Oxford pond ciliate reads two stop codons as two different amino acids

Because PL0344 was picked to test a pipeline ^[3], its code was found without anyone searching through genomes for an unusual one. The team was trying to improve tools for organisms that are hard to grow in a laboratory, and genetic code evolution was not the question ^[8]. "It's sheer luck we chose this protist to test our sequencing pipeline, and it just shows what's out there, highlighting just how little we know about the genetics of protists," McGowan said ^[10].

The best part of the result is that the two reassignments differ. Two signals that normally end a protein have both been converted to coding, and each now stands for its own amino acid ^[1]. According to the Earlham Institute's account, researchers had thought those signals evolve together, and treated the rule as nearly universal ^[2]. In this ciliate both changed, and they changed to different things ^[1].

The denominator is one organism, a ciliate ^[11], from a group that is hard to generalise about. "The definition of a protist is loose -- essentially it is any eukaryotic organism which is not an animal, plant, or fungus," McGowan said ^[13]. "Basically, we can make very few generalizations," McGowan added ^[14]. The thing a single species doesn't tell you is how often the split happens, among ciliates or across protists. The release also does not name the two codons or the amino acids they now encode, or describe the evidence used to assign them.

The release says the result revealed "an unexpected level of flexibility in one of life's most fundamental biological systems" ^[12]. I think the narrow version of that holds: in at least one eukaryote, two signals expected to change together changed apart ^[1][2]. The broad version, a genetic code that is generally more pliable than assumed, depends on how many other lineages show the same split. The variants reported since in related microorganisms ^[16] are where that count starts.