Escherichia coli varies the synthesis of many of its respiratory enzymes in response to oxygen availability. These enzymes include cytochrome o oxidase (cyoABCDE) and cytochrome d oxidase (cydAB), used during aerobic cell growth, and a fumarate reductase (frdABCD), dimethyl sulfoxide/trimethylamine oxide reductase (dmsABC), and nitrate reductase (narGHJI), used during anaerobic respiratory conditions. To determine how different levels of oxygen affect the expression of each operon, strains containing cyo-lacZ, cyd-lacZ, frdA-lacZ, dmsA-lacZ, and narG-lacZ fusions were grown in continuous culture at various degrees of air saturation. The basal-level expression of the anaerobic respiratory genes, frdABCD, dmsABC, and narGHJI, occurred when the air saturation of the medium was above 20%; as the saturation was reduced to below 10% (ca. 2% oxygen), the expression rapidly increased and reached a maximal level at 0% air. In contrast, cyoABCDE gene expression was lowest under anaerobic conditions while cyd-lacZ expression was about 40% of its maximum level. When the oxygen level was raised into the microaerophilic range (ca. 7% air saturation) cyd-lacZ expression was maximal while cyo-lacZ expression was elevated by about fivefold. As the air level was raised to above 20% saturation, cyd-lacZ expression fell to a basal level while cyo-lacZ expression was increased to its maximum level. The role of the Fnr and ArcA regulatory proteins in this microaerophilic control of respiratory gene expression was documented: whereas Fnr function as an aerobic/anaerobic switch in the range of 0 to 10% air saturation, ArcA exerted its control in the 10 to 20% range. These two transcriptional regulators coordinate the hierarchial control of respiratory pathway gene expression in E. coli to ensure the optimal use of oxygen in the cell environment
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