Unlike other amino acids present in biological proteins, however, it is not coded for directly in the genetic code. Selenocysteine is encoded by a UGA codon, which is normally a stop codon but, in these exceptional proteins, is modified by a subsequent sequence in the mRNA molecule that encodes the enzyme. When cells are grown in the absence of selenium, protein translation terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme.
The primary and secondary structure of selenocysteine tRNA differ from those of standard tRNAs in several respects, most notably in having an 8-base pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions.
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