Effect of redox imbalance on the fitness of Escherichia coli strains engineered for production of biofuels and biochemicals

Bacterial metabolism is a set of inter-connnected reactions in the cell upon which a substrate molecule is converted to another molecule and the energy generated is utilized for the multiplication of the organism. For a bacterial fermentation process to function, the number of electron between the substrate and the desired product need to be balanced. Redox balancing is thus an important consideration in metabolic engineering and strain development as well as fermentation design and control. The internal redox state of the cell is known to affect a broad range of genes and cellular functions, metabolite profiles, and membrane transport and other key functions. In our first study, we observed that the redox state of the cell also affects another key cellular function: genetic stability. By measuring the mutation rate in different E. coli clones, we observed a correlation between correlation between the intracellular redox ratio (NADH/NAD+) and the mutagenesis rate of the organism. Further studies revealed that this increase in mutation was caused by the stress induced downregulation of the DNA repair. In our next study, we sought to expand our toolbox for the redox engineering by identifying a set of gene targets whose activity could be modulated to induce changes in the intracellular redox ratio. We next applied our understanding to improve the fitness of the redox-impaired strains of Escherichia coli engineered for the production of succinic acid. These efforts lead to the creation of a strain capable of producing 60% higher succinate than previous reports.