A Genetic Switch for Stable, Long-Term Fermentative Production of Anabolic Products in Yeast
The strategic partnership between Total and Amyris is focused on producing biomolecules for renewable fuels and chemicals, and in particular for blending with petroleum-derived products to create diesel and jet fuel with reduced carbon footprint and pollution. Production and marketing of the flagship molecule farnesene (Biofene®) has already been commercialized with production scale in some markets. Farnesene has many potential applications as a renewable feedstock for the aforementioned diesel and jet fuels, as well as polymers, lubricants, solvents, and cosmetics. To reduce the production cost of farnesene, at Amyris we engineer strains using state-of-the art industrial synthetic biology platform to have high titer, yield, and productivity, and we perform fermentations in 200 m3 vessels over the course of many days or weeks. The challenge is that high-producer cells grow more slowly than spontaneous mutant low- or non-producer cells, especially in the nutrient-unlimited conditions of the seed train expansion, and yet must comprise the vast majority of the population. We have successfully addressed this challenge by developing an industrially-scalable genetic switch to successfully maintain high performance throughout lengthy fermentations. This genetic switch uses maltose (a cheap, non-toxic and metabolizable molecule) to control transcription such that when maltose is added in the seed train, product formation is shut off. This increased the growth of high-producer cells, resulting in higher inoculum purity and improved performance in bioreactors. The construction of the switch required the development of synthetic promoters that are tightly turned OFF in the absence of maltose.