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 a-subunit (lysine transferase) and beta-subunit (q(0)kN-tRNA lyase), respectively.