A microscopic organism in a freshwater pond is governed by a different set of genetic rules. The newly identified protist, Oligohymenophorea sp. PL0344, which scientists stumbled upon while testing DNA sequencing technology, does something researchers haven’t seen before. It uses two genetic signals that normally tell cells to stop making proteins to insert two completely different amino acids in their place. This might seem like a little biological technicality, but it isn’t. The discovery, published in PLOS Genetics in October 2023, challenged a long-held assumption about how the genetic code evolves. Since then, further research has discovered more unusual genetic codes in related microorganisms, suggesting that the microscopic world still has many surprises in store. These genetic instructions are not supposed to work this way. Your cells are constantly building proteins, the molecules responsible for everything from transporting 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 are assembled into proteins. Three codons usually have 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 periods in a sentence. Without them, the cell’s protein-making machinery wouldn’t know where to end up. But in Oligohymenophorea sp. PL0344, two of those supposed points have taken on completely different jobs. TAA encodes lysine, while TAG encodes glutamic acid. Both are amino acids commonly used to form proteins, but they have different chemical properties. TGA remains the body’s stop signal. But scientists already knew that some organisms did that. What is unusual is that TAA and TAG have been reassigned to two different amino acids instead of changing them together to specify the same one. In general, researchers had observed that these two codons evolved together when they stopped functioning as stop signals. This body demonstrated 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, in collaboration with researchers at the institute and a team led by Professor Thomas Richards of the University of Oxford. Its original goal was to test a DNA sequencing method capable of working with extremely small amounts of genetic material, potentially from a single cell. They did not set out to challenge one of the basic assumptions of biology. The sample came from a pond in 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 revealed 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 error. The team also found many TGA stop codons immediately beyond the protein-coding regions. Those additional signals could provide useful protection, helping to prevent proteins from becoming too elongated if the cell’s machinery reads past the intended endpoint. The researchers were unable to establish exactly how the organism developed its unusual genetic code. But evidence showed that their protein production system could function with a set of instructions that did not follow the expected pattern. What exactly is a protist? Protists are a tremendously 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 characteristics. The protist at the center of this discovery belongs to a group called ciliates. These organisms often use tiny hair-like structures called cilia to move through water. Ciliates are particularly interesting to geneticists because some have developed unusual ways of reading genetic instructions. That makes them useful for exploring how the genetic code can change without preventing an organism from surviving. Furthermore, the 2023 discovery was not the end of the story. More Microorganisms Are Rewriting the Rules In December 2024, McGowan and colleagues published another study in PLOS Genetics that describes additional changes to the genetic code in several ciliate lineages. Using genomic data, including material collected through the TARA Oceans project, the team identified three previously uncultured ciliate species associated with the Arctic and Southern Oceans. In these organisms, the 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 encode glutamine. These were not identical copies of the original discovery. In the five ciliates examined in the 2024 study, the UAA remained a stop signal, while the UAG had taken on 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 at 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 bacteria living within the organism. That study did not find the same changes in the genetic code. 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 laboratory. Focus is on cracking the genetic code The genetic code is remarkably consistent across most of life on Earth. That coherence is one of the reasons scientists have been able to study genes from one organism and use the information to understand, or sometimes design, another. The unusual genetic codes offer a way to investigate how that shared system evolved, what changes cells can tolerate, and why some genetic instructions remain stable while others are remapped. 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 operating outside the standard configuration. However, researchers still don’t fully understand what drives these changes, how they arise, or how widespread they might be among organisms that haven’t been studied in detail. As they examine more microorganisms, they may discover more variations in the way cells interpret genetic instructions. Source: ScienceDaily
