Enhancing the biosynthesis of riboflavin in the recombinant BL21 strain by metabolic engineering.

In this study, to construct the riboflavin-producing strain R1, five key genes, , , , , and , were cloned and ligated to generate the plasmid pET-AE, which was overexpressed in BL21. The R1 strain accumulated 182.65 ± 9.04 mg/l riboflavin. Subsequently, the R2 strain was constructed by the overexpression of harboring the constructed plasmid pAC-Z in the R1 strain. Thus, the level of riboflavin in the R2 strain increased to 319.01 ± 20.65 mg/l (74.66% increase). To further enhance transcript levels and riboflavin production, the FMN riboswitch was deleted from BL21 with CRISPR/Cas9 to generate the R3 strain. The R4 strain was constructed by cotransforming pET-AE and pAC-Z into the R3 strain. Compared to those of BL21, the transcript levels of R2 and R4 improved 2.78 and 3.05-fold, respectively. The R4 strain accumulated 437.58 ± 14.36 mg/l riboflavin, increasing by 37.17% compared to the R2 strain. These results suggest that the deletion of the FMN riboswitch can improve the transcript level of and facilitate riboflavin production. A riboflavin titer of 611.22 ± 11.25 mg/l was achieved under the optimal fermentation conditions. Ultimately, 1574.60 ± 109.32 mg/l riboflavin was produced through fed-batch fermentation with 40 g/l glucose. This study contributes to the industrial production of riboflavin by the recombinant BL21.