All steps from primers purchase to cell lysate preparation were carried out as previously described. The enzyme was purified by loading the cell-free extract onto a Ni-NTA column (QIAGEN) according to the manufacturer's instructions. The elution buffer was 50 mM sodium potassium phosphate (pH 7.5), 50 mM NaCl, 250 mM imidazole and 10 % glycerol and the desalting buffer was 50 mM phosphate (pH 7.5), 50 mM NaCl and 10 % glycerol. Large-scale purification was conducted from a 500 mL culture by nickel affinity chromatography in tandem with desalting (HiPrep 26/10 17-5087-01) as described elsewhere. Protein concentration was determined by the Bradford method with bovine serum albumin as the standard. The samples were analyzed by SDS-PAGEs using the Invitrogen NuPage system. The purified proteins were stored at -80 °C.