Difficulty: Intermediate | Prerequisites: Lac operon, trp operon, basics of transcription and translation, RNA structure.
Tags: riboswitch, ara operon, AraC, merT operon, MerR, sigma factor, sigma factor switching, alternative sigma factor, sporulation, feedback translational control, ribosomal protein operon, Shine-Dalgarno, 5' UTR, catabolite repression, arabinose, mercury regulation, Bacillus subtilis, E. coli
Beyond the classic lac and trp operons, bacteria use a wide range of regulatory strategies to adapt gene expression to shifting environments. This set of notes covers five additional mechanisms: riboswitches (RNA-level sensing), the ara operon (a dual-function regulator), the merT operon (metal-responsive regulation), sigma factor switching (swapping the promoter-recognition subunit of RNA polymerase), and feedback translational control (where ribosomal proteins regulate their own synthesis). Each illustrates a different point in the gene-expression pipeline where bacteria can exert control, from mRNA folding to polymerase assembly to translation.
Bacteria regulate genes at every level. Riboswitches use mRNA structure to sense metabolites directly. The ara operon has a regulator (AraC) that can act as both an activator and a repressor. The merT operon responds to mercury via conformational changes in DNA. Sigma factor switching redirects RNA polymerase to entirely different promoters. Ribosomal proteins can block their own translation when rRNA runs low.
Riboswitch
A regulatory element in the 5' untranslated region (UTR) of an mRNA that directly binds a small molecule (metabolite), causing a conformational change that affects transcription or translation. In simple terms, it is an RNA-based sensor built into the message itself, with no protein required.
Shine-Dalgarno sequence
The ribosome-binding site on bacterial mRNA, located upstream of the start codon. Riboswitches can regulate translation by sequestering this sequence inside a secondary structure so the ribosome cannot attach.
AraC
The regulatory protein of the ara (arabinose) operon. It acts as a repressor in the absence of arabinose (forming a DNA loop) and as an activator in the presence of arabinose. Think of it as a single protein that can play both roles depending on which small molecule is around.
Arabinose
A five-carbon sugar whose metabolism is controlled by the ara operon. When arabinose is present (and glucose is absent), the operon is activated.
MerR
The regulatory protein of the merT operon, which controls mercury resistance genes. When mercury binds MerR, the protein induces a conformational change in the DNA that allows RNA polymerase to access the promoter.
Sigma (σ) factor
The subunit of bacterial RNA polymerase responsible for promoter recognition. Different sigma factors recognise different promoter sequences, so swapping sigma factors redirects the polymerase to a new set of genes.
Alternative sigma factor
Any sigma factor other than the housekeeping σ70 (in E. coli). Examples include σ32 (heat shock), σ38 (stationary phase), and the cascade of sigma factors used during Bacillus subtilis sporulation.
Feedback translational control
A mechanism by which ribosomal proteins, when produced in excess of available rRNA, bind to their own mRNA and block its translation. In simple terms, the protein senses that it has no rRNA partner and shuts down its own production until rRNA catches up.
Riboswitches are entirely RNA-based regulatory elements. No protein factor is needed for sensing.
They reside in the 5' UTR of the mRNA they regulate, upstream of the coding sequence.
When a specific small molecule (the ligand) binds the aptamer domain of the riboswitch, the RNA refolds into a different conformation.
Two main regulatory outcomes:
Transcription termination: Ligand binding stabilises a terminator hairpin in the mRNA, causing RNA polymerase to stop before it reaches the structural genes.
Translation inhibition: Ligand binding sequesters the Shine-Dalgarno sequence within a secondary structure, preventing ribosome binding and translation initiation.
Riboswitches have been found controlling genes for vitamin biosynthesis, amino acid metabolism, and nucleotide pathways, among others.
The ara operon encodes enzymes for arabinose catabolism (araB, araA, araD).
AraC is the sole regulatory protein, but it behaves differently depending on whether arabinose is present.
Without arabinose (and without glucose):
AraC forms a dimer that binds two distant DNA sites (araO2 and araI1), creating a DNA loop.
