Effect of overexpression of nicotinic acid mononucleotide adenylyltransferase on succinic acid production in Escherichia coli NZN111

Escherichia coli NZN111 is a promising strain with ldhA and pflB genes inactivated for the production of succinic acid. However, with these mutations, NAD⁺ could not be regenerated from NADH, and an unbalanced NADH/NAD⁺ ratio eliminated cell growth and glucose utilization under anaerobic conditions. Nicotinic acid mononucleotide adenylyltransferase (NAMNAT), encoded by the nadD gene, catalyzes the reaction from nicotinic acid mononucleotide (NaMN) to nicotinic acid adenine dinucleotide (NaAD) during the synthetic pathway of NAD(H). Overexpression of the nadD gene could enhance the concentration of NAD(H) and maintain a suitable NADH/NAD⁺ ratio. In this study, we constructed a recombinant strain E. coli NZN111/pTrc99a-nadD, and overexpressed NAMNAT with 1.0 mmol/L of IPTG under anaerobic conditions in sealed bottles. Compared to E. coli NZN111, the concentrations of NAD⁺ and NADH in the recombinant strain increased by 3.21-fold and 1.67-fold, respectively. The total concentration of NAD(H) was increased by 2.63-fold, and the ratio of NADH/NAD⁺ decreased from 0.64 to 0.42. The recombinant strain restored the cell growth and glucose utilization under anaerobic conditions. After 72 h, the recombinant strain could consume 14.0 g/L of glucose to produce 6.23 g/L of succinic acid, and the concentration of succinic acid was 19-fold higher than in E. coli NZN111.