Milbemycins and their semisynthetic derivatives are recognized as effective and eco-friendly pesticides, whereas the high price limits their widespread applications in agriculture. One of the pivotal questions is the accumulation of milbemycin-like by-products, which not only reduces the yield of the target products milbemycin A3/A4, but also brings difficulty to the purification. With other analogous by-products abolished, α9/α10 and β-family milbemycins remain to be eliminated. Herein, we solved these issues by engineering of post-modification steps. First, Cyp41, a CYP268 family cytochrome P450, was identified to participate in α9/α10 biosynthesis. By deleting cyp41, milbemycin α9/α10 was eliminated with an increase of milbemycin A3/A4 titer from 2382.5 ± 55.7 mg/L to 2625.6 ± 64.5 mg/L. Then, MilE, a CYP171 family cytochrome P450, was determined to be responsible for the generation of the furan ring between C6 and C8a of milbemycins. By further overexpression of milE, the production of β-family milbemycins was reduced by 77.2%. Finally, the titer of milbemycin A3/A4 was increased by 53.1% to 3646.9 ± 69.9 mg/L. Interestingly, overexpression of milE resulted in increased transcriptional levels of milbemycin biosynthetic genes and production of total milbemycins, which implied that the insufficient function of MilE was a limiting factor to milbemycin biosynthesis. Our research not only provides an efficient engineering strategy to improve the production of a commercially important product milbemycins, but also offers the clues for future study about transcriptional regulation of milbemycin biosynthesis.
Keywords: CYPs; Cytochromes P450; Milbemycins; Post-PKS step; Streptomyces bingchenggenis.