E. coli M15 harboring the gene of d-pflp, d-pflp-GC or dpflp-A549 was incubated in LB medium (Difco, USA) with 100 mg/ml ampicillin and 25 mg/ml kanamycin at 37 8C respectively. When the culture reached an absorbance of 0.6 at A600, isopropylthio-b-D-galactoside induction was given to a final concentration of 1 mM. Cells were harvested after 4 h incubation at 37 8C. Protein was purified with Ni-NTA resin spin kit (Qiagen) in native condition according to the manufacturer instruction. The imidazole residue was removed by P6 gel spin column (Bio-Rad) and the eluting product was dialyzed against 10 mM sodium phosphate buffer (pH 7.0). Purified protein was analyzed in 15% SDS-PAGE.