A non-classical biosynthetic route enables tocopherol synthesis in bacteria

Tocopherols have been widely applied in healthcare, medicine, animal feed and other fields. However, microbial synthesis of tocopherols with industrial potential has not been accomplished yet. Here we report a non-classical biosynthetic route for tocopherol synthesis in Rhodobacter species that bypasses the classical α-tocopherol pathway requiring the 2,3-dimethyl-5-phytyl-1,4-benzoquinone (DMPBQ) as an intermediate. In particular, a Rhodobacter methyltransferase with bifunctional γ/β-tocopherol methyltransferase activities is identified, which can directly synthesize α-tocopherol from δ-tocopherol and γ/β-tocopherols. Furthermore, geranylgeranyl pyrophosphate reductase (GR) plays an important role in reducing tocotrienols into tocopherols. Through systematic metabolic engineering, the engineered Rhodobacter sphaeroides produced a total titer of 508.21 mg l^-1 tocopherols in shake flasks, and 3.51 g l^-1 under the fed-batch fermentation after process optimization. Taken together, these results offer a non-classical biosynthetic pathway for efficient tocopherol biosynthesis, which would pave the way for future commercial production of tocopherols by microbial fermentation.