Investigation into the effect of soxR and soxS genes deletion on the central metabolism of Escherichia coli based on gene expressions and enzyme activities

Effect of soxR and soxS genes deletion on the central metabolism of Escherichia coli was investigated by combining the information of fermentation characteristics, gene expressions and enzyme activities. Twenty-one different enzyme activities in the central metabolic pathways were assayed in both mutants and its parent E. coli. Semi-quantitative RT-PCR was performed to detect the transcriptional levels of 79 different genes. The deletion of soxR and soxS genes reduced the specific glucose uptake rate and specific growth rate. On the other hand, acetate production rate was increased significantly in the soxS mutant as compared to that in the parent strain, which is partly due to down-regulation of tricarboxylic acid (TCA) cycle. It was found that the pntA (membrane bound transhydrogenase) transcripts, which is involved in NADPH generation were up-regulated in both soxR and soxS mutants by 2.0-fold. The reason was mainly due to the down-regulation of NADPH generating enzymes such as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) in pentose phosphate (PP) pathway, and isocitrate dehydrogenase (ICDH) in TCA cycle, since NADPH plays a significant role to reduce oxidative stress. On the other hand, udhA (soluble transhydrogenase) transcripts which is involved in the reoxidation of NADPH were found to be down-regulated by 0.8-fold in the soxR mutant but up-regulated by 1.7-fold in the soxS mutant compared to that in the parent strain. This result was consistent with the measurement of transhydrogenase activity, indicating that soxR mutant suffered from insufficient reoxidation of reducing power. This caused the slow growth for the soxR mutant. It was also found that the expression levels of cyoA and ndh genes were down-regulated in both mutants, demonstrating that soxR and soxS genes deletion adversely affected respiratory system and electron transport chain.