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 β-cyclodextrin glycosyltransferase (β-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 β-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 β-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 α-CGTase expression (9.5 U/mL) during shake-flask cultivation, demonstrating the general applicability of this system. Finally, the production of β-CGTase using the dual-promoter P(HpaII)–P(amyQ′) system was investigated in a 3-L fermenter. Extracellular expression of β-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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12934-017-0649-1) contains supplementary material, which is available to authorized users.