Metabolic engineering of Corynebacterium glutamicum for high-level ectoine production - design, combinatorial assembly and implementation of a transcriptionally balanced heterologous ectoine pathway.

Ectoine is formed in various bacteria as cell protectant against all kinds of stress. Its preservative and protective effects have enabled various applications in medicine, cosmetics, and biotechnology, and ectoine therefore has high commercial value. Industrially, ectoine is produced in a complex high-salt process, which imposes constraints on the costs, design, and durability of the fermentation system. Here, Corynebacterium glutamicum is upgraded for the heterologous production of ectoine from sugar and molasses. To overcome previous limitations, the ectoine pathway taken from Pseudomonas stutzeri is engineered using transcriptional balancing. An expression library with 185,193 variants is created, randomly combining 19 synthetic promoters and three linker elements. Strain screening discovers several high-titer mutants with an improvement of almost fivefold over the initial strain. High production thereby particularly relies on a specifically balanced ectoine pathway. In an optimized fermentation process, the new top producer C. glutamicum ectABC(opt) achieves an ectoine titer of 65 g L-1 and a specific productivity of 120 mg g(-1) h(-1). This process is the first reported example of a simple fermentation process under low-salt conditions using well-established feedstocks to produce ectoine with industrial efficiency. There is a compelling case for more intensive implementation of transcriptional balancing in future metabolic engineering of C. glutamicum.