Involvement of ArcA and Fnr in expression of Escherichia coli thiol peroxidase gene

To explore the oxygen response regulators involved in thiol peroxidase gene (tpx) expression in Escherichia coli, we constructed a single-copy tpx-lacZ operon fusion and monitored tpx-lacZ expression in various genetic backgrounds. Expression of the tpx-lacZ fusion was increased 4-fold by aerobic growth. Anaerobic expression of tpx-lacZ in either (delta)arcA or delta(fnr) strains was 2.5-fold depressed compared with that of the wild-type strain. The results of immunoblotting experiments also demonstrated that ArcA and Fnr regulatory proteins repressed thiol peroxidase gene expression during anaerobic growth. Inspection of the tpx promoter region revealed putative binding sites for ArcA and Fnr. It thus appears that ArcA and Fnr function as repressors by blocking the binding of RNA polymerase to the tpx promoter in E. coli under anaerobic growth conditions.     

Citrate utilization under anaerobic environment in Escherichia coli is under direct control of Fnr and indirect control of ArcA and Fnr via CitA-CitB system

Most Escherichia coli (E. coli) strains do not cause disease, naturally living in the lower intestine and is expelled into the environment within faecal matter. Escherichia coli can utilize citrate under anaerobic conditions but not aerobic conditions. However, the underlying regulatory mechanisms are poorly understood. In this study, we explored regulatory mechanisms of citrate fermentation genes by global regulators ArcA and Fnr under anaerobic conditions. A gel mobility shift assay showed that the regulator proteins ArcA and Fnr binded to the promoter region localized between the citAB and citCDEFXGT operons. Subsequent assays confirmed that ArcA indirectly controled the expression of citrate fermentation genes via regulating CitA-CitB system, while Fnr directly regulated but also indirectly modulated citrate fermentation genes via controling CitA-CitB system. Deletions of arcA and fnr significantly reduced the growth of Escherichia coli in M9 medium with a citrate carbon source. We conclude that both ArcA and Fnr can indirectly control the citrate utilization via CitA-CitB system, while Fnr can also directly regulate the expression of citrate fermentation genes in E. coli under anaerobic conditions.     

Effect of oxygen on the Escherichia coli ArcA and FNR regulation systems and metabolic responses

Escherichia coli has several elaborate sensing mechanisms for response to the availability of oxygen and the presence of other electron acceptors. The adaptive responses are coordinated by a group of global regulators, which include the one-component Fnr protein, and the two-component Arc system. To quantitate the contribution of Arc and Fnr-dependent regulation in catabolism, arcA and fnr mutant strains were constructed using the recently developed lambda derived recombination system. The metabolic activity of wildtype E. coli, an arcA mutant, an fnr mutant, and a double arcA-fnr mutant, via the fermentative pathways in glucose-limited cultures and different oxygen concentrations was studied in chemostat cultures at steady state. It was found that the most significant role of ArcA is under microaerobic conditions, while that of FNR is under more strictly anaerobic conditions. The FNR protein is normally inactive during microaerobic conditions. However, our results indicate that in the arcA mutant strain the cells behave as if a higher level of the FNR regulator is in the activated form compared to the wildtype strain during the transition from aerobic to microanaerobic growth. The results show a significant increase in the flux through pyruvate formate lyase (PFL) in the presence of oxygen. The activity of FNR-regulated pathways in the arcA mutant strain is correlated with the high redox potential obtained under microaerobic growth.