Romier C, Reuter K, Suck D, Ficner R
European Molecular Biology Laboratory, Structural Biology Programme, Heidelberg, Germany.
EMBO J. 1996 Jun 3;15(11):2850-7.
tRNA-guanine transglycosylases (TGT) are enzymes involved in the modification of the anticodon of tRNAs specific for Asn, Asp, His and Tyr, leading to the replacement of guanine-34 at the wobble position by the hypermodified base queuine. In prokaryotes TGT catalyzes the exchange of guanine-34 with the queuine (.)precursor 7-aminomethyl-7-deazaguanine (preQ1). The crystal structure of TGT from Zymomonas mobilis was solved by multiple isomorphous replacement and refined to a crystallographic R-factor of 19% at 1.85 angstrom resolution. The structure consists of an irregular (beta/alpha)8-barrel with a tightly attached C-terminal zinc-containing subdomain. The packing of the subdomain against the barrel is mediated by an alpha-helix, located close to the C-terminus, which displaces the eighth helix of the barrel. The structure of TGT in complex with preQ1 suggests a binding mode for tRNA where the phosphate backbone interacts with the zinc subdomain and the U33G34U35 sequence is recognized by the barrel. This model for tRNA binding is consistent with a base exchange mechanism involving a covalent tRNA-enzyme intermediate. This structure is the first example of a (beta/alpha)-barrel protein interacting specifically with a nucleic acid.
转运RNA-鸟嘌呤转糖基酶（TGT）是参与修饰天冬酰胺、天冬氨酸、组氨酸和酪氨酸特异性转运RNA反密码子的酶，导致摆动位置的鸟嘌呤-34被超修饰碱基queuine取代。在原核生物中，TGT催化鸟嘌呤-34与queuine（.）前体7-氨甲基-7-脱氮鸟嘌呤（preQ1）的交换。运动发酵单胞菌TGT的晶体结构通过多同晶置换法解析，并在1.85埃分辨率下精修至晶体学R因子为19%。该结构由一个不规则的（β/α）8桶状结构和一个紧密相连的含锌C末端亚结构域组成。亚结构域与桶状结构的堆积由靠近C末端的一个α螺旋介导，该α螺旋取代了桶状结构的第八个螺旋。TGT与preQ1复合物的结构表明了转运RNA的一种结合模式，其中磷酸主链与锌亚结构域相互作用，U33G34U35序列被桶状结构识别。这种转运RNA结合模型与涉及共价转运RNA-酶中间体的碱基交换机制一致。该结构是（β/α）桶状蛋白与核酸特异性相互作用的第一个例子。
