LAUSR

Significance Lignin valorization is critical for economic viability of future biorefineries but is hindered due to the challenges of engineered bio-chassis such as the slow kinetics of substrate uptake, aromatics toxicity, and cost. Here, an autoregulatory system involving a vanillin autoinducible promoter is demonstrated with an aromatics transporter in Escherichia coli that is induced by lignin-derived aromatics and simultaneously converted to value-added platform chemical with diverse applications. In addition to alleviating toxicity, the engineered E. coli strain eliminates the need for an external inducer such as isopropyl β-d-1-thiogalactopyranoside during fermentation, thereby significantly reducing the process cost. This study demonstrates an autoregulatory system for aromatics bioconversion and serves as a platform for future strain development for lignin valorization. Efficient lignin valorization could add more than 10-fold the value gained from burning it for energy and is critical for economic viability of future biorefineries. However, lignin-derived aromatics from biomass pretreatment are known to be potent fermentation inhibitors in microbial production of fuels and other value-added chemicals. In addition, isopropyl-β-d-1-thiogalactopyranoside and other inducers are routinely added into fermentation broth to induce the expression of pathway enzymes, which further adds to the overall process cost. An autoregulatory system that can diminish the aromatics’ toxicity as well as be substrate-inducible can be the key for successful integration of lignin valorization into future lignocellulosic biorefineries. Toward that goal, in this study an autoregulatory system is demonstrated that alleviates the toxicity issue and eliminates the cost of an external inducer. Specifically, this system is composed of a catechol biosynthesis pathway coexpressed with an active aromatic transporter CouP under induction by a vanillin self-inducible promoter, ADH7, to effectively convert the lignin-derived aromatics into value-added chemicals using Escherichia coli as a host. The constructed autoregulatory system can efficiently transport vanillin across the cell membrane and convert it to catechol. Compared with the system without CouP expression, the expression of catechol biosynthesis pathway with transporter CouP significantly improved the catechol yields about 30% and 40% under promoter pTrc and ADH7, respectively. This study demonstrated an aromatic-induced autoregulatory system that enabled conversion of lignin-derived aromatics into catechol without the addition of any costly, external inducers, providing a promising and economically viable route for lignin valorization.