An unprecedented codon reassignment and two new genetic code variants in ciliates

Licnophora spp. in Spirotrichea shows an unprecedented genetic code variant where UAA encodes glutamate, UAG glutamine, and UGA is homonymous, encoding arginine in-frame.

• Why it matters: Ciliate nuclear genomes have frequent genetic code changes, but key variants were previously proposed without solid evidence, limiting accurate protein prediction and understanding of code evolution.
• What they did: The authors analyzed genomes and transcriptomes of Litostomatea and deeply branching Spirotrichea lineages, identifying tRNAs for reassigned UAA/UAG in Licnophora and UGA in Balantidium ctenopharyngodoni.
• The result: Litostomateans mostly use the standard genetic code except Balantidium ctenopharyngodoni, where UGA encodes leucine; the mechanism for UGA decoding as arginine in Licnophora remains unresolved, requiring an updated genetic code variant list.

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