3.5 Stability and versatility of BL21 (DE3-lac1G) strain
Stability and versatility are key indicators for investigating the quality of protein expression hosts. As shown in Fig. 6A, overall, the ration of plasmid-carrying cells in BL21 (DE3) continued to decline with increasing fermentation time, reaching a low of 40% at 36h and decreasing further to 0 at 43h. By contrast, the ratio of plasmid-carrying cells in BL21 (DE3-lac1G) remained about 96.5% at 43h, and about 49% of the cells maintained enzyme activity at normal levels (~ 350 U / mL) (Fig. 6B). Therefore, the stability of the new protein expression host is much higher than that of the parent strain. Moreover, the SDS-PAGE images showed that BL21 (DE3-lac1G) is also resistant to autolysis that is triggered by a further 10 tested recombinant proteins, including amide hydrolase, cephalosporin C acylase, formate dehydrogenase, R-carbonyl reductase, L-lactate dehydrogenase, L-2-hydroxyisocaproate dehydrogenase, L-phenylalanine dehydrogenase, secondary alcohol dehydrogenase, D-2-hydroxyisocaproate dehydrogenase, S-carbonyl reductase (Fig. 7).