Difficulty: Intermediate | Prerequisites: Sigma factor and promoter recognition (see companion notes), RNA polymerase structure (alpha₂ beta beta-prime sigma), closed complex concept.
Once RNA polymerase has found and bound the promoter (the closed complex), it must pry apart the DNA strands to access the template before transcription can begin. This section covers how the enzyme transitions from the closed complex to the open complex through a process called isomerization, the dramatic structural rearrangements that accompany it, and how RNA polymerase initiates RNA synthesis without needing a primer. This is where promoter binding becomes actual transcription, so it bridges recognition (covered in the companion notes) and elongation.
The closed-to-open complex transition (isomerization) involves melting of DNA between positions -11 and +2 and large structural changes in the polymerase, including clamping of the downstream DNA and displacement of sigma region 1.1 from the active site cleft. Unlike the closed complex, the open complex is essentially irreversible. RNA polymerase, unlike DNA polymerase, can start a new chain without a primer.
Closed complex
The initial complex formed when RNA polymerase binds the promoter with the DNA still fully double-stranded. In simple terms, the enzyme is sitting on the promoter but has not yet opened the DNA. Formation of the closed complex is readily reversible: polymerase can dissociate just as easily as it can proceed.
Open complex
The complex in which RNA polymerase has melted (separated) the DNA strands around the transcription start site, exposing the template strand for RNA synthesis. Think of it as the enzyme having unzipped the DNA and locked itself in place, ready to start transcribing.
Isomerization
The spontaneous, energetically favourable conformational change that converts the closed complex into the open complex. For sigma-70-containing holoenzyme, this does not require ATP hydrolysis. In simple terms, the enzyme and DNA rearrange themselves into a lower-energy state that happens to separate the strands.
Molecular mimic
A structural element that resembles another molecule closely enough to occupy the same binding site. Sigma region 1.1 is highly negatively charged, mimicking DNA, and occupies the positively charged active centre cleft until real DNA displaces it.
Active centre cleft
The catalytic core of RNA polymerase, located at the base of the enzyme's pincers (between beta and beta-prime subunits). This is where phosphodiester bond formation occurs.
Transcription bubble
The region of melted (single-stranded) DNA within the open complex, spanning positions -11 to +2 relative to the transcription start site. The double helix re-forms at -11 behind the enzyme.
In the closed complex, RNA polymerase sits on the promoter with the DNA fully double-stranded.
To begin transcription, the enzyme must become more intimately engaged with the promoter by melting the DNA.
The melting ("transcription bubble") spans positions -11 to +2 relative to the transcription start site.
For sigma-70 holoenzyme, the closed-to-open transition is called isomerization.
Isomerization is a spontaneous conformational change; it does not require energy from ATP hydrolysis.
It is driven by the enzyme-DNA complex falling into a more energetically favourable state.
Two specific bases in the non-template strand of the -10 element (A11 and T7) flip out of their base-stacking interactions and insert into pockets in the sigma protein, where they form more favourable contacts.
By stabilising the single-stranded form of the -10 element, these contacts drive melting of the surrounding promoter region.
The closed complex is readily reversible: polymerase can dissociate from the promoter just as easily as it can proceed to the open complex.
Isomerization (open complex formation) is essentially irreversible. Once complete, transcription will typically initiate.
Regulation can still be imposed after isomerization in some cases, but the default outcome is that transcription proceeds.
RNA polymerase is a claw-shaped enzyme with a channel running between its pincers.
The active site sits at the base of the pincers, in the active centre cleft, and is formed by regions of both the beta and beta-prime subunits.
Five channels service the enzyme:
NTP uptake channel: ribonucleotides enter the active centre.
RNA exit channel: the growing RNA chain leaves the enzyme during elongation.
Downstream DNA channel: double-stranded DNA (ahead of the enzyme, yet to be transcribed) enters the active centre cleft between the pincers. Strands separate from position +3.
Non-template strand (NT) channel: the non-template strand exits the cleft and travels across the enzyme surface.
Template strand (T) channel: the template strand exits through this channel. The double helix re-forms at -11 in the upstream DNA behind the enzyme.
Two major structural changes occur upon isomerization:
The pincers clamp tightly down on the downstream DNA, locking the enzyme onto the template.
Sigma region 1.1 shifts approximately 50 Å from inside the active centre cleft to the outside of the enzyme, freeing the cleft for the template DNA strand.
When not bound to DNA, sigma region 1.1 sits in the active centre cleft of the holoenzyme.
Region 1.1 is highly negatively charged, just like DNA.
The active centre cleft is highly positively charged.
Region 1.1 therefore acts as a molecular mimic of DNA, occupying the cleft until the real template displaces it during open complex formation.
This is a built-in placeholder mechanism: the cleft is never "empty" and waiting, which may protect it or regulate access.
RNA polymerase can initiate a new RNA chain de novo on a DNA template. It does not need a primer.
This contrasts with DNA polymerase, which can only extend an existing polynucleotide chain and therefore always requires a primer (typically a short RNA molecule).
For RNA polymerase to initiate, the DNA template must be brought into the active site and held stably in a helical conformation. The first ribonucleotide must be positioned on the template, and the next NTP must be presented with the correct geometry for phosphodiester bond formation.
