Selenomodification of tRNA in archaea requires a bipartite rhodanese enzyme

5-Methylaminomethyl-2-selenouridine (mnm5Se2U) is found in the first position of the anticodon in certain tRNAs from bacteria, archaea and eukaryotes. This selenonucleoside is formed in Escherichia coli from the corresponding thionucleoside mnm5S2U by the monomeric enzyme YbbB. This nucleoside is present in the tRNA of Methanococcales, yet the corresponding 2-selenouridine synthase is unknown in archaea and eukaryotes. Here we report that a bipartite ybbB ortholog is present in all members of the Methanococcales. Gene deletions in Methanococcus maripaludis and in vitro activity assays confirm that the two proteins act in trans to form in tRNA a selenonucleoside, presumably mnm5Se2U. Phylogenetic data suggest a primal origin of seleno-modified tRNAs.

► A bipartite archaeal tRNA 2-selenouridine synthase (SelU) was identified in silico.
► Both proteins are required to catalyze tRNA selenation in vivo and in vitro.
► SelU presumably was present in the last common ancestor of bacteria and archaea.