While testing a new DNA sequencing technique on a microscopic protist collected from a freshwater pond at Oxford University Parks, researchers discovered a genetic code that breaks a rule thought to be nearly universal. The organism, a previously unknown ciliate named Oligohymenophorea sp. PL0344, uses two codons that normally signal "stop" to instead encode two different amino acids. The finding, published in PLOS Genetics, came from a team at the Earlham Institute and the University of Oxford led by Dr. Jamie McGowan and Professor Thomas Richards.
In the standard genetic code, the codons TAA, TAG, and TGA act as stop signals that tell the ribosome to end protein synthesis. In this protist, two of those signals have been reassigned to amino acids, and each one now codes for a different amino acid. That is particularly surprising because researchers had believed the two signals evolve together, so their independent reassignment suggests an unexpected flexibility in how genetic information can be interpreted.
The discovery was accidental: the project was designed to test a sequencing pipeline for analyzing extremely small amounts of genetic material, not to study genetic code evolution. "It's sheer luck we chose this protist to test our sequencing pipeline," said McGowan, adding that it highlights "just how little we know about the genetics of protists." Subsequent work has found additional genetic code variations in related microorganisms, hinting that more surprises may be waiting in the microscopic world.