Difficulty: Intermediate | Prerequisites: RNA polymerase subunit composition (alpha, beta, beta-prime, sigma), basic promoter anatomy (-10, -35 elements), Chapter 13 Part 1 notes.
This material covers how bacterial RNA polymerase finds and locks onto a promoter, with particular focus on the sigma factor's role. You need to understand that the sigma subunit is the component of the holoenzyme responsible for recognising specific DNA sequences at the promoter, and that different regions of sigma contact different promoter elements. This sits at the heart of transcription initiation and feeds directly into gene regulation, since swapping sigma factors changes which genes get transcribed.
The sigma-70 factor has four regions, each responsible for recognising different parts of the promoter. Region 4 uses a helix-turn-helix motif to bind the -35 element, while region 2 recognises the -10 element and plays a key role in DNA melting. The UP-element is the exception: it is recognised by the alpha-CTD, not by sigma.
Sigma factor (sigma-70)
The dissociable subunit of bacterial RNA polymerase holoenzyme that directs the enzyme to promoter sequences. Sigma-70 is the primary, "housekeeping" sigma in E. coli. In simple terms, sigma is the sat-nav that tells RNA polymerase where to start transcribing.
Helix-turn-helix (HTH)
A common DNA-binding structural motif consisting of two alpha helices connected by a short turn. One helix (the recognition helix) inserts into the major groove and reads the DNA sequence; the other makes backbone contacts. Think of it as a molecular hand whose fingers reach into the groove of the DNA.
Alpha-CTD (alpha C-terminal domain)
The carboxy-terminal domain of the alpha subunit of RNA polymerase. Connected to the alpha NTD by a flexible linker, the alpha-CTD recognises the UP-element of the promoter. In simple terms, this is the part of the enzyme that reaches upstream to grab extra promoter contacts for stronger binding.
UP-element
An AT-rich sequence upstream of the -35 element that enhances transcription by providing an additional contact point for the alpha-CTD. Think of it as a "bonus grip" that helps polymerase hold on more tightly.
Sigma region 3.2 (sigma 3/4 linker)
The flexible region of sigma between regions 2 and 4 that accommodates the large physical spacing (~75 Å) needed to bridge the -10 and -35 promoter elements simultaneously.
Extended -10 element
An additional promoter element (upstream of the -10 box) recognised by an alpha helix in sigma region 3. Some promoters that lack a strong -35 element rely on this element instead.
Discriminator
A promoter element recognised by sigma region 1.2, located between the -10 element and the transcription start site.
Sigma-70 is divided into four regions (1 through 4), each with distinct functions.
Regions 2 and 4 are the primary DNA-contact regions: region 2 recognises the -10 element, and region 4 recognises the -35 element.
When sigma is bound in the holoenzyme, regions 2 and 4 are separated by approximately 75 Å, which closely matches the physical distance between the centres of the -10 and -35 elements on a typical sigma-70 promoter.
Region 3 (especially region 3.2, the sigma 3/4 linker) spans the gap between regions 2 and 4, acting as a flexible bridge.
Two helices in sigma region 4 form a helix-turn-helix motif.
One helix inserts into the major groove and makes sequence-specific contacts with bases in the -35 region.
The other helix lies across the top of the groove and contacts the DNA backbone.
The HTH motif is extremely common in DNA-binding proteins; nearly all bacterial transcriptional activators and repressors use it.
The -35 element primarily contributes binding energy to anchor polymerase at the promoter.
The -10 element is recognised by an alpha helix in sigma region 2, but the interaction goes well beyond simple binding.
The -10 region is where DNA melting is initiated during the transition from the closed complex to the open complex, so sigma's contact here has a dual function: recognition and strand separation.
The alpha helix that contacts the -10 region contains several essential aromatic amino acids that interact with bases on the non-template strand and stabilise the melted (single-stranded) DNA.
Two bases in the non-template strand flip out of the helix and insert into pockets in the sigma protein, making favourable contacts that stabilise the unwound state.
Where present, the extended -10 element is recognised by an alpha helix in sigma region 3, which contacts the two specific base pairs that make up this element.
The discriminator element is recognised by sigma region 1.2.
The UP-element is the one promoter element not recognised by sigma.
It is contacted by the alpha-CTD, which is connected to the alpha NTD by a flexible linker.
