Fig. 8. Proposed model for SelB function in stop codon read through. (A) SelB binds GTP. (B) SelB·GTP binds Sec-tRNASec. (C) SelB·GTP·Sec-tRNASec binds to an mRNA hairpin in a selenoprotein gene. Upon binding, SelB-C undergoes a conformational change so that the L is opened. SelB is ready to deliver Sec-tRNASec to the A-site of the approaching translating ribosome. (D) When the UGA codon is available in the A-site, SelB-N swings in the tRNA. If a correct codon–anticodon match occurs, GTP is hydrolysed. (E) SelB changes conformation to the GDP form and releases the tRNA that is accommodated in the peptidyl-transfer site. (F) SelB leaves the mRNA hairpin, perhaps simultaneously with the combined unwinding and translocation of mRNA.