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#tRNA Sec

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Eukaryotes gene regulation

Enzyme charging tRNA Sec with serine is

Serine loading onto tRNASec uses ubiquitous seryl-tRNA synthetase SerRS, same dimeric class II enzyme charging canonical tRNASer isoacceptors. SerRS identity elements include long variable arm and discriminator base G73 recognized in tRNASec acceptor stem, compensating absence of typical G3:U70 marker. Reaction consumes ATP forming seryl-adenylate intermediate then transferring serine to A76 3' end. No separate SerRS isoform required for Sec pathway, reflecting economical reuse of translation machinery. Fidelity depends on downstream modifying enzymes PSTK and SepSecS rejecting unmodified tRNASec and tRNASer species. Hence canonical synthetase moonlights in selenoprotein pathway linking selenium utilization to central tRNA aminoacylation infrastructure across domains.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SerRS Charging tRNASec

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Eukaryotes gene regulation

tRNA Sec is initially charged with

Selenocysteine biosynthesis uniquely occurs on its tRNA scaffold because free selenocysteine is unstable and oxidized spontaneously. Initial aminoacylation attaches serine, since no amino acid pool provides selenocysteine directly. Seryl-tRNA synthetase SerRS recognizes tRNASec despite unusual structure, esterifying serine to terminal A76 2' hydroxyl. This serine adduct provides hydroxyl substrate for subsequent phosphorylation reaction and replacement by selenophosphate donor. Maintaining amino acid covalently attached to tRNA throughout conversion avoids release of reactive intermediate, channels toxic selenium safely into protein, and ensures only tRNASec-dependent proteins undergo modification, preserving specificity across translation apparatus.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 5: tRNASec Aminoacylation with Serine

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