Identification of a radical SAM enzyme involved in the synthesis of archaeosine

Archaeosine (G + ), 7-formamidino-7-deazaguanosine, is an archaea-specific modified nucleoside found at the 15th position of tRNAs. In Euryarchaeota, 7-cyano-7-deazaguanine (preQ 0 )-containing tRNA (q 0 N-tRNA), synthesized by archaeal tRNA-guanine transglycosylase (ArcTGT), has been believed to be converted to G + -containing tRNA (G + -tRNA) by the paralog of ArcTGT, ArcS. However, we found that several euryarchaeal ArcSs have lysine transfer activity to q 0 N-tRNA to form q 0 kN-tRNA, which has a preQ 0 lysine adduct as a base. Through comparative genomics and biochemical experiments, we found that ArcS forms a robust complex with a radical S -adenosylmethionine (SAM) enzyme named RaSEA. The ArcS–RaSEA complex anaerobically converted q 0 N-tRNA to G + -tRNA in the presence of SAM and lysine via q 0 kN-tRNA. We propose that ArcS and RaSEA should be considered an archaeosine synthase α-subunit (lysine transferase) and β-subunit (q 0 kN-tRNA lyase), respectively.