Unveiling the role of selenocysteine, the mysterious 21st amino acid

Unveiling the role of selenocysteine, the mysterious 21st amino acid

News from CRG

There exist a total of 61 codons that code for 20 amino acids, and three codons that act as stop signals in the translation process.

Nevertheless, certain organisms use an extra amino acid, selenocysteine, dubbed the 21st amino acid, which lacks its own codon and uses a stop codon after modifying it. For this purpose, it avails itself of complex machinery, with specific enzymes and RNA; this process can prove to be very costly for the cell. But why? What function does this amino acid have in proteins? Why is it present in humans and in the other vertebrates whereas, on the other hand, other species have lost it? Now, researchers from the Centre for Genomic Regulation (CRG) in Barcelona, part of the Barcelona Institute of Science and Technology (BIST), in collaboration with CRG Alumni Marco Mariotti and Vadim N. Gladyshev, from the Harvard Medical School (USA), and Gustavo Salinas, from the University of the Republic in Uruguay, have shed some light on these questions.

The fungi were the only organism kingdom in which a species with selenocysteine had never been found, and the researchers decided to focus on them, leveraging the recent publication of a thousand fungi genomes in public-access databases. On analysing them, they discovered, as they reported in the article published in Nature Microbiology, that nine of the 1,000 species actually did have this amino acid.

Understanding why selenocysteine is important in fungi and other branches of the tree of life may help us to understand why it is so important to our species and to define what makes selenium essential to human health”, concludes the the ICREA Research Professor Toni Gabaldón, head of the CRG’s Comparative Genomics group.


Mariotti M, Salinas G, Gabaldón T, Gladyshev VN. “Utilization of selenocysteine in early-branching fungal phyla.” Nat Microbiol. 2019 Feb 11. DOI: 

More information:
CRG website