Engineering a PhrC-RapC-SinR quorum sensing molecular switch for dynamic fine-tuning of menaquinone-7 synthesis in Bacillus subtilis.

Background: Menaquinone-7 (MK-7) is a valuable vitamin K₂ produced by Bacillus subtilis. Although many strategies have been adopted to increase the yield of MK-7 in B. subtilis, the effectiveness of these common approaches is not high because long metabolic synthesis pathways and numerous bypass pathways competing for precursors with MK-7 synthesis. Regarding the modification of bypass pathways, studies of common static metabolic engineering method such as knocking out genes involved in side pathway have been reported previously. Since byproductsphenylalanine(Phe), tyrosine (Tyr), tryptophan (Trp), folic acid, dihydroxybenzoate, hydroxybutanone in the MK-7 synthesis pathway are indispensable for cell growth, the complete knockout of the bypass pathway restricts cell growth, resulting in limited increase in MK-7 synthesis. Dynamic regulation via quorum sensing (QS) provides a cost-effective strategy to harmonize cell growth and product synthesis, eliminating the need for pricey inducers. SinR, a transcriptional repressor, is crucial in suppressing biofilm formation, a process closely intertwined with MK-7 biosynthesis. Given this link, we targeted SinR to construct a dynamic regulatory system, aiming to modulate MK-7 production by leveraging SinR's regulatory influence.

Results: A modular PhrC-RapC-SinR QS system is developed to dynamic regulate side pathway of MK-7. In this study, first, we analyzed the SinR-based gene expression regulation system in B. subtilis 168 (BS168). We constructed a promoter library of different abilities, selected suitable promoters from the library, and performed mutation screening on the selected promoters. Furthermore, we constructed a PhrC-RapC-SinR QS system to dynamically control the synthesis of Phe, Tyr, Trp, folic acid, dihydroxybenzoate, hydroxybutanone in MK-7 synthesis in BS168. Cell growth and efficient synthesis of the MK-7 production can be dynamically balanced by this QS system. Using this system to balance cell growth and product fermentation, the MK-7 yield was ultimately increased by 6.27-fold, from 13.95 mg/L to 87.52 mg/L.

Conclusion: In summary, the PhrC-RapC-SinR QS system has been successfully integrated with biocatalytic functions to achieve dynamic metabolic pathway control in BS168, which has potential applicability to a large number of microorganisms to fine-tune gene expression and enhance the production of metabolites.