No mathematical formulas in this section, but be prepared to sketch or label:
A diagram of the open complex showing the transcription bubble (-11 to +2), the five channels of RNA polymerase, and the paths of the template and non-template strands.
The position of sigma region 1.1 in the closed complex (inside the cleft) versus the open complex (outside the enzyme, shifted ~50 Å).
Understanding isomerization is relevant to antibiotic design. Rifampicin, for instance, targets the beta subunit of bacterial RNA polymerase and blocks the RNA exit channel, trapping short transcripts. Knowing the structural details of the enzyme's channels and conformational changes helps explain why such drugs are effective and how resistance mutations arise.
Students frequently assume that open complex formation requires ATP. For sigma-70 holoenzyme, isomerization is spontaneous and does not require ATP hydrolysis. (Note: some alternative sigma factors do require ATP for this step.)
A common error is confusing the reversibility of the two stages. The closed complex is readily reversible; the open complex is essentially irreversible. Not the other way round.
Students sometimes think sigma region 1.1 is discarded or degraded during isomerization. It is not. It simply relocates from inside the active centre cleft to the outside of the enzyme.
Another mix-up: students may say RNA polymerase needs a primer. It does not. DNA polymerase requires a primer; RNA polymerase initiates de novo.
⚠️ The reversibility distinction (closed = reversible, open = essentially irreversible) is a classic exam question.
⚠️ Know the five channels of RNA polymerase and what each one carries (NTP uptake, RNA exit, downstream DNA, NT strand, T strand).
⚠️ The molecular mimic concept (sigma region 1.1 mimicking DNA in the active centre cleft) is a frequently tested detail.
⚠️ Be prepared to explain why RNA polymerase does not need a primer but DNA polymerase does.
⚠️ The specific bases that flip out (A11 and T7 in the non-template strand) and their role in driving melting may appear as short-answer or fill-in-the-blank questions.
True or False: Isomerization from the closed to the open complex requires ATP hydrolysis when sigma-70 is the sigma factor.
Fill in the blank: DNA melting in the open complex spans positions _____ to _____ relative to the transcription start site.
True or False: Formation of the closed complex is essentially irreversible.
Fill in the blank: Sigma region 1.1 acts as a molecular mimic of ________ because it is highly negatively charged.
True or False: RNA polymerase requires a primer to initiate transcription.
Answers: 1. False (it is spontaneous, no ATP required). 2. -11 to +2. 3. False (the closed complex is readily reversible; the open complex is essentially irreversible). 4. DNA. 5. False (RNA polymerase initiates de novo).
Q: What is isomerization in the context of transcription initiation, and does it require energy input?
A: Isomerization is the spontaneous conformational change that converts the closed complex (DNA double-stranded) into the open complex (DNA melted around the start site). For sigma-70 holoenzyme, it does not require ATP hydrolysis; it is driven by the complex falling into a more energetically favourable state.
Q: Describe the two major structural changes in RNA polymerase that occur during the closed-to-open complex transition.
A: First, the pincers at the front of the enzyme clamp tightly down on the downstream DNA. Second, sigma region 1.1 shifts approximately 50 Å from inside the active centre cleft to the outside of the enzyme, freeing the cleft for the template DNA strand.
Q: Why is sigma region 1.1 described as a "molecular mimic" of DNA?
A: Region 1.1 is highly negatively charged, just like DNA. It occupies the positively charged active centre cleft in the holoenzyme (before DNA binding), effectively standing in for DNA until the template strand displaces it during open complex formation.
Q: Name the five channels in RNA polymerase and state what each carries.
A: NTP uptake channel (ribonucleotides in), RNA exit channel (growing RNA out), downstream DNA channel (dsDNA entering the cleft), non-template strand channel (NT strand out, across enzyme surface), template strand channel (template strand out). The double helix re-forms at -11 upstream.
Q: Why does RNA polymerase not require a primer, while DNA polymerase does?
A: RNA polymerase can initiate a new RNA chain de novo by positioning the first ribonucleotide on the template and catalysing the first phosphodiester bond. DNA polymerase can only extend an existing strand and therefore needs a pre-existing primer (usually a short RNA) with a free 3' OH to add nucleotides to.
Q: Which two specific bases flip out of the non-template strand during isomerization, and what is the consequence?
A: A11 and T7 in the non-template strand of the -10 element flip out and insert into pockets in the sigma protein. These favourable contacts stabilise the single-stranded form and drive melting of the promoter region.
Isomerization connects to gene regulation: some repressors work by blocking the closed-to-open transition rather than preventing initial binding. The concept of a molecular mimic (sigma region 1.1) appears again in anti-sigma factors and phage proteins that hijack host transcription. The primer requirement distinction between RNA and DNA polymerase is foundational to understanding DNA replication (Chapter 9) and ties into why Okazaki fragments on the lagging strand each need a new RNA primer.
open complex, closed complex, isomerization, transcription bubble, DNA melting, promoter melting, sigma region 1.1, molecular mimic, active centre cleft, RNA polymerase channels, NTP uptake channel, RNA exit channel, downstream DNA channel, non-template strand channel, template strand channel, pincers, claw-shaped enzyme, base flipping, A11, T7, de novo initiation, primer-independent, RNA polymerase vs DNA polymerase, transcription initiation, bacterial transcription, sigma-70