Use of selenocysteine, the 21st amino acid, in the fungal kingdom
Selenoproteins are a diverse class of proteins containing selenocysteine (Sec), the 21st amino acid, incorporated into proteins during translation through a unique recoding mechanism. Selenoproteins fulfil essential roles in several lineages, including vertebrates; yet, they are not ubiquitous across the tree of life. In particular, the fungal kingdom was thought to lack selenoproteins, a paradigm that we defy here. We show that the genetic machinery for Sec utilization is present in the genomes of five species belonging to diverse early-branching fungal phyla (Chytridiomycota, Zoopagomycota, and Mucoromycota). We identified several selenoprotein genes in each of these genomes, and we detected canonical Sec insertion RNA structures (SECIS elements) for some of them. The identified selenoproteins are known or predicted oxidoreductases, some of which are conserved in mammals. Phylogenetic analyses support a scenario of vertical inheritance of the Sec trait within eukaryotes and fungi. Sec was then lost in numerous independent events in various fungal lineages, even within Sec-containing phyla. Notably, Sec was lost at the base of Dikarya, resulting in the absence of this trait in Saccharomyces cerevisiae and other well studied fungi. Our results indicate that, despite scattered occurrence, selenoproteins are found in all kingdoms of life.