Increasing the secretion ability of the kil gene for recombinant proteins in Escherichia coli by using a strong stationary-phase promoter

By using a β-glucanase from Bacillus as a model protein, we investigated whether the secretion competence based on the action of the kil gene can be improved using stronger promoters for the expression of the kil gene. Since the production of extracellular target proteins also depends on the promoter strengths of the target gene, we constructed four expression vectors with all possible combinations of a weak and a strong stationary-phase promoter for the kil gene, and a weak and a strong constitutive promoter, respectively, for the β-glucanase gene. The results of batch fermentations showed that the use of stronger promoters generally decreased the cell density. However, a drastic increase of productivity of the cells to produce and secrete β-glucanase resulted in a significantly higher activity of extracellular β-glucanase. The yield of extracellular β-glucanase can be increased (to 168 %) by using a strong promoter for the β-glucanase alone. However, the increase was much higher when the weak promoter of the kil gene was replaced by a strong stationary-phase promoter (to 221 %). An even higher yield of extracellular β-glucanase was reached when β-glucanase was expressed by a strong promoter in addition indicating a combinatorial effect. This shows that the extracellular production of a recombinant target gene can be optimized by tuning the promoter strengths of components, the kil gene and the target gene.