Discovered in1986, selenocysteine is the 21st naturally occuring amino acid, and is coded for by the RNA codon UGA, which is normally a "stop" signal, but is modified in some organisms to create selenocysteine by a subsequent RNA loop, which is interpreted by a group of genes called the sel group, which are activated by the loop in the mRNA and produce molecule of tRNA for selenocysteine. The molecule itself is effectively a cysteine amino acid with the sulphur atom replaced by a selenium atom. It occurs in several enzymes and in a large number of organisms, including humans, but is much rarer than most of the other amino acids.
Interestingly a protein containing selenocysteine has been found in HIV-1, and may shed new light on the action of the virus, as it is known to deplete selenium levels within the body
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