The specific transfer RNA used for the incorporation of selenocysteine in proteins recognizes the UGA codon, which is normally a translation stop codon. What prevents this tRNA from reading through all the other "legitimate" UGA stop codons in the cells and causing a massive, disastrous recoding?
A) The cells that have selenocysteine in their proteins do not normally use the UGA stop codon and always use the other two stop codons (UAA and UAG) instead.
B) This tRNA is made in such a small amount that its side effects are negligible.
C) This tRNA only interacts with the UGA codon in the P site of the ribosome, and therefore does not interfere with the normal function of the codon in translation termination, which takes place in the A site.
D) The mRNAs encoding the selenocysteine-containing proteins also contain additional sequences required for the recoding event.

Question

Quizplus · Accepted Answer

The presence of the UGA codon is only one of the signals required for the recoding event. Other sequences (that can form stem-loop structures) in the vicinity of the UGA codon on the mRNA collaborate with a specific protein factor to incorporate the amino acid into the growing protein chain only for particular mRNAs and not any mRNA with a UGA codon.

The Specific Transfer RNA Used for the Incorporation of Selenocysteine