The ArcBA two-component system of Escherichia coli is regulated by the redox state of both the ubiquinone and the menaquinone pool

Abstract

ArcBA is a two-component regulatory system of Escherichia coli involved in sensing oxygen availability and the concomitant transcriptional regulation of oxidative and fermentative catabolism. Based on in vitro data, it has been postulated that the redox state of the ubiquinone pool is the determinant for ArcB kinase activity. Here we report on the in vivo regulation of ArcB activation, as determined using a lacZ reporter specifically responsive to phosphorylated ArcA. Our results indicate that upon deletion of a ubiquinone biosynthetic enzyme, regulation of ArcB in the anaerobic-aerobic transition is not affected. In contrast, interference with menaquinone biosynthesis leads to inactivation of ArcB during anaerobic growth; this phenotype is fully rescued by addition of a menaquinone precursor. This clearly demonstrates that the menaquinones play a major role in ArcB activation. ArcB shows a complex pattern of regulation when E. coli is titrated through the entire aerobiosis range; ArcB is activated under anaerobic and subaerobic conditions and is much less active under fully aerobic and microaerobic conditions. Furthermore, there is no correlation between ArcB activation and the redox state of the ubiquinone pool, but there is a restricted correlation between the total cellular ubiquinone content and ArcB activity due to the considerable increase in the size of the ubiquinone pool with increasing degrees of aerobiosis. These results lead to the working hypothesis that the in vivo activity of ArcB in E. coli is modulated by the redox state of the menaquinone pool and that the ubiquinone/ubiquinol ratio in vivo surely is not the only determinant of ArcB activity.

FIG. 1.

(A) Schematic representation of part of the transcriptional regulatory elements upstream of the cydAB operon (not to scale). (B) Schematic representation of the constructed ArcA∼P-dependent promoter, based on P1 of cydAB (not to scale).

FIG. 2.

Concentration dependence of binding of ArcA and phosphorylated ArcA (ArcA∼P) to the DNA fragment containing ArcA-binding site II (from position −59 to position −175 relative to the start of cydAB P1). (A) Radiolabeled DNA fragment (0.32 nM) incubated with different amounts of either phosphorylated or unphosphorylated ArcA. Protein concentrations are indicated above the lanes. (B) Quantitative evaluation of the results of the gel retardation assays shown in panel A. Each point indicates the mean of four to eight independent experiments. (C) Double-reciprocal plot for binding of ArcA and ArcA∼P to the DNA fragment with n = 2 for ArcA and n = 6 for ArcA∼P.

FIG. 3.

Activity of the ArcA∼P-dependent-lacZ reporter construct (λRSS2). The construct was tested in the wild-type (JA001), ΔubiC (JA023), and ΔarcB (JA032) backgrounds after batch culture growth in mineral medium supplemented with glucose. Cultures were grown aerobically (open bars) and anaerobically (filled bars).

FIG. 4.

Activity of the ArcA∼P-dependent-lacZ reporter construct (λRSS2). The construct was tested in the wild type (JA001) and ΔmenB (JA022) backgrounds after growth in anaerobic batch cultures in medium with (bars with diagonal lines) or without (filled bars) 2 μM 1,4-dihydroxy-2-naphthoic acid or after aerobic growth without 1,4-dihydroxy-2-naphthoic acid (open bars).

FIG. 5.

Activity of the ArcA∼P-dependent-lacZ reporter construct (ASA12) (filled squares) and redox state of the UQ₈ pool (open circles) under various aerobiosis conditions. The construct was tested in the wild-type ASA12 background after glucose-limited continuous growth in mineral medium supplemented with various amounts of oxygen.

FIG. 6.

Contents of total UQ₈ (black triangles), DMK₈ (open diamonds), and MK₈ pool (open squares) under various aerobiosis conditions. Strain ASA12 was tested in glucose-limited continuous growth conditions in mineral medium supplemented with various amounts of oxygen. The values are data from single experiments.

FIG. 7.

Simplified model for modulation of ArcB activity by the degree of aerobiosis. Upon a shift from anaerobic growth conditions to low-aerobiosis growth conditions, the menaquinone pool oxidizes rapidly, resulting in an inactive ArcB kinase. Too little ubiquinol is present to prevent binding of the oxidized form of ubiquinone. A further increase in aerobiosis to high-microaerobiosis conditions (80% aerobiosis) results in an increase in the total ubiquinone pool and therefore an increase in ubiquinol, allowing binding of ubiquinol, which brings back the cysteine in the reduced form. In completely aerobic conditions the content of the quinone pool decreases, which results in oxidation of the key cysteines and in inactivation of ArcB. Adapted from the work of Malpica et al. (26). H, histidine kinase domain; D, receiver domain.