Practice question

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.