High-level extracellular protein production in Bacillus subtilis using an optimized dual-promoter expression system

Background: We recently constructed a Bacillus subtilis strain (CCTCC M 2016536) from which we had deleted the srfC, spoIIAC, nprE, aprE and amyE genes. This strain is capable of robust recombinant protein production and amenable to high-cell-density fermentation. Because the promoter is among the factors that influence the production of target proteins, optimization of the initial promoter, P-amyQ from Bacillus amyloliquefaciens, should improve protein expression using this strain. This study was undertaken to develop a new, high-level expression system in B. subtilis CCTCC M 2016536. Results: Using the enzyme beta-cyclodextrin glycosyltransferase (beta-CGTase) as a reporter protein and B. subtilis CCTCC M 2016536 as the host, nine plasmids equipped with single promoters were screened using shake-flask cultivation. The plasmid containing the P-amyQ' promoter produced the greatest extracellular beta-CGTase activity; 24.1 U/mL. Subsequently, six plasmids equipped with dual promoters were constructed and evaluated using this same method. The plasmid containing the dual promoter P-HpaII-P-amyQ' produced the highest extracellular beta-CGTase activity (30.5 U/mL) and was relatively glucose repressed. The dual promoter P-HpaII-P-amyQ' also mediated substantial extracellular pullulanase (90.7 U/mL) and alpha-CGTase expression (9.5 U/mL) during shake-flask cultivation, demonstrating the general applicability of this system. Finally, the production of beta-CGTase using the dual-promoter P-HpaII-P-amyQ' system was investigated in a 3-L fermenter. Extracellular expression of beta-CGTase reached 571.2 U/mL (2.5 mg/mL), demonstrating the potential of this system for use in industrial applications. Conclusions: The dual-promoter P-HpaII-P-amyQ' system was found to support superior expression of extracellular proteins in B. subtilis CCTCC M 2016536. This system appears generally applicable and is amenable to scale-up.