascent RNA, translocates 5' to 3' using ATP hydrolysis, and unwinds the RNA-DNA hybrid when it catches a paused polymerase. NusG stimulates Rho activity.
Q: What are backtracking and pyrophosphorolysis, and how do they contribute to transcription fidelity?
A: Both are proofreading mechanisms. Pyrophosphorolysis is the reverse of the synthesis reaction: the polymerase removes the last-added nucleotide by re-incorporating pyrophosphate. Backtracking involves the polymerase sliding backwards along the template, extruding the 3' end of the RNA; the mismatched segment is then cleaved (stimulated by Gre factors). Together, they reduce the transcription error rate.
Q: Why can transcription tolerate a higher error rate than DNA replication?
A: Transcription errors affect individual RNA molecules, which are transient and produced in many copies. A single defective mRNA does not alter the heritable genome or permanently change the cell. Replication errors, by contrast, are passed to all daughter cells and become permanent mutations.
Q: How does rifampicin inhibit bacterial transcription, and why does it not affect eukaryotic cells?
A: Rifampicin binds to the β subunit of bacterial RNA polymerase and physically blocks the RNA exit channel, preventing the enzyme from synthesising transcripts longer than 2–3 nucleotides (blocking initiation). Eukaryotic RNA polymerases are structurally distinct and do not bind rifampicin, so human cells are not affected.
This material connects to Part 1 (Central Dogma) as the detailed molecular mechanism behind the DNA → RNA step in bacteria. It feeds directly into prokaryotic gene regulation topics (lac operon, trp operon, attenuation) where activators, repressors, and termination mechanisms are the key regulatory tools. The bacterial system also serves as a comparative framework for Part 3 (eukaryotic transcription), where the machinery is more complex but the underlying logic of initiation, elongation, and termination is conserved.
bacterial transcription, RNA polymerase, holoenzyme, core enzyme, sigma factor, σ70, sigma 70, promoter, -10 element, Pribnow box, -35 element, UP element, regulon, abortive initiation, promoter escape, open complex, closed complex, elongation, two-metal ion mechanism, proofreading, pyrophosphorolysis, backtracking, Gre factors, GreA, GreB, NusA, NusG, Rho factor, rut site, Rho-dependent termination, Rho-independent termination, intrinsic termination, hairpin terminator, rifampicin, transcription fidelity, transcription error rate, Molecular Biology of the Gene Watson, UCF Molecular Biology II