Mutagenesis and crystallographic studies of Zymomonas mobilis tRNA-guaninetransglycosylase to elucidate the role of serine 103 for enzymatic activity

The tRNA modifying enzyme tRNA-guanine transglycosylase (TGT) is involved i n the exchange of guanine in the first position of the anticodon with preQ( 1) as part of the biosynthesis of the hypermodified base queuine (Q). Mutat ion of Ser(90) to an alanine in Escherichia coli TGT leads to a dramatic re duction of enzymatic activity (Reuter, K. et al. (1994) Biochemistry 33, 70 41-7046). To further clarify the role of this residue in the catalytic cent er, we have mutated the corresponding Ser(103) of the crystallizable Zymomo nas mobilis TGT into alanine. The crystal structure of a TGT(S103A)/preQ(1) complex combined with biochemical data presented in this paper suggest tha t Ser(103) is essential for substrate orientation in the TGT reaction. (C) 1999 Federation of European Biochemical Societies.