373: Base by Base 373 — A ciliate rewrites UAA and UAG

McGowan J et al., PLOS Genetics - Genome and transcriptome sequencing of an uncultured Oligohymenophorea ciliate (PL0344) reveals a novel nuclear genetic code in which UAA translates as lysine and UAG as glutamic acid, supported by suppressor tRNAs and phylogenomic context. Key terms: genetic code, ciliate, UAA, UAG, suppressor tRNA.

Study Highlights:
The authors assembled a macronuclear genome from low-input G&T-Seq data for Oligohymenophorea sp. PL0344 and found widespread in-frame UAA and UAG codons. Codetta and PhyloFisher analyses predict UAA→lysine and UAG→glutamic acid, and multiple corresponding suppressor tRNA genes were identified. UGA remains as a stop and is enriched as tandem stops in the 3' UTR, suggesting selection to limit readthrough. Phylogenomic mapping shows numerous independent genetic code changes across ciliates, making this the first reported case where UAA and UAG specify different amino acids.

Conclusion:
This study documents a previously unknown nuclear genetic code variant in a ciliate where UAA and UAG have distinct sense meanings, expanding known genetic code diversity and highlighting the need to consider noncanonical codes in genome annotation and evolutionary analyses.

Music:
Enjoy the music based on this article at the end of the episode.

Article title:
Identification of a non-canonical ciliate nuclear genetic code where UAA and UAG code for different amino acids

First author:
McGowan J

Journal:
PLOS Genetics

DOI:
10.1371/journal.pgen.1010913

Reference:
McGowan J, Kilias ES, Alacid E, Lipscombe J, Jenkins BH, Gharbi K, et al. (2023) Identification of a non-canonical ciliate nuclear genetic code where UAA and UAG code for different amino acids. PLoS Genet 19(10): e1010913. https://doi.org/10.1371/journal.pgen.1010913

License:
This episode is based on an open-access article published under the Creative Commons Attribution 4.0 International License (CC BY 4.0) – https://creativecommons.org/licenses/by/4.0/

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Episode link: https://basebybase.com/episodes/non-canonical-ciliate-code-373

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2026-05-21.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Substantively audited the transcript sections that discuss genetic code reassignments (UAA/UAG), suppressor tRNAs, UGA handling (stop/Sec), tandem stop codons, sequencing approach (G&T-Seq), and phylogenetic context.
- transcript topics: Canonical vs noncanonical genetic code in ciliates; UAA and UAG reassignment to Lys and Glu; Suppressor tRNA genes and wobble binding; G&T-Seq single-cell genome/transcriptome methodology; UGA stop codon usage and tandem stop codons; Phylogenomics and independent genetic code changes in ciliates

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 6
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- UAA codon reassigned to lysine and UAG codon reassigned to glutamic acid in PL0344
- Presence of multiple suppressor tRNA genes corresponding to UAA (Lys) and UAG (Glu)
- tRNA-SeC(UCA) identified, suggesting UGA can encode selenocysteine in addition to stop
- UGA codons enriched in the 3'-UTR downstream of genes as tandem stops to mitigate readthrough
- Genome assembly size around 69.7 Mb with thousands of transcripts and tRNA genes
- Wobble-coupling constraint circumvented; UAA and UAG have different meanings in this organism

QC result: Pass.