Blending of selected yeast extract and peptone for inducible and constitutive protein production in Escherichia coli using the pET system

pET vectors allow inducible expression of recombinant proteins in Escherichia coli. In this system, isopropyl β-d-1-thiogalactopyranoside (IPTG) drives lacUV5 promoter to produce T7 RNA polymerase, simultaneously releasing the suppression of T7lac promoter. T7 RNA polymerase then strongly transcribes the target gene. A lac repressor encoded by lacI in the vector represses the promoters. Despite stringent repression and inducible expression achieved with the pET system, unexpected leaky expression can occur without IPTG induction. Here, by evaluating leaky expression in recombinant cells cultured in various Luria–Bertani (LB) media, prepared using yeast extract and peptone from different suppliers, as well as in five commercial premix-LB media, we confirmed the presence of unknown lac inducers in LB. To explore these inducers, we examined E. coli growth in media comprising yeast extract or peptone. At 4% concentration, five commercial yeast extract and six peptone samples individually allowed E. coli growth equivalent to that in LB medium. We determined the luciferase activity of the luxCDABE operon in the pET vector under these conditions. The presence of different concentrations of inducers was detected in both the yeast extract and peptone. Furthermore, we blended yeast extract and peptone with low or high concentrations of lac inducers. The low-expression blend, used as a basal medium before IPTG addition, allowed leak-free, tightly controlled expression. The high-expression blend was used for constitutive high-expression and pET induction with the basal medium, in lieu of IPTG. These blended media can be used for well-controlled inducible and constitutive expression using the pET system.