Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 1 / Section B / 4

RNA structure controls which ribosome-binding and decoding events are kinetically accessible. A bacterial messenger RNA can base-pair with itself or other RNA molecules, forming alternative structures whose populations depend on sequence, ligand concentration, temperature and translating ribosomes. Its sequence consequently encodes both a polypeptide and part of the mechanism controlling production of that polypeptide.

At initiation, a stem can sequester the Shine-Dalgarno sequence or start region, reducing pairing with the anti-Shine-Dalgarno region of the small-subunit ribosomal RNA and preventing productive initiation. Opening that structure makes the same coding sequence more accessible. Start-site accessibility also matters in bacteria whose transcripts do not use a canonical Shine-Dalgarno sequence; not all bacterial initiation can be reduced to this one motif. The regulatory variable is often competition between folding, unfolding and ribosome binding rather than a permanently closed structure.

A riboswitch couples a ligand-binding RNA domain to an expression platform that can alternatively expose or hide a translation-initiation region. Binding of a metabolite can stabilize one structure without requiring a separate regulatory protein. Direct binding of thiamine derivatives to bacterial messenger RNA established an RNA-based route to metabolic regulation.^[ref] In an RNA thermometer, heating destabilizes a stem and exposes the initiation site. The FourU thermometer provides a concrete example in which uridines help pair with the ribosome-binding region of a heat-responsive transcript.^[ref] Sequence specificity and a biologically relevant transition range distinguish such a sensor from the nonspecific fact that all RNA eventually denatures on heating.

Trans-acting small regulatory RNAs can remodel target messenger RNA structure by base pairing. Some block initiation directly, often aided by the RNA-binding chaperone Hfq; others disrupt an inhibitory stem and activate translation. Pairing can also expose an ribonuclease cleavage site or recruit degradation machinery. Protein-binding regulators provide another route: a ribosomal protein can recognize a structured element in its own polycistronic transcript and repress excess ribosomal-protein production. Occupancy of a structured site thus links translation to stoichiometric demand.

In a polycistronic messenger RNA, translation of an upstream coding region can unwind a structure masking a downstream initiation site, producing translational coupling. During elongation, the ribosome can unwind many structures, but a sufficiently resistant downstream RNA pseudoknot or stem, combined with an appropriate slippery sequence, can promote programmed ribosomal frameshifting. This changes the reading frame and hence the ratio of distinct protein products. Structure can also influence recoding or pause positions; a predicted stable hairpin alone does not establish that a physiologically meaningful pause occurs.

RNA structure further couples translation to transcription. In the tryptophan attenuation in bacterial gene regulation mechanism, rapid or stalled translation of a leader peptide changes which alternative RNA stems form. This decides formation of a transcription terminator and therefore how much downstream transcript is available. The immediate switch is termination of transcription, even though ribosome movement and structured RNA generate the decision. Likewise, intrinsic terminator formation, transcript degradation and ribosome protection change protein output without necessarily changing translation efficiency per intact transcript.

A convincing experiment measures both transcript abundance and protein output. Fuse the regulatory region to a reporter while retaining a control promoter; compare ligand or temperature conditions with a construct lacking the proposed structural element. A mutation disrupting one side of a stem, followed by a compensatory mutation restoring pairing with a changed sequence, distinguishes pairing from a single nucleotide-specific contact. RNA structure probing can test accessibility directly, and toeprinting can report initiation-complex occupancy. Include controls for reporter folding, growth, RNA half-life and effects of the perturbation on the ribosome itself. These tests connect a structural transition to the appropriate mechanistic step rather than assuming every change in protein abundance is a change in initiation.