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

The tRNA modifying enzyme tRNA-guanine transglycosylase (TGT) is involved in 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). Mutation of Ser(90) to an alanine in Escherichia coli TGT leads to a dramatic reduction of enzymatic activity (Reuter, K. et al. (1994) Biochemistry 33, 7041-7046). To further clarify the role of this residue in the catalytic center, we have mutated the corresponding Ser(103) of the crystallizable Zymomonas mobilis TGT into alanine. The crystal structure of a TGT(S103A)/preQ(1) complex combined with biochemical data presented in this paper suggest that Ser(103) is essential for substrate orientation in the TGT reaction. (C) 1999 Federation of European Biochemical Societies.