This loop blocks RNA polymerase access to the promoter. The operon is off.
With arabinose (and without glucose):
Arabinose binds AraC and changes its conformation. The dimer now binds araI1 and araI2 (adjacent sites at the promoter) instead of looping.
In this configuration, AraC acts as an activator, recruiting RNA polymerase to the promoter.
CAP-cAMP is also required for maximal expression, just as in the lac operon. If glucose is present, cAMP is low, CAP is inactive, and expression stays minimal.
The ara operon is a useful exam contrast to the lac operon because AraC is a single protein that toggles between repressor and activator roles, whereas the lac system uses separate proteins for each function (lac repressor for repression, CAP for activation).
The merT operon encodes genes for mercury detoxification and resistance.
MerR protein is the regulator. It is unusual because it remains bound to the promoter whether mercury is present or not.
Without mercury:
MerR sits on the DNA but holds the promoter in a suboptimal conformation. RNA polymerase binds but cannot initiate transcription effectively. The operon is silenced.
With mercury:
Mercury ions bind to MerR, causing a conformational change in the MerR-DNA complex.
This change untwists the DNA at the promoter, realigning the -10 and -35 elements so that RNA polymerase can now form an open complex and begin transcription.
This is a distinctive mechanism: regulation occurs not by blocking or recruiting polymerase, but by physically reshaping the DNA to make an already-bound polymerase functional.
The core RNA polymerase (α2ββ'ω) is the same across conditions, but the sigma subunit determines which promoters it recognises.
By replacing one sigma factor with another, the cell redirects transcription to an entirely different regulon.
Examples:
E. coli σ32: Produced under heat shock, directing polymerase to heat-shock gene promoters (chaperones, proteases).
Bacillus subtilis sporulation: A cascade of alternative sigma factors (σF, σE, σG, σK) activates successive stages of sporulation in a precise temporal order, with different factors active in the mother cell versus the forespore.
Phage infection: Some bacteriophages encode their own sigma factors (or anti-sigma factors) to redirect the host polymerase away from host genes and toward phage genes.
Sigma factor switching is a powerful strategy because it reprogrammes the entire transcriptional output of the cell in one step, rather than adjusting operons one at a time.
Ribosomal proteins and rRNA must be produced in balanced amounts to assemble functional ribosomes.
Each ribosomal protein operon contains a "sensor" protein: one of the ribosomal proteins encoded by that operon that can also bind its own mRNA.
How it works:
Under normal conditions, newly made ribosomal proteins bind to newly made rRNA and are incorporated into ribosomes. Translation of the operon mRNA proceeds freely.
If rRNA becomes limiting (e.g. during slow growth), free ribosomal proteins accumulate. The sensor protein binds a structure on its own mRNA that mimics the rRNA binding site, blocking the ribosome from translating the operon.
When rRNA production recovers, the sensor protein shifts back to rRNA, freeing the mRNA for translation again.
This is an elegant example of molecular mimicry: the mRNA binding site resembles the rRNA binding site, and the same protein recognises both. The result is automatic balancing of ribosomal protein output to match rRNA availability.
Riboswitches are targets for novel antibiotics. If a drug can mimic the natural ligand and lock a riboswitch in the "off" position, it can shut down essential bacterial genes. Mercury-resistance operons like merT are studied in bioremediation, where engineered bacteria are used to clean up mercury-contaminated environments. Sigma factor engineering is a growing tool in synthetic biology for building multi-stage gene-expression programmes in industrial microorganisms.
Students sometimes think riboswitches require a protein sensor. They do not. The mRNA itself is both the sensor and the switch.
AraC is often mistakenly described as only an activator. In reality, without arabinose it actively represses the operon through DNA looping.
MerR is unusual in that it does not leave the DNA when inactive. Students used to the lac repressor model may assume MerR dissociates; it does not. It stays bound and changes the DNA's shape upon mercury binding.
