The genetic code showing that Sec is the 21st amino acid and that Sec is coded by UGA. ∗, UGA and Sec; ▴, AUG, the other codon in the genetic code that serves a dual function.

SECIS element consensus structure. Conserved nucleotides in the SECIS core (Quartet) are shown in boldface. Adenosines that are conserved in most, but not all, selenoprotein genes are also indicated.

Animal Sec-containing proteins. All currently known selenoproteins are listed (left). The relative sizes of selenoproteins (empty boxes) and the locations of Sec (red box) and an α-helix immediately downstream of Sec (green box) in the selenoprotein sequences are indicated (right).

Mammalian Sec tRNA^[Ser]Sec shown in a 9/4 and a 7/5 cloverleaf model (see text). Brackets around bases at positions 11, 12, and 47c indicate those bases that have been shown to vary in different mammals, as discussed in the text.

Mechanism of Sec insertion in eukaryotes (see text). Selenocysteyl-tRNA (in orange with Sec in yellow) is shown in a complex with EFsec (in blue) and SBP2 (in green) and the SECIS element (shown as a hairpin loop in black) that is ready for donation to the ribosomal A site to be decoded by UGA (shown in the selenoprotein mRNA in black). Once the Sec tRNA^[Ser]Sec complex is donated to the A site, Sec tRNA^[Ser]Sec is transferred to the peptidyl site and Sec is incorporated into the nascent selenopeptide. The growing selenopeptide is shown as alternating gold and blue balls attached to the tRNA in the peptidyl site. The mRNA (shown in black with its start and stop codons indicated) is attached to the smaller of the two ribosomal subunits, and the unacylated tRNA is shown leaving the ribosomal exit site.