Stop codon readthrough happens when the cell’s protein-building machinery skips past a “stop” signal in the genetic message and keeps adding amino acids, producing a longer-than-expected protein. This is not random noise. The efficiency of readthrough depends on a tug-of-war at the ribosome between termination factors that want to end translation and certain transfer RNAs (tRNAs) that sneak in to keep it going. The identity of the stop codon itself, the surrounding sequence, RNA structures further downstream, metabolic conditions, and even prion states can all tip the balance. Understanding these factors matters not only for basic biology but also for drug development aimed at diseases caused by premature stop mutations.
The Core Competition at the Ribosome
When a ribosome reaches a stop codon, two things can happen. A release factor called eRF1 can recognize the stop signal and trigger termination, or a “near-cognate” tRNA, one whose anticodon almost but doesn’t perfectly match the stop codon, can wedge itself into the ribosome’s decoding site and add an amino acid instead. Readthrough occurs when the tRNA wins that race. The outcome depends on the relative concentrations of release factors and these near-cognate tRNAs, along with modifications on the tRNA that affect how well it interacts with the stop codon and the ribosome itself.1PubMed Central. Recognition of 3′ nucleotide context and stop codon readthrough are determined during mRNA translation elongation Several studies in yeast and human cells have shown that specific near-cognate tRNAs are more readthrough-inducing than others, influenced by how abundant they are in the cell and by chemical modifications in the part of the tRNA that contacts the stop codon.2Nature Communications. Extended stop codon context predicts nonsense codon readthrough efficiency in human cells
The tRNAs involved must outcompete release factors and settle into the ribosome’s decoding center to prevent termination and allow translation to continue.3PubMed Central. Ribosomal A-site interactions with near-cognate tRNAs drive stop codon readthrough This is not a vague process. Mass spectrometry experiments have confirmed that, in some cases, a specific amino acid like glutamine is inserted cleanly at the readthrough site, with no errors at neighboring positions, showing that what happens is a precise molecular substitution driven by a particular tRNA competing with the release factor.4bioRxiv. Glutamine codon-driven translational readthrough reveals context-dependent stop codon decoding fidelity
Which Stop Codon and What Comes After It
Not all stop codons are created equal when it comes to readthrough. There are three stop codons, UGA, UAG, and UAA, and they differ substantially in how “leaky” they are. Large-scale measurements in human cells show a clear hierarchy: UGA is the most readthrough-permissive, followed by UAG, with UAA being the tightest.2Nature Communications. Extended stop codon context predicts nonsense codon readthrough efficiency in human cells This pattern holds across multiple experimental systems and has been reproduced by different research groups.
The single nucleotide immediately after the stop codon, often called the +4 position, has an outsized influence. A cytosine at +4 promotes readthrough, while a guanine tends to suppress it, likely because purines at that position stabilize the interaction between eRF1 and the mRNA, making termination more efficient.5eLife. Stop codon context influences genome-wide stimulation of termination codon readthrough by aminoglycosides The combination of the stop codon and +4 nucleotide creates a four-letter signal that strongly predicts readthrough levels. In human kidney cells, the most readthrough-permissive four-letter combination is UGAC, while the most inhibitory is UAAG.2Nature Communications. Extended stop codon context predicts nonsense codon readthrough efficiency in human cells
The influence extends beyond just one nucleotide. Work in plant viruses first showed that a “leaky” UAG stop codon depends on the two downstream codons, with sequences of the form CAR-YYA (where R is a purine and Y is a pyrimidine) conferring readthrough, and that the right downstream context can make even UAA and UGA stop codons leaky.6Journal of Molecular Biology. The signal for a leaky UAG stop codon in several plant viruses includes the two downstream codons More broadly, A’s and U’s in the region downstream of the stop tend to increase readthrough probability, while C’s and G’s decrease it.5eLife. Stop codon context influences genome-wide stimulation of termination codon readthrough by aminoglycosides
RNA Structures That Stall Termination
Sequence context is only part of the story. Some readthrough events depend on three-dimensional RNA structures sitting downstream of the stop codon. The best-studied example comes from retroviruses. In murine leukemia virus, a structure called an RNA pseudoknot, where a strand of RNA loops back and forms a knot-like fold, sits eight nucleotides downstream of the UAG stop codon separating the viral gag and pol genes. This pseudoknot is required for readthrough; without it, the virus cannot produce the Gag-Pol fusion protein it needs to replicate.7PubMed. Evidence that a downstream pseudoknot is required for translational read-through of the Moloney murine leukemia virus gag stop codon Structural studies have confirmed that this pseudoknot has a very specific architecture, with a one-nucleotide loop and a seven-base-pair stem, and that nucleotides downstream of the stem also contribute to function.8PubMed. Structural studies of the RNA pseudoknot required for readthrough of the gag-termination codon of murine leukemia virus
RNA structures are not limited to viruses. Computational predictions in fruit flies and mosquitoes found structured RNA elements in roughly a tenth of genes with conserved readthrough, a rate far higher than in genes without readthrough.9Molecular Biology and Evolution. Evolutionary Dynamics of Abundant Stop Codon Readthrough These structures can appear or disappear over evolutionary time while readthrough itself persists, suggesting that an RNA fold is one of several interchangeable tools evolution uses to tune readthrough levels.
