This illustrates the ease by which the genetic code may evolve new coding schemes, possibly aiding organisms to adapt to changing environments, and show the genetic code is much more flexible than previously thought.
In 2014 researchers from the Joint Genome Institute and Yale University found out that some organisms interpret the one of three "stop" codons which terminate translation to mean anything but. In a computational study they scanned several trillion base pairs of metagenomic data and revealed a large number of stop codon reassignments in bacteria and bacteriophages. In some of those organisms, the three-letter codon UGA, which normally signals the end of a protein-coding gene, is hijacked to code for a the rarely encoded amino acid called selenocysteine.
The same team now discovered that some microorganisms actually recognize more than one codon for selenocysteine. From approximately 6.4 trillion bases of metagenomic sequences and 25,000 microbial genomes, the team identified several species that recognize the stop codons UAG and UAA, in addition to 10 sense codons, as acceptable variants for the selenocysteine codon UGA.
What about eukaryotic organisms? Although we found nine tRNASec variants of algal origin, they need further validation, because they are almost identical to canonical tRNASec species. A similar search of 92 mammalian genomes (215 Gbp) and of the Drosophila melanogaster genome (139 Mbp) showed no exception to the use of UGA as the Sec codon. Whether this is related to the necessity of selenoproteins in high-level redox signaling pathways or due to the sophisticated backup systems remains to be investigated. However, in the lower eukaryote Euplotes crassus UGA serves both as a Cys and also as a SECIS-dependent Sec codon