Fig. 1.

Influence of protein overexpression on cell growth and culture pH. A, Growth curves in LB medium at 37 °C. E. coli MG1655 cells harboring the indicated pQE-derived expression vector and the pREP4 LacI-expressing plasmid were induced with 1 mm IPTG at an OD₆₀₀ of 0.4–0.6, and growth was continued for 3 h. Measurements were conducted on culture aliquots diluted with LB to OD₆₀₀ < 1.0. Two control cultures harboring the empty pQE-60 vector were evaluated, one induced with IPTG and the other not induced. B, Culture pH was measured using a standard electrode at the indicated times. C, Coomassie-Blue-stained 15% SDS-PAGE gel of cellular fractions at the end of the induction period. After lysis by sonication with a microtip probe, the extract was centrifuged for 1 h at 14,000 rpm to separate pellet and soluble fractions.