Fungi—at least some of them—use selenocysteine after all

Selenocysteine is found in many organisms scattered across the tree of life. Biologists have thought that fungi missed out on using the amino acid because they haven’t been able to find any with selenoproteins. But they may have reached that conclusion too soon. Vadim N. Gladyshev and Marco Mariotti of Harvard Medical School and coworkers have found selenocysteine-containing proteins in nine species of fungi across three phyla (Nat. Microbiol. 2019, DOI: 10.1038/s41564-018-0354-9). In most organisms that use selenocysteine, including humans, the amino acid is added by repurposing a stop codon and using special RNA machinery. A few of the genes for fungal selenoproteins coded for RNA structures similar to the ones other organisms use, but the rest of the selenoprotein genes lacked such machinery, suggesting that those selenocysteines are inserted through a distinct, currently unknown mechanism. Despite the different presumed synthesis mechanisms, all the fungal selenoproteins identified so far belong to known selenoprotein families. Microbiologists are interested in these selenoproteins from an evolutionary perspective. But the findings also suggest that scientists may now be able to engineer yeast to make sufficient quantities of selenoproteins to be able to obtain crystal structures, something that has not previously been possible.