A microscopic organism from a freshwater pond is playing by a different set of genetic rules. The newly identified protist, Oligohymenophorea sp. PL0344, which scientists stumbled across while testing DNA sequencing technology, does something researchers hadn’t seen before. It uses two genetic signals that normally tell cells to stop making proteins to insert two completely different amino acids instead.
That might sound like a tiny biological technicality, but it isn’t. The discovery, published in PLOS Genetics in October 2023, challenged a long-standing assumption about how the genetic code evolves. Follow-up research has since uncovered more unusual genetic codes in related microorganisms, suggesting that the microscopic world still has plenty of surprises in store.
These genetic instructions aren’t supposed to work this way
Your cells constantly build proteins, the molecules responsible for everything from carrying oxygen to digesting food. To make them, cells read genetic instructions in groups of three letters called codons. Most codons specify an amino acid, the building blocks that get assembled into proteins. Three codons usually serve a different purpose: telling the cell when to stop.
Known as stop codons, TAA, TAG and TGA mark the end of a protein-coding sequence in DNA. Their corresponding RNA versions are UAA, UAG and UGA. They are like full stops in a sentence. Without them, the cell’s protein-making machinery would not know where to finish.
But in Oligohymenophorea sp. PL0344, two of those supposed full stops have taken on entirely different jobs. TAA codes for lysine, while TAG codes for glutamic acid. Both are amino acids commonly used to build proteins, but they have different chemical properties.
TGA remains the organism’s stop signal. But scientists already knew some organisms did that. What is unusual is that TAA and TAG have been reassigned to two different amino acids rather than changing together to specify the same one. Researchers had generally observed those two codons evolving in tandem when they stopped functioning as stop signals. This organism showed that the pattern was not an unbreakable rule.
Scientists weren’t even looking for this
The discovery came from an experiment led by Dr. Jamie McGowan, then a postdoctoral scientist at the Earlham Institute in England, working with researchers at the institute and a team led by Professor Thomas Richards at the University of Oxford. Their original goal was to test a DNA sequencing method capable of working with extremely small quantities of genetic material, potentially from a single cell. They were not setting out to challenge one of biology’s basic assumptions.
The sample came from a pond at Oxford University Parks. When the team assembled and examined the organism’s genome, they realized they were looking at a previously unidentified species with an unusual way of interpreting genetic instructions. Further analysis turned up specialized transfer RNA genes associated with the reassigned codons. Transfer RNA, or tRNA, helps the cell match each codon to the appropriate amino acid during protein production. These genes supported the conclusion that the unusual code was a real biological feature, not a sequencing glitch.
The team also found plenty of TGA stop codons immediately beyond protein-coding regions. Those extra signals could provide a useful safeguard, helping prevent proteins from becoming too long if the cell’s machinery reads beyond the intended endpoint.
The researchers could not establish exactly how the organism evolved its unusual genetic code. But the evidence showed that its protein-making system could function with a set of instructions that didn’t follow the expected pattern.
What exactly is a protist?
Protists are a wildly diverse collection of organisms whose cells contain a nucleus, like those of animals, plants and fungi. The category includes familiar microscopic life such as amoebas and many algae, as well as organisms with very different lifestyles and biological features. The protist at the center of this discovery belongs to a group called ciliates. These organisms typically use tiny, hair-like structures called cilia to move through water.
Ciliates are particularly interesting to geneticists because some have evolved unusual ways of reading genetic instructions. That makes them useful for exploring how the genetic code can change without preventing an organism from surviving. Also, the 2023 discovery wasn’t the end of the story.
More microorganisms are rewriting the rules
In December 2024, McGowan and colleagues published another study in PLOS Genetics describing additional changes to the genetic code across several ciliate lineages. Using genomic data, including material gathered through the TARA Oceans project, the team identified three previously uncultivated ciliate species associated with the Arctic and Southern Oceans. In these organisms, UAG, normally a stop codon, appeared to specify the amino acid leucine instead.
The researchers also identified two other ciliates, Hartmannula sinica and Trochilia petrani, in which UAG appeared to code for glutamine. These were not identical copies of the original discovery. In the five ciliates examined in the 2024 study, UAA remained a stop signal while UAG had acquired a new meaning. Evolutionary comparisons suggested that these changes had arisen independently at least three times.
Scientists are also finding new species by looking at organisms that are difficult to grow in laboratories. In March 2026, researchers from the Earlham Institute and the University of Oxford reported identifying three previously unrecognized evolutionary lineages of Bodo, a group of protists found in freshwater, brackish water and soil.
The study, published in Microbial Genomics, analyzed just seven uncultured cells and found that each lineage was associated with its own distinct bacterial partner living inside the organism. That study did not find the same genetic code changes. Instead, it offered another example of how much biological diversity can remain hidden when researchers focus on organisms that are easy to study in a lab.
The focus on breaking the genetic code
The genetic code is remarkably consistent across most life on Earth. That consistency is one reason scientists have been able to study genes from one organism and use the information to understand, or sometimes engineer, another. Unusual genetic codes offer a way to investigate how that shared system evolved, which changes cells can tolerate and why some genetic instructions remain stable while others get reassigned.
There could also be implications for synthetic biology, where researchers engineer cells to produce proteins with unusual properties or perform new functions. Natural organisms that already use alternative genetic codes offer real-world examples of biological systems working outside the standard setup.
Nevertheless, researchers still do not fully understand what drives these changes, how they emerge or how widespread they might be among organisms that haven’t been studied in detail. As they examine more microorganisms, they may uncover further variations in how cells interpret genetic instructions.
Source: ScienceDaily