Because of that flexible linker, the alpha-CTD can reach the UP-element even when it is located well upstream of the -35 region, not immediately adjacent.
The DNA-binding regions of sigma point outward, away from the body of the enzyme, rather than being buried inside it. This orientation makes promoter recognition physically feasible.
The spacing between binding regions on sigma matches the spacing between corresponding promoter elements on DNA.
Sigma factor swapping is one of the ways bacteria respond to environmental stress. For example, during heat shock, an alternative sigma factor (sigma-32 in E. coli) replaces sigma-70 and redirects RNA polymerase to heat-shock gene promoters. Understanding sigma-promoter contacts is essential for designing synthetic promoters in synthetic biology.
Students often assume the -10 and -35 regions serve the same purpose. They do not: the -35 element is primarily about binding energy, while the -10 element is where strand separation begins.
Students sometimes think all promoter elements are recognised by sigma. The UP-element is the exception; it is recognised by the alpha-CTD.
It is a common error to say the HTH motif is unique to sigma. In fact, it is one of the most widespread DNA-binding motifs in biology.
Students may confuse sigma regions with promoter elements. Sigma region 2 contacts the -10 element and sigma region 4 contacts the -35 element, not the other way round.
⚠️ Know which sigma region contacts which promoter element (region 2 → -10; region 4 → -35; region 3 → extended -10; region 1.2 → discriminator).
⚠️ The UP-element / alpha-CTD distinction is a favourite exam question because it breaks the pattern.
⚠️ Be ready to explain why the -10 region's role is "more complicated" than simple binding (it initiates melting, aromatic amino acids stabilise ssDNA, base flipping occurs).
⚠️ The ~75 Å spacing between sigma regions 2 and 4 and its match to the -10/-35 distance is the kind of structural detail examiners love.
True or False: The helix-turn-helix motif in sigma region 4 inserts into the minor groove of DNA.
Fill in the blank: The promoter element recognised by the alpha-CTD rather than sigma is the ________.
True or False: Sigma regions 2 and 4 are separated by approximately 75 Å in the holoenzyme.
Fill in the blank: Two bases in the non-template strand flip out and insert into ________ in the sigma protein during -10 element recognition.
True or False: The discriminator element is recognised by sigma region 4.
Answers: 1. False (major groove). 2. UP-element. 3. True. 4. Pockets. 5. False (sigma region 1.2).
Q: Which sigma-70 regions are responsible for recognising the -10 and -35 promoter elements, respectively?
A: Region 2 recognises the -10 element; region 4 recognises the -35 element.
Q: What structural motif does sigma region 4 use to bind the -35 element, and how does it interact with DNA?
A: A helix-turn-helix motif. One helix inserts into the major groove and makes base-specific contacts; the other lies across the top of the groove and contacts the backbone.
Q: Why is the interaction between sigma and the -10 element described as "more complicated" than simple DNA binding?
A: Because the -10 region is where DNA melting is initiated. The alpha helix in region 2 contains aromatic amino acids that interact with the non-template strand and stabilise the single-stranded form. Two bases flip out and insert into sigma pockets, driving the unwound state.
Q: How does the alpha-CTD reach the UP-element even when it is far upstream of the -35 region?
A: The alpha-CTD is connected to the alpha NTD by a flexible linker, allowing it to extend a considerable distance from the body of the enzyme.
Q: What is the functional significance of the ~75 Å separation between sigma regions 2 and 4?
A: It matches the distance between the centres of the -10 and -35 promoter elements, enabling simultaneous recognition of both elements when sigma is bound in the holoenzyme.
This material connects directly to gene regulation (Chapter 16+), since activator and repressor proteins often work by enhancing or blocking sigma-promoter contacts. It also ties into the broader theme of protein-DNA recognition motifs, which you will encounter again in eukaryotic transcription factors. The concept of base flipping reappears in DNA repair mechanisms.
sigma factor, sigma-70, sigma subunit, RNA polymerase holoenzyme, promoter recognition, helix-turn-helix, HTH motif, -10 element, -35 element, Pribnow box, UP-element, alpha-CTD, alpha C-terminal domain, extended -10, discriminator, sigma region 2, sigma region 4, sigma region 3.2, sigma 3/4 linker, DNA-binding motif, base flipping, promoter binding, transcription initiation, bacterial transcription