It Transcribes and Makes No Protein: Check the RBS Is Not Paired Shut
6 min read · Updated October 30, 2026
A construct that transcribes fine and makes no protein has a short list of causes, and the cheapest one to rule out is that something in the transcript is base-paired over the ribosome binding site.
It is also the mechanism every engineered translational switch runs on. The same question starts both jobs: what in this transcript is complementary to the ribosome's footprint, and how strongly?
The footprint, and why it is −20 to +13
The 30S subunit covers a defined window around the start codon, measured by toeprinting: roughly 20 nucleotides upstream and 13 downstream. That window contains the Shine-Dalgarno sequence, the spacer and the first few codons, and it is the region that has to be single-stranded for initiation.
Anchoring on the start codon is what makes the question answerable. A real 5' UTR usually contains several ATG, GTG or TTG triplets, and the footprint is a different window for each of them — so the start codon you mean has to be the one the analysis uses.
Two transcripts, one base-paired
Take a 70-nucleotide transcript: a leader, then AGGAGG, a six-base spacer and the start codon. The Shine-Dalgarno sits at 26–31, the spacing is 6, the footprint runs 18–50 — and nothing in the transcript is complementary to it. Zero arms, not occluded.
Now insert twelve bases at the 5' end: TGTTGTCCTCCT, the reverse complement of the SD and its spacer. The transcript is 82 nucleotides, the Shine-Dalgarno has moved to 38–43 with the same spacing of 6, and there is now an arm at positions 1–12 pairing with 38–49 — a 12 bp duplex with a melting temperature of 50.0 °C, covering the Shine-Dalgarno.
Nothing about the ribosome binding site changed. The site is the same; what changed is whether it is available.
What this is and is not
This is base-pairing arithmetic over the letters: every stretch that is reverse-complementary to the footprint, extended as far as the pairing holds, with its length, loop size and nearest-neighbour melting temperature. It is exact and checkable, and every arm it reports is a real complementarity.
It is not a folding prediction. Whether a given stem actually wins in the ensemble at 37 °C, against every other structure the transcript could adopt, is a different question that needs a folding model. A long complementary arm with a high melting temperature is a strong candidate and not a verdict.
Use the two together: this to find what could pair with the site, a fold to ask whether it does.
What counts as a stem worth worrying about
Below about 4 bp every sequence pairs with everything, so a minimum stem length is part of the question rather than a filter on the answer. A duplex that actually holds a site shut is usually 8 bp or more, and the two numbers to read are the stem length and the melting temperature — a 12 bp stem at 50 °C is stable at the temperature a cell grows at.
Where the arm sits matters as much as how strong it is. The report distinguishes an arm covering the Shine-Dalgarno core from one covering the start codon from one merely nearby, because the first two block initiation and the third may do nothing at all.
The same analysis designs a switch
A riboswitch, a toehold switch, an RNA thermometer and an antisense repressor all work the same way: the Shine-Dalgarno and the start codon are paired into a stem in one state and free in the other. Designing one is choosing an arm deliberately, with a stem stable enough to hold the site shut and openable by whatever the sensor responds to.
The region 5' of the binding site is where a sensor domain goes — in the open transcript above, positions 1–25, and in the closed one 1–37, because the insertion pushed the site downstream and the window grew with it. Putting the aptamer there keeps it clear of the site itself.
What no sequence analysis gives you is a switching ratio. The ON and OFF states are both ensembles, and the ratio between them depends on the cell, the temperature and the ligand concentration — it is measured, not computed.
Frequently asked questions
My construct transcribes but makes no protein. Where do I start?
With the ribosome binding site's accessibility, because it is the cheapest to check and a common cause. Look for sequence elsewhere in the transcript that is reverse-complementary to the window from 20 nucleotides before the start codon to 13 after it; a stem of 8 bp or more over the Shine-Dalgarno or the start codon is a strong candidate.
Is this a secondary structure prediction?
No. It finds every stretch complementary to the ribosome's footprint and reports each duplex's length and melting temperature — exact arithmetic over the sequence. Whether a given stem wins against every alternative structure at 37 °C needs a folding model, so use a fold to confirm what this finds.
Can a transcript initiate without a Shine-Dalgarno sequence?
It can. Many E. coli transcripts initiate with a weak or absent Shine-Dalgarno, and some do so efficiently. The footprint is still reported around the start codon, so the accessibility question is unchanged — what is lost is the anchor the SD normally gives, which usually means initiation is more sensitive to structure rather than less.
Can a switching ratio be predicted from the sequence?
Because that number cannot be computed from the sequence. Both states of a switch are structural ensembles, and the ratio between them depends on temperature, ligand concentration and the cellular context. What the sequence supports is which arms could hold the site shut and how strongly — the ratio is an experiment.
Related tools
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