Practice question

Bacterial selenocysteine insertion employs elongation factor SelB, homologous to EF-Tu yet extended with C-terminal winged-helix domain directly binding SECIS hairpin immediately downstream of in-frame UGA within mRNA coding region. SelB-GTP complex loaded with Sec-tRNASec specifically interacts with SECIS, forming quaternary complex delivered to ribosome stalled at UGA, competing with release factor RF2 for A-site access. GTP hydrolysis triggers accommodation into peptidyl transferase center. Eukaryotic system separates functions between SBP2 for SECIS binding and EFSec for tRNA delivery, but bacteria fuse functions into single multifunctional SelB enabling proximity-mediated recoding with high efficiency and selectivity ensuring faithful selenium incorporation under nutrient limitation.