Optimizing the expression of a monoclonal antibody fragment under the transcriptional control of the Escherichia coli lac promoter

The expression of a monoclonal antibody Fab fragment in Escherichia coli st rain RB791/pComb3, induced with either lactose or isopropyl-beta-D-thiogala ctoside (IPTG), was compared to determine if lactose might provide an inexp ensive alternative to induction with IPTG. Induction of Fab expression impo sed a metabolic load on the recombinant cells, resulting in lower final cel l yields compared to the non-induced controls. An IPTG concentration of 0.0 5 mM was sufficient to achieve maximal expression of soluble Fab protein wh en inducing in the early-, mid-, or late-log phases of batch cultures grown using either glucose or glycerol as a carbon source. The largest overall y ield of Fab fragments when using 0.05 mM IPTG was achieved by increasing th e final yield of cells through glycerol feeding following induction in late -log phase. Lactose was as effective as IPTG for inducing Fab expression in E. coli RB791/pComb3. The greatest overall level of Fab expression was fou nd when cells grown on glycerol were induced with 2 g/L lactose in late-log phase. Since the cost of 0.05 mM of IPTG is significantly greater than the cost of 2 g/L lactose, lactose provides an inexpensive alternative to IPTG for inducing the expression of Fab fragments, and possibly other recombina nt proteins, from the E. coli lac promoter.