Toehold Switch-Based Approach for Engineering Acid-Tolerance Modules to Enhance Production Robustness of Industrial E. coli Strains at Low pH

Enhancing acid tolerance of industrial microorganisms is critical for improving fermentation efficiency and sustainability. This study presents a synthetic biology approach that employs toehold switch-based acid-tolerance modules to engineer acid-tolerant strains. This toehold switch-based approach enables the construction of modules consisting of a trigger block and a switch block, generating a synthetic module library of ~10⁵ constructs that integrate four acid-responsive promoters and 18 acid-resistance genes. Through stepwise evaluation, we identified two best synthetic modules, RE-6 and RE-38, which enabled an industrial lysine-producing strain to maintain lysine titers and yields at pH 5.5 comparable to those observed in the parent strain at pH 6.8. Transcriptional analyses revealed that upregulation of key acid-resistance genes involved in protein quality control, reactive oxygen species scavenging and redox homeostasis contributed to the enhanced acid tolerance of the engineered strains. Our study offers a powerful toehold switch-based approach for constructing synthetic modules of interest, particularly for enhancing the robustness and productivity of industrial strains.