Supplementation of substrate uptake gene enhances the expression of rhIFN-β in high cell density fed-batch cultures of Escherichia coli.

Over-expression of recombinant proteins in Escherichia coli triggers a metabolic stress response which causes a sharp decline in both growth and product formation rates post induction. We identified a key down-regulated substrate utilization gene, glycerol kinase (glpK), whose up-regulation could help alleviate this stress response. In a proof of principal study conducted in shake flask cultures, the glpK gene under the "ara" promoter in a pPROLar.A122 vector was co-transformed along with the recombinant interferon-β (rhIFN-β) gene in a pET22b vector into E. coli BL-21(DE3) cells. Co-expression of glpK improved the expression levels of rhIFN-β in glycerol containing medium, while no such gain was observed in medium without glycerol. This study was extended to high cell density fed-batch cultures where exponential feeding of complex substrates was done to increase biomass and hence product titers. For this we first constructed a modified E. coli strain BL-21(glpK (+)) where the glpK gene was inserted downstream of the ibpA promoter in the host chromosome. There was a significant improvement in growth as well as expression levels of rhIFN-β in this modified strain when the feed medium contained high glycerol. A final product concentration of 4.8 g/l of rhIFN-β was obtained with the modified strain which was 35 % higher than the control.