Screening of high glucose tolerant Escherichia coli for L-valine fermentation by autonomous evolutionary mutation

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L-valine, also known as L-2,6-diaminocaproic acid, stands out as a superior nutritional supplement (Nielsen 2019 ; Olin-Sandoval et al. 2019 ). Its production primarily relies on microbial fermentation. Key microbial strains for L-valine production include Escherichia coli (Savrasova and Stoynova 2019 ), Corynebacterium glutamicum (Liu et al. 2021 ; Wang et al. 2024a ), and Saccharomyces cerevisiae (Ohashi et al. 2020 ; Takpho et al. 2018a , b ). However, the metabolic network of the high-yielding L-valine strain C. glutamicum is intricate (Zhang et al. 2018 ), posing significant challenges for genetic modifications due to limited genetic information. When S. cerevisiae is used as a host for metabolic engineering, it lacks a strongly regulated promoter, leading to a prolonged fermentation period, complex nutritional requirements, and unsuitability for high-density culturing. Additionally, the accumulation of nutrients and monosaccharides in the later stages of fermentation increases the cost of separation and purification (Takpho et al. 2018a , b ). On the other hand, E. coli has emerged as a preferred strain for amino acid production due to its well-defined genetics, rapid fermentation cycle, high glucose utilization, and advanced gene manipulation techniques (Hao et al. 2020 ; Zheng et al. 2018 ). Glucose serves as the primary carbohydrate source for microbial fermentation-based L-valine production (Xu et al. 2019 ). An insufficient glucose supply during fermentation can slow strain growth and reduce product yields, while excessively high glucose concentrations can increase osmotic pressure, hindering glucose and nutrient absorption, thus limiting strain growth and product synthesis. Wild-type E. coli exhibits limited glucose tolerance and metabolic synthesis capabilities. The inherent complexities of the biological system pose significant challenges in engineering novel phenotypes in E. coli (Xu et al. 2019 ). To overcome these challenges, further research is needed to enhance the glucose tolerance and metabolic efficiency of E. coli for improved L-valine production.

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