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#UAG codon

3 public questions tagged with this topic.

Eukaryotes gene regulation

tRNA recognizing UAG for pyrrolysine is

tRNAPyl is unusual tRNA encoded by pylT with small D-loop, extended anticodon stem, and CUA anticodon complementary to amber UAG. Substrate for class IIc pyrrolysyl-tRNA synthetase, identity lies in acceptor stem not anticodon, permitting suppression. tRNAPyl lacks many canonical modifications yet efficiently aminoacylated and delivered by EF-Tu variant. Structural studies show elongated variable loop altering elbow angle influencing ribosome kinetics. Its orthogonal nature enables amber suppression technology widely exploited for site-specific noncanonical amino acid incorporation in synthetic biology, expanding protein engineering beyond natural twenty residues effectively.

Ref: NCBI Bookshelf, tRNA Families: tRNAPyl Structure and Amber Suppression

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Eukaryotes gene regulation

Pyrrolysine is encoded by which codon?

In most organisms UAG amber codon terminates translation via release factor RF1 or eRF1 mediated hydrolysis. Methanogenic archaea capable of methylamine metabolism recode specific UAG codons to pyrrolysine when downstream PYLIS element and high PylRS levels present. Release factor competition determines outcome; high Pyl-tRNAPyl availability favors elongation producing full-length methyltransferase essential for methanogenesis from methylamines. Remaining genomic UAG instances still terminate translation normally, ensuring proteome integrity. This context-dependent reassignment parallels selenocysteine UGA recoding, illustrating dynamic codon redefinition regulated by metabolic state. Genome annotation reveals specialized amber codons within mtmB, mtbB, mtcB genes persisting despite termination signal in standard tables.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Pyrrolysine Encoding by UAG Codon

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Eukaryotes gene regulation

Pyrrolysine is known as the

Genetic code expansion natural example pyrrolysine discovered in methylamine methyltransferase genes of methanogenic archaea and certain Gram-positive bacteria demonstrates additional proteinogenic amino acid beyond canonical twenty. Featuring bulky pyrroline ring connected via isopeptide bond to lysine epsilon-amino group, pyrrolysine designated twenty-second amino acid. Coding requires dedicated amber suppressor tRNAPyl and pyrrolysyl-tRNA synthetase PylRS recognizing UAG codon only within specific mRNAs. Incorporation expands catalytic versatility for methanogenesis from methylamines, permitting methane formation. Limited distribution but strong catalytic benefit suggests evolutionarily recent codon capture where stop reassignment provided selective advantage for specialized anaerobic metabolism in niche environments.

Ref: NCBI Bookshelf, Genetic Code Expansion: Pyrrolysine – 22nd Amino Acid Discovery

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