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 (preQ₀)-containing tRNA (q₀N-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 q₀N-tRNA to form q₀kN-tRNA, which has a preQ₀ 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 q₀N-tRNA to G⁺-tRNA in the presence of SAM and lysine via q₀kN-tRNA. We propose that ArcS and RaSEA should be considered an archaeosine synthase α-subunit (lysine transferase) and β-subunit (q₀kN-tRNA lyase), respectively.

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