Scientists have discovered a microscopic pond organism that uses a unique genetic code, challenging long-held assumptions about gene translation. This protist, identified as *Oligohymenophorea* sp. PL0344, reassigns the function of genetic stop signals. This discovery occurred during a routine experiment testing a new single-cell deoxyribonucleic acid (DNA) sequencing method.
Jamie McGowan, a postdoctoral scientist at the Earlham Institute, studied the genome of the protist. The goal was to test a DNA sequencing pipeline for extremely small amounts of DNA. The team found an unexpected genetic outlier. The organism is a previously unknown species with a rare change in how it reads DNA instructions and builds proteins.
The study, published in *PLOS Genetics*, reported that two codons normally associated with gene stopping signals had been reassigned. These codons, TAA and TAG, typically signal the end of protein construction. In *Oligohymenophorea* sp. PL0344, only TGA functions as a stop codon. TAA specifies lysine, while TAG specifies glutamic acid. This combination was previously unreported.
Protists are a diverse group of eukaryotic organisms that are not animals, plants, or fungi. Many are microscopic and single-celled. *Oligohymenophorea* sp. PL0344 belongs to ciliates, a group known for genetic code changes. The genetic code is usually described as nearly universal. Variations are rare. In most known variants, TAA and TAG change together and specify the same amino acid.
This organism's unique genetic code suggests that the genetic code is more flexible than previously thought. The team's genome and transcriptome analysis identified suppressor transfer ribonucleic acid (tRNA) genes matching the reassigned codons. This supports the conclusion that the organism reads these former stop signals as amino acids. Later work has shown that ciliates are rich sources of genetic code surprises. Multiple independent reassignments of the UAG stop codon have been found in phyllopharyngean ciliates.
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