Exchange of the l-cysteine exporter after in-vivo metabolic control analysis improved the l-cysteine production process with engineered Escherichia coli

Background: l-Cysteine is a proteinogenic amino acid of high pharmaceutical and industrial interest. However, the fermentation process for l-cysteine production is faced with multiple obstacles, like the toxicity of l-cysteine for the cells, the low carbon yield of the product, and the low selectivity of the l-cysteine exporter. In previous work, in-vivo metabolic control analysis (MCA) applied to an l-cysteine fed-batch production process with E. coli, followed by the targeted metabolic engineering to reduce an intracellular O-acetylserine (OAS) deficiency, resulted in a significant improvement of the l-cysteine production process with the new producer strain. Results: In this work, in-vivo MCA was applied to the l-cysteine fed-batch production process with the new producer strain (E. coli W3110 pCysK). The MCA indicated that a simultaneous increase in the exporter's expression and selectivity is required to increase the l-cysteine production further. The exchange of the l-cysteine exporter YdeD present in the plasmid pCysK for the potentially more selective exporter YfiK led to an increase of the maximal l-cysteine concentration by the end of the fed-batch process of 37% to a final concentration of 33.8 g L⁻¹. The l-cysteine production could also be extended for at least 20 h due to conserved cellular activity as a result of the reduction of carbon loss as OAS. Conclusions: It could be shown that the in-vivo MCA methodology can be utilised iteratively with cells from the production process to pinpoint targets for further strain optimisation towards a significant increase in the l-cysteine production with E. coli. The use of this technology in combination with process engineering to adapt the fed-batch process to the modified strain may achieve a further improvement of the process performance.