Sigma factor switching does not destroy or degrade the previous sigma factor (necessarily). Both can coexist; the new sigma simply competes for binding to core polymerase, often aided by higher expression levels or anti-sigma factors that sequester the old one.
⚠️ Be able to distinguish riboswitch regulation at the transcription level (terminator hairpin) from regulation at the translation level (Shine-Dalgarno sequestration). Exams may give you a scenario and ask which type is operating.
⚠️ Know how AraC switches from repressor (DNA loop) to activator (binding adjacent sites) depending on arabinose. This is a favourite exam comparison with the lac operon.
⚠️ Understand that MerR regulation works by DNA distortion, not by blocking or recruiting polymerase.
⚠️ Be able to name at least two examples of alternative sigma factors and the conditions that trigger them (heat shock, sporulation, phage infection).
⚠️ For feedback translational control, know the concept of molecular mimicry: the mRNA site resembles the rRNA site, and the same protein recognises both.
True or false: Riboswitches require a protein co-factor to sense their ligand.
Fill in the blank: In the ara operon, AraC acts as a ______ in the absence of arabinose and an ______ in its presence.
True or false: MerR dissociates from the DNA when mercury is absent.
Fill in the blank: Alternative sigma factors redirect RNA polymerase to different ______.
True or false: In feedback translational control, ribosomal proteins bind their own mRNA at a site that mimics rRNA.
Answers: 1. False. 2. Repressor; activator. 3. False (MerR stays bound). 4. Promoters. 5. True.
Q: A riboswitch in the 5' UTR of a vitamin biosynthesis gene binds the vitamin product and forms a terminator hairpin. What happens to gene expression when vitamin levels are high?
A: High vitamin levels mean more ligand is available to bind the riboswitch. The terminator hairpin forms, and RNA polymerase terminates transcription before reaching the biosynthesis genes. Gene expression is shut off, preventing wasteful overproduction.
Q: How does the ara operon's regulation differ from the lac operon's regulation in terms of the number of regulatory proteins involved?
A: The lac operon uses two separate regulatory proteins: the lac repressor (negative control) and CAP (positive control). The ara operon uses a single protein, AraC, which toggles between a repressor role (DNA looping without arabinose) and an activator role (adjacent-site binding with arabinose). Both operons additionally require CAP-cAMP for full activation.
Q: Explain why sigma factor switching is described as a "global" regulatory strategy.
A: Because replacing one sigma factor with another changes which promoters the entire pool of RNA polymerase can recognise. Rather than adjusting individual operons one by one, the cell reprogrammes transcription genome-wide in a single step.
Q: In feedback translational control, what would happen if a ribosomal protein lost its ability to bind its own mRNA but could still bind rRNA?
A: The protein would continue to be incorporated into ribosomes normally, but it could no longer autoregulate. Its mRNA would be translated constitutively, producing excess protein whenever rRNA levels dropped. This would waste cellular resources and could potentially interfere with ribosome assembly stoichiometry.
Riboswitches connect to the attenuation mechanism in the trp operon: both use RNA secondary structure to regulate transcription, but riboswitches sense a metabolite directly while attenuation senses amino acid levels indirectly via ribosome stalling. The ara operon's use of CAP-cAMP links it to the lac operon and the broader theme of catabolite repression. Sigma factor switching during the SOS response connects to lambda phage prophage induction (covered in the lambda phage notes), where DNA damage triggers a cascade affecting both host and phage gene expression. Feedback translational control is a specific case of autogenous regulation, a principle that also appears in the cI autoregulatory loop of lambda phage.
riboswitch, aptamer, 5' UTR, Shine-Dalgarno, terminator hairpin, RNA regulatory element, ara operon, AraC, arabinose, DNA looping, dual regulator, merT operon, MerR, mercury resistance, DNA distortion, sigma factor, alternative sigma factor, sigma-32, sigma-70, heat shock, sporulation, Bacillus subtilis, phage sigma factor, feedback translational control, ribosomal protein operon, molecular mimicry, autogenous regulation, rRNA, catabolite repression, CAP-cAMP, prokaryotic gene regulation, molecular biology