ACS Chem Biol. 2012 Feb 17;7(2):300-5. doi: 10.1021/cb200361w. Epub 2011 Nov 11.
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.
考古核苷 (G(+)) 存在于许多古菌 tRNA 的第 15 位。在广古菌中，G(+) 前体，7-氰基-7-脱氮鸟嘌呤（preQ(0)），由 tRNA 鸟嘌呤转糖基酶（arcTGT）插入 tRNA，然后由 Archaeosine Synthase（ArcS）转化为 G(+)。然而，许多已知含有 G(+)的泉古菌缺乏 ArcS 同源物。通过比较基因组学方法，在这些生物体中发现了两种可能具有替代 ArcS 功能的家族：(1) GAT-QueC，一种具有 N 端谷氨酰胺酰胺转移酶 II 类结构域融合到与 QueC 同源的酶的双结构域家族，QueC 酶产生 preQ(0)和 (2) QueF-like，一种与细菌酶同源的家族，催化 preQ(0)还原为 7-氨基甲基-7-脱氮鸟嘌呤。在这里，我们表明这两种蛋白质家族能够在异源系统中催化 G(+)的形成。泉古菌和广古菌 arcTGT 的结构和序列比较表明，泉古菌酶具有更广泛的底物特异性。这些结果为泉古菌中 G(+)的合成和回收提供了新的模型。
