Experimental evolution reveals an effective avenue for d-lactic acid production from glucose-xylose mixtures via enhanced Glk activity and a cAMP-independent CRP mutation.

ABSTRACT

d-Lactic acid holds significant industrial importance due to its versatility and serves as a crucial component in the synthesis of environmentally friendly and biodegradable thermal-resistant poly-lactic acid. This polymer exhibits promising potential as a substitute for nonbiodegradable, petroleum-based plastics. The production of d-lactic acid from lignocellulosic biomass, a type of biorenewable and nonfood resources, can lower costs and improve product competitiveness. Glucose and xylose are the most abundant sugar monomers in lignocellulosic biomass materials. Despite Escherichia coli possessing native xylose catabolic pathways and transport, their ability to effectively utilize xylose is often hindered in the presence of glucose. Here, the E. coli strain Rec1.0, previously engineered to overcome carbon catabolite repression, was selected as the initial strain for reengineering to produce d-lactic acid. An adaptive evolution approach was employed to achieve highly efficient fermentation of glucose-xylose mixtures. The resulting strain, QJL010, could produce d-lactic acid of 87.5 g/L with a carbon yield of 0.99 mol/mol. Notably, the consumption rates of glucose and xylose reached 0.75 and 0.82 g/gDCW/h, respectively. Further analysis revealed that increased Glk activity, resulting from glk mutations (A142V and R188H), along with their upregulated expression, contributed to an elevated glucose consumption rate. Additionally, a CRP G141D mutation, cAMP-independent, stimulated the expression of the xylR, xylE, and galABC* genes, resulting in an accelerated xylose consumption rate. These findings provide valuable support for the utilization of E. coli platform strains in the production of value-added chemicals from lignocellulosic biomass.