Diversity of archaeosine synthesis in crenarchaeota

Archaeosine (G(+)) is found at position 15 of many archaeal tRNAs. In Euryarchaeota, the G(+) precursor, 7-cyano-7-deazaguanine (preQ(0)), is inserted into tRNA by tRNA-guanine transglycosylase (arcTGT) before conversion into G(+) by ARChaeosine Synthase (ArcS). However, many Crenarchaeota known to harbor G(+) lack ArcS homologues. Using comparative genomics approaches, two families that could functionally replace ArcS in these organisms were identified: (1) GAT-QueC, a two-domain family with an N-terminal glutamine amidotransferase class-II domain fused to a domain homologous to QueC, the enzyme that produces preQ(0) and (2) QueF-like, a family homologous to the bacterial enzyme catalyzing the reduction of preQ(0) to 7-aminomethyl-7-deazaguanine. Here we show that these two protein families are able to catalyze the formation of G(+) in a heterologous system. Structure and sequence comparisons of crenarchaeal and euryarchaeal arcTGTs suggest the crenarchaeal enzymes have broader substrate specificity. These results led to a new model for the synthesis and salvage of G(+) in Crenarchaeota.