Crystallization and preliminary crystallographic studies of putative RNA 3′-terminal phosphate cyclase from the crenarchaeon Sulfolobus tokodaii

RNA 3′-terminal phosphate cyclase from the crenarchaeon S. tokodaii has been overexpressed and purified. The apo form, its selenomethionine derivative and their complexes with ATP under different conditions have been crystallized. The structure of the selenomethionine-labelled form has been successfully solved by the Se-MAD method. RNA 3′-terminal phosphate cyclase (Rtc) is an enzyme involved in RNA splicing that converts the 3′-terminal hydroxyl group of truncated RNA to 2′,3′-cyclic phosphate, which is required just before its ligation. This reaction may occur in the following two steps: (i) Rtc + ATP → Rtc-AMP + PP_i and (ii) RNA-N3′p + Rtc-AMP → RNA-N>p + Rtc + AMP. In order to reveal the reaction mechanism, Rtc of Sulfolobus tokodaii (St-Rtc) overexpressed in Escherichia coli was purified and crystallized in the following states: St-Rtc, St-Rtc+Mn, St-Rtc+ATP, St-Rtc+AMP and St-Rtc-AMP. The crystals diffracted to 2.25–3.00 Å resolution and preliminary solutions of their structures have been obtained by molecular replacement using the structure of a selenomethionine-labelled St-Rtc crystal which was solved in advance using the MAD method as a model. These crystals grew in two different space groups (P3₁ and P4₂), with the former space group displaying two distinct packing modes