Development of an efficient autoinducible expression system by promoter engineering in Bacillus subtilis

Background: Bacillus subtilis, a Gram-positive organism, has been developed to be an attractive expression platform to produce both secreted and cytoplasmic proteins owing to its prominent biological characteristics. We previously developed an auto-inducible expression system containing the srfA promoter (P-srfA) which was activated by the signal molecules acting in the quorum-sensing pathway for competence. The P-srfA promoter exhibited the unique property of inducer-free activity that is closely correlated with cell density. Results: To improve the P-srfA-mediated expression system to the high-cell-density fermentation for industrial production in the B. subtilis mutant strain that is unable to sporulate, a spore mutant strain BSG1682 was developed, and the P-srfA promoter was enhanced by promoter engineering. Using green fluorescent protein (GFP) as the reporter, higher fluorescent intensity was observed in BSG1682 with expression from either plasmid or chromosome than that of the wild type B. subtilis 168. Thereafter, the P-srfA was engineered, yielding a library of P-srfA derivatives varied in the strength of GFP expression. The P23 promoter exhibited the best performance, almost twofold stronger than that of P-srfA. Two heterologous proteins, aminopeptidase (AP) and nattokinase (NK), were successfully overproduced under the control of P23 in BSG1682. Finally, the capacity of the expression system was demonstrated in batch fermentation in a 5-L fermenter. Conclusions: The expression system demonstrates prominence in the activity of the auto-inducible promoter. Desired proteins could be highly and stably produced by integrating the corresponding genes downstream of the promoter on the plasmid or the chromosome in strain BSG1682. The expression system is conducive to the industrial production of pharmaceuticals and heterologous proteins in high-cell-density fermentation in BSG1682.