A Surprising Cellular Partner
You might expect that readthrough would be controlled mainly by the factors that handle termination, the release factors. But research has revealed a surprising player: eIF3, a factor normally associated with starting translation, not ending it. eIF3 physically associates with ribosomes that have reached a stop codon and interferes with how eRF1 reads the third position of the stop codon when the surrounding sequence is unfavorable for termination. This interference allows near-cognate tRNAs to slip in. Critically, eIF3 promotes readthrough at all three stop codons.10Nucleic Acids Research. Translation initiation factor eIF3 promotes programmed stop codon readthrough The involvement of an initiation factor in a termination decision underscores how interconnected the steps of translation really are.
What Gets Inserted at the Readthrough Site
When readthrough occurs, the amino acid that gets plugged in at the stop codon position is not always the same, and it is not random either. It depends on which near-cognate tRNA wins the competition. Studies of the cystic fibrosis gene (CFTR) illustrate this well. At one premature UGA stop codon, three amino acids were found: cysteine (most common, at about 44% of events), tryptophan (36%), and arginine (20%). But at a different premature UGA in the same gene, the proportions shifted dramatically: leucine became the most common insertion at 58%, followed by cysteine at 38%, with tryptophan dropping to just 4%, and arginine not detected at all. The only difference between the two sites was the six codons flanking the stop, showing that local sequence context influences not just whether readthrough happens but what amino acid is inserted.11Human Molecular Genetics. Identification of the amino acids inserted during suppression of CFTR nonsense mutations and determination of their functional consequences
This has practical importance. If a therapy is designed to push readthrough at a disease-causing premature stop codon, the resulting protein will carry a substitution at that position. Whether the protein functions normally depends on which amino acid lands there. Different substitutions can have very different effects on protein folding and activity.
How Viruses Exploit Readthrough
Readthrough is not just a quirk cells tolerate; viruses have evolved to depend on it. In the gammaretroviruses, typified by Moloney murine leukemia virus, the gag and pol genes sit in the same reading frame, separated by a UAG stop codon. The virus needs the ribosome to stop most of the time to make the structural Gag protein but to read through occasionally to produce the Gag-Pol fusion protein, which contains the enzymes needed for replication. The ratio of Gag to Gag-Pol matters: disrupting it interferes with viral assembly.12PubMed Central. Modulation of stop codon read-through efficiency and its effect on the replication of murine leukemia virus
Plant viruses use similar tricks. Barley yellow dwarf virus reads through the stop codon of its coat protein gene at a low rate, producing an extended polypeptide that becomes part of the virus particle. Both local and distant sequences in the RNA are required for this readthrough to work properly.13PubMed Central. Local and distant sequences are required for efficient readthrough of the barley yellow dwarf virus PAV coat protein gene stop codon This means antiviral strategies that target readthrough could, in principle, disrupt viral replication without directly attacking the virus’s proteins.
Metabolic Stress and Environmental Shifts
Readthrough is not fixed at a constant rate. Environmental conditions can dial it up or down. When yeast cells experience metabolic stress caused by excess carbon, acid metabolites accumulate, lowering the pH inside the cell. This reduced pH impairs the activity of release factors, tipping the balance in favor of near-cognate tRNAs and substantially increasing both the level and the cell-to-cell variability of readthrough.14PubMed Central. Metabolic stress promotes stop-codon readthrough and phenotypic heterogeneity The variability part is interesting: under stress, some cells in a population experience far more readthrough than others, creating a diversity of protein forms that could help the population adapt.
