2.1 Strains and culture condition
The strains and plasmids used in this study are shown in Table S1 . E. coli JM109 is used for plasmid construction, and BL21 is used for protein expression. E. coli JM109 was cultured in LB medium or on LB plate containing 1.5% (w/v) agar at 37 °C. E. coli BL21 was cultured in LB medium or TBSB medium (yeast extract 24 g, tryptone 12 g, KH2PO4 2.31 g, K2HPO4·3H2O 12.55 g, glycerin 10 g, and 1 L H2O) at 37 °C. The final concentration of kanamycin for E. coli JM109 and BL21 is 50 mg/L.