Genetically switched d-lactate production in Escherichia coli

During a fermentation process, the formation of the desired product during the cell growth phase competes with the biomass for substrates or inhibits cell growth directly, which results in a decrease in production efficiency. A genetic switch is required to precisely separate growth from production and to simplify the fermentation process. The ldhA promoter, which encodes the fermentative d-lactate dehydrogenase (LDH) in the lactate producer Escherichia coli CICIM B0013-070 (ack-pta pps pflB dld poxB adhE frdA), was replaced with the λ pR and pL promoters (as a genetic switch) using genomic recombination and the thermo-controllable strain B0013–070B (B0013-070, ldhAp::kan-cIts857-pR–pL), which could produce two-fold higher LDH activity at 42 °C than the B0013-070 strain, was created. When the genetic switch was turned off at 33 °C, strain B0013-070B produced 10% more biomass aerobically than strain B0013-070 and produced only trace levels of lactate which could reduce the growth inhibition caused by oxygen insufficiency in large scale fermentation. However, 42 °C is the most efficient temperature for switching on lactate production. The volumetric productivity of B0013-070B improved by 9% compared to that of strain B0013-070 when it was grown aerobically at 33 °C with a short thermo-induction at 42 °C and then switched to the production phase at 42 °C. In a bioreactor experiment using scaled-up conditions that were optimized in a shake flask experiment, strain B0013-070B produced 122.8 g/l d-lactate with an increased oxygen-limited productivity of 0.89 g/g·h. The results revealed the effectiveness of using a genetic switch to regulate cell growth and the production of a metabolic compound.

► The λ pR–pL promoters were used as a genetic switch for metabolic regulation.
► Switching off lactate production in growth phase improved biomass accumulation.
► Switching on the genetic switch improved lactate productivity in production phase.