Glycolate is widely used in industry, especially in the fields of chemical cleaning, cosmetics, and medical materials, and has broad market prospects for the future. Recent advances in metabolic engineering and synthetic biology have significantly improved the titer and yield of glycolate. However, an expensive inducer was used in previous studies that is not feasible for use in large-scale industrial fermentations. To constitutively biosynthesize glycolate, the expression level of each gene of the glycolate synthetic pathway needs to be systemically optimized. The main challenge of multi-gene pathway optimization is being able to select or screen the optimum strain from the randomly assembled library by an efficient high-throughput method within a short period of time. To overcome these challenges, we firstly established a glycolate-responsive biosensor and developed agar plate- and 48-well deep well plate-scale high-throughput screening methods for rapid screening of superior glycolate producers from a large library. A total of 22 gradient strength promoter-5′-UTR complexes were randomly cloned upstream of the genes of the glycolate synthetic pathway, generating a large random assembled library. After rounds of screening, the optimum strain was obtained from 6×105 transformants in a week, and it achieved a titer of 40.9 ± 3.7 g/L glycolate in a 5-L bioreactor. Furthermore, high expression levels of the enzymes YcdW and GltA were found to promote glycolate production, whereas AceA has no obvious impact on glycolate production. Overall, the glycolate biosensor-based pathway optimization strategy presented in this work provides a paradigm for other multi-gene pathway optimizations.
Importance
The use of strong promoters, such as pTrc and T7, to control gene expression not only need adding expensive inducers but also results in excessive protein expression that may be resulting in unbalanced metabolic flux and the waste of cellular building blocks and energy. To balance the metabolic flux of glycolate biosynthesis, the expression level of each gene needs to be systemically optimized in a constitutive manner. However, the lack of a high-throughput screening methods restricted the glycolate synthetic pathway optimization. Our work firstly established a glycolate-response biosensor, then agar plate and 48-well plate scale high-throughput screening methods were developed for rapid screening of optimum pathways from a large library. Finally, we obtained a glycolate producing strain with good biosynthetic performance, and the use of the expensive inducer IPTG was avoided, which broadens our understanding about the mechanism of glycolate synthesis.
Droplet-based microfluidic platform for high-throughput screening of Streptomyces
