What do we know about the coding of mRNA that makes E crassus the exception to the rule of protein synthesis

Answer 1

Messenger RNA is used as a template to assemble proteins by means of three-letter sequences called codons. Each codon corresponds to either single amino acid or a 'stop' signal. However, Vadim Gladyshev of the University of Nebraska in Lincoln and his colleagues found that in E. Crassus, the codon UGA could encode two amino acids: cysteine or selenocysteine.

The codon could be read in both ways within the same gene, depending on its location within the mRNA strand and the presence and exposure of a specific sequence near the end of the mRNA molecule. The results suggest that the genetic code can be evolutionarily expanded.

This was taken from an article here: https://www.nature.com/articles/457238e