2.3 EGFP intensity measurement
After successful construction in E. coli JM109, all plasmids were transformed into E. coli BL21. Three transformants were picked up for each expression strain and activated in 24 deep-well plate containing 2 mL LB medium and 50 mg/L kanamycin at 37 ℃, 220 rpm for 12 h. Then, the cultures were transferred to 2 mL fresh LB medium with a 10% (v/v) inoculation rate, and 1 mM IPTG was directly added after inoculation. After 24 h of induction, the cells were collected and diluted at about 0.5. The fluorescence intensity was measured by a fluorescence spectrophotometer at a 488 nm excitation wavelength and a 507 emission wavelength. Each sample’s unit fluorescence intensity was calculated by normalizing the fluorescence intensity by OD600.