Protein expression systems combining a flavonoid-inducible promoter and T7 RNA polymerase in Bacillus subtilis

Recombinant protein production must be tightly controlled when overproduction adversely affects the host bacteria. We developed a flavonoid-inducible T7 expression system in Bacillus subtilis using the qdoI promoter to control the T7 RNA polymerase gene (T7 pol). Using the egfp reporter gene controlled by the T7 promoter in a multicopy plasmid, we confirmed that this expression system is tightly regulated by flavonoids, such as quercetin and fisetin. Altering the qdoI promoter for T7 pol control to its hybrid derivative increased the expression level by 6.6-fold at maximum values upon induction. However, faint expression leakage was observed under a noninducing condition. Therefore, the two expression systems with the original qdoI promoter and the hybrid construct can be used selectively, depending on the high control accuracy or production yield required.