A "plug-n-play" modular metabolic system for the production of apocarotenoids.

Apocarotenoids, such as α-, β-ionone, and retinol, have high commercial values in the food and cosmetic industries. The demand for natural ingredients has been increasing dramatically in recent years. However, attempts to overproduce β-ionone in microorganisms have been limited by the complexity of the biosynthetic pathway. Here, an Escherichia coli-based modular system was developed to produce various apocarotenoids. Incorporation of enzyme engineering approaches (N-terminal truncation and protein fusion) into modular metabolic engineering strategy significantly improved α-ionone production from 0.5 mg/L to 30 mg/L in flasks, producing 480 mg/L of α-ionone in fed-batch fermentation. By modifying apocarotenoid genetic module, this platform strain was successfully re-engineered to produce 32 mg/L and 500 mg/L of β-ionone in flask and bioreactor, respectively (>80-fold higher than previously reported). Similarly, 33 mg/L of retinoids was produced in flask by reconstructing apocarotenoid module, demonstrating the versatility of the "plug-n-play" modular system. Collectively, this study highlights the importance of the strategy of simultaneous modular pathway optimization and enzyme engineering to overproduce valuable chemicals in microbes.