CELLULAR AND MOLECULAR PHYSIOLOGY OF ESCHERICHIA-COLI IN THE ADAPTATION TO AEROBIC ENVIRONMENTS

Upon exposure to oxygen, Escherichia coli increases the expression of enzymes essential for aerobic respiration, such as components of the T CA cycle and terminal oxidase complexes. This increase requires the el imination of repression mediated by the Are regulatory system under an aerobic conditions. Coordinately, the synthesis of enzymes that functi on in anaerobic processes such as fermentation decreases, partly due t o the inactivation of the transcription factor Fm. E. coli is thus abl e to adjust the levels of respiratory enzymes to fit its environmental circumstances, and in this case, reduces the production of the less e nergy efficient fermentation enzymes in favor of the aerobic pathways. In contrast to the advantage in energy production, aerobiosis brings a disadvantage to E. coli: the production of reactive oxygen species ( ROS), i.e. superoxide anion radical (O-2 .(-)), hydrogen peroxide (H2O 2), and hydroxyl radical (. OH). These byproducts of aerobic respirati on damage many biological molecules, including DNA, proteins, and lipi ds. To alleviate the toxicity of these compounds, E. coli induces the synthesis of protective enzymes, such as Mn-dependent superoxide dismu tase (SodA) and catalase I (HP I), and this induction is controlled by the regulatory proteins SoxRS, OxyR, and ArcAB. Thus, ArcAB, Fnr, Sox RS, and OxyR function in concert so that E. coli can optimize its ener gy production and growth rate. Fnr and SoxRS are cytoplasmic, DNA-bind ing proteins, and these regulatory systems utilize iron-sulfur cluster s as cofactors which may directly sense the redox environment. OxyR is also a cytoplasmic, DNA-binding protein, and appears to respond to re dox potential through the oxidation state of a specific cysteine resid ue. In the ArcAB system (which belongs to the family of two-component regulatory systems), ArcB, a membrane protein, functions as the sensor , and ArcA, a DNA-binding protein, directly controls target gene expre ssion. Under anaerobic conditions, ArcB undergoes autophosphorylation and transphosphorylates ArcA, stimulating ArcA's DNA-binding activity. During aerobic growth, the transphosphorylation of ArcA does not occu r, In this signal transduction mechanism, the ArcB C-terminal or ''rec eiver'' domain plays a critical role; that is, it stimulates or abolis hes the transphosphorylation depending on the metabolic state of the c ell, which in turn is influenced by the availability of oxygen. E. col i thus employs at least four global regulatory systems which monitor t he cellular oxidative/metabolic conditions, and adjust the expression of more than 70 operons to give the organism a better aerobic life.