Engineering Methyltransferase and Sulfoxide Synthase for High-Yield Production of Ergothioneine

Ergothioneine (ERG) is an unusual sulfur-containing amino acid with antioxidant activity that can be synthesized by certain bacteria and fungi. Microbial fermentation is a promising method for ERG production. In this study, the bifunctional enzyme methyltransferase–sulfoxide synthase NcEgt1 from Neurospora crassa was truncated to obtain sulfoxide synthase TNcEgt1, which showed a higher expression level in Escherichia coli BL21(DE3). Then, the genes egtD encoding methyltransferase EgtD and egtE encoding C–S lyase EgtE from Mycobacterium smegmatis were cloned with TncEgt1 into E. coli BL21(DE3) to produce 70 mg/L ERG. To improve ERG production, TNcEgt1 and EgtD were modified, and the resulting mutants were screened with an established high-throughput method which could directly analyze the ERG content in culture broths. After several rounds of mutation and screening, the optimal mutant MD4 was obtained and produced 290 mg/L ERG. Furthermore, a fed-batch culture was conducted in a 5 L bioreactor. After optimizing the fermentation process, the ERG yield reached 5.4 g/L after 94 h of cultivation supplemented with amino acids and glycerol, which is the highest ERG yield reported to date. The results showed that ERG production was significantly improved by modifying the key enzymes, and the engineered strains constructed in this study have potential industrial application prospects.