Effect of culture medium on protein expression in green fluorescent protein-expressing Escherichia coli BL21(DE3)

Abstract

A protein expression system using Escherichia coli is widely used to obtain useful proteins.　This study examined the effect of culture medium on the expression of green fluorescent protein (GFP) by E. coli BL21(DE3).　The bacteria were transformed using the pET19b plasmid, which inserted the GFP gene following the T7lac promoter. 　When the transformed E. coli BL21(DE3) was cultured in Luria Bertani (LB)-HI medium (the LB medium with hipolypepton as peptone), the GFP fluorescence was detected in the absence of protein expression inducers, such as isopropyl-β-D-thiogalactopyranoside. 　Moreover, compared to E. coli transformed with pET19b plasmid without the GFP insert, autonomous GFP expression was slightly detected when BL21(DE3) was grown in a culture medium without hipolypepton, such as LBBact medium, using Bacto™ Tryptone as peptone.　Therefore, galactose and lactose induced GFP expression in the culture medium without hipolypepton.　This suggests that hipolypepton contains an unknown substance that induces protein expression. 　Additionally, autonomous protein expression can be induced without adding a protein expression inducer to a culture medium containing hipolypepton.