Prions as Readthrough Switches in Yeast
One of the more dramatic ways readthrough can be cranked up involves prions, self-propagating protein aggregates, in yeast. The yeast translation termination factor Sup35 can adopt a prion state called [PSI+]. In this state, Sup35 gets trapped in aggregates, depleting the pool of functional termination factor available to ribosomes. With less termination factor around, stop codon readthrough increases across the genome.15PubMed Central. Prion induction involves an ancient system for the sequestration of aggregated proteins and heritable changes in prion fragmentation This uncovers hidden genetic variation, sequences that sit beyond normal stop codons and are normally silent, giving rise to new traits that are sometimes beneficial.16Cell Reports. Genome-wide Translational Changes Induced by the Prion [PSI+] The prion state is heritable, meaning daughter cells inherit the elevated readthrough. This has been proposed as a mechanism for rapid, reversible phenotypic switching, a kind of bet-hedging strategy in fluctuating environments.
Readthrough Sends Proteins to New Destinations
Some of the most elegant examples of functional readthrough come from cases where the extended protein tail acts as a postal code, redirecting the protein to a different part of the cell. In mammals, the enzyme lactate dehydrogenase B (LDHB) normally works in the cytosol. But a small fraction of LDHB molecules undergo readthrough, producing an extended version called LDHBx. The extra tail contains a peroxisomal targeting signal, a short sequence recognized by the cell’s sorting machinery, which diverts LDHBx into peroxisomes. When researchers mutated the targeting signal, the protein stopped going to peroxisomes, confirming that the readthrough extension is what redirects it.17PubMed Central. Peroxisomal lactate dehydrogenase is generated by translational readthrough in mammals
LDHB is not the only enzyme with this trick. Malate dehydrogenase 1 (MDH1) also uses readthrough to generate a peroxisomal isoform from the same gene that encodes the cytosolic version.18PLOS Genetics. Ribosomal Readthrough at a Short UGA Stop Codon Context Triggers Dual Localization of Metabolic Enzymes in Fungi and Animals This mechanism allows a single gene to serve two cellular compartments without needing a separate gene or alternative splicing. It is an economical solution to a logistical problem, and it has been conserved across fungi and animals, suggesting it provides a real fitness advantage.
How Readthrough Connects to mRNA Surveillance
Cells have a quality-control system called nonsense-mediated mRNA decay (NMD) that destroys mRNA molecules containing premature stop codons. But readthrough and NMD are in tension: if a ribosome reads through a premature stop, the mRNA looks less like it has a defect, and NMD is less likely to destroy it. Diverse readthrough-promoting elements have been shown to inhibit NMD with similar effectiveness, and the level of readthrough needed to protect the mRNA from destruction depends on how far the suppressed stop codon is from the end of the message.19PLOS ONE. A system for coordinated analysis of translational readthrough and nonsense-mediated mRNA decay This interplay matters for therapeutic readthrough approaches: a drug that increases readthrough not only produces full-length protein directly but may also stabilize the mRNA, amplifying the effect.
Physiological Roles in Mammals
For a long time, readthrough was thought to be mostly a viral strategy or a peculiarity of lower organisms. That view has shifted. Mouse experiments have shown that readthrough of the gene MTCH2, which encodes a mitochondrial protein, helps regulate the protein’s levels and, through that, mitochondrial energy production and ATP levels in skeletal muscle. Mice engineered to lack MTCH2 readthrough showed decreased musculoskeletal activity and lost protection against diet-induced obesity, suggesting readthrough plays a role in adipose tissue function as well.20etd@IISc. Physiological Significance of Stop Codon Readthrough These findings imply that readthrough in mammals is not just tolerated error but part of a regulatory toolkit.
Evolutionary Scale and the Debate Over Function
How widespread is functional readthrough? Ribosome profiling in fruit flies, a technique that maps where ribosomes are actively translating, has revealed that readthrough is far more pervasive than predicted by sequence conservation alone. One study identified over 300 readthrough events not anticipated by comparative genomics.21PubMed Central. Ribosome profiling reveals pervasive and regulated stop codon readthrough in Drosophila melanogaster A more recent integration of ribosome profiling, proteomics, and functional screening in fruit flies identified roughly 1,400 genes with evidence of readthrough, including both constitutive and tissue-restricted modes.22bioRxiv. Regulatory landscape of widespread stop codon readthrough in Drosophila
Comparative genomics across mosquito species identified 353 genes in Anopheles gambiae with evolutionary signatures of conserved, functional readthrough. These readthrough regions showed population-level signs of purifying selection, meaning mutations that would change the amino acid sequence of the readthrough extension were weeded out, consistent with the extensions doing something useful. Stop codons at readthrough sites strongly favored UGA and, to a lesser extent, UAG over UAA, mirroring patterns seen in fruit flies, and most changes to readthrough repertoires between species came from gains or losses in existing genes rather than the birth of new ones.9Molecular Biology and Evolution. Evolutionary Dynamics of Abundant Stop Codon Readthrough
The picture is not universally rosy, though. A study examining readthrough in yeast and fruit flies argued that much of it arises from molecular errors and is generally non-adaptive, finding that post-stop-codon regions in readthrough genes were not more conserved than comparable untranslated regions when controlling for overall gene conservation.23PLoS Genetics. Stop-codon read-through arises largely from molecular errors and is generally nonadaptive The disagreement likely reflects a real mix: some readthrough events are clearly functional and under selection, while others are background noise the cell tolerates. Distinguishing the two remains an active challenge.
Therapeutic Readthrough for Genetic Diseases
About 10 to 15 percent of inherited disease cases are caused by premature stop codons, mutations that introduce a stop signal in the middle of a gene and truncate the protein. If you could force the ribosome to read through that premature stop and produce a full-length or near-full-length protein, you could potentially treat the disease. This idea, called nonsense suppression therapy, has driven considerable drug development. Compounds like aminoglycoside antibiotics (gentamicin, G418) and purpose-designed molecules like ataluren have been tested for their ability to increase readthrough at premature stop codons across a range of conditions, including cystic fibrosis, Duchenne muscular dystrophy, and certain cancers.24PubMed Central. Nonsense suppression therapies in human genetic diseases
The challenges are significant. The inserted amino acid may not be the original one, meaning the readthrough protein could be less functional. The efficiency of readthrough drugs varies dramatically depending on the stop codon identity and surrounding context, as discussed above. And aminoglycosides have serious side effects at the doses needed. Still, the therapeutic logic is sound, and understanding the factors that control readthrough efficiency, from the +4 nucleotide to the local codon context to the involvement of factors like eIF3, directly informs efforts to make these therapies work better.
When Stop Codons Are Not Stop Codons at All
Readthrough assumes the stop codon is still functioning as a stop most of the time, with occasional bypass. But some organisms have gone further: they have permanently reassigned what were once stop codons to encode amino acids. Ciliates, a group of single-celled organisms, offer some of the most extreme examples. In several ciliate species, all three standard stop codons have been reassigned to code for amino acids such as glutamine and arginine or cysteine. In the ciliate Uroleptopsis setigera, TAA, TAG, and TGA appeared thousands of times within protein-coding regions, with roughly half showing allele frequencies above 90 percent, confirming genuine reassignment rather than sporadic readthrough. Ciliate genomes carry far more in-frame stop codons than related organisms, on the order of 40 per gene compared to about 4 in close relatives.25Oxford Academic. Stop or Not: Genome-Wide Profiling of Reassigned Stop Codons in Ciliates These organisms represent the logical extreme of what happens when the boundary between “stop” and “sense” erodes, a boundary that readthrough exploits in a more measured way in other species.
Engineering Readthrough for New Functions
The molecular competition at the stop codon has also attracted the attention of synthetic biologists. Because readthrough depends on tRNAs outcompeting release factors, engineering tRNAs that are better at recognizing stop codons can be used to insert unnatural amino acids, molecules not found in nature’s standard set, into proteins at specific sites. This technology, known as genetic code expansion, was initially focused on engineering the enzymes that load amino acids onto tRNAs. More recently, attention has shifted to optimizing the tRNAs themselves, improving both the efficiency of unnatural amino acid incorporation and the specificity of the engineered system so it does not interfere with normal translation elsewhere in the cell.26PubMed Central. tRNA engineering strategies for genetic code expansion By fine-tuning the same competition that underlies natural readthrough, researchers can build proteins with chemical properties that biology never invented on its own.