EFFECT OF MICROAEROPHILIC CELL-GROWTH CONDITIONS ON EXPRESSION OF THEAEROBIC (CYOABCDE AND CYDAB) AND ANAEROBIC (NARGHJI, FRDABCD, AND DMSABC) RESPIRATORY PATHWAY GENES IN ESCHERICHIA-COLI

Escherichia coli varies the synthesis of many of its respiratory enzym es in response to oxygen availability, These enzymes include cytochrom e o oxidase (cyoABCDE) and cytochrome d oxidase (cydAB), used during a erobic cell growth, and a fumarate reductase (frdABCD), dimethyl sulfo xide/trimethylamine oxide reductase (dmsABC), and nitrate reductase (n arGHJI), used during anaerobic respiratory conditions. To determine ho w different levels of oxygen affect the expression of each operon, str ains containing cyo-lacZ, cyd-lacZ, frdA-lacZ, dmsA-lacZ, and narG-lac Z fusions were grown in continuous culture at various degrees of air s aturation, The basal-level expression of the anaerobic respiratory gen es,frdABCD, dmsABC, and narGHJI, occurred when the air saturation of t he medium was above 20%; as the saturation was reduced to below 10% (c a, 2% oxygen), the expression rapidly increased and reached a maximal level at 0% air, In contrast, cyoABCDE gene expression was lon est und er anaerobic conditions while cyd-lacZ expression was about 40% of its maximum level, When the oxygen level was raised into the microaerophi lic range (ca, 7% air saturation) cyd-lacZ expression was maximal whil e cyo-lacZ expression was elevated by about fivefold, As the air level was raised to above 20% saturation, cyd-lacZ expression fell to a bas al level while cyo-lacZ expression was increased to its maximum level. The role of the Fnr and ArcA regulatory proteins in this microaerophi lic 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 resp iratory pathway gene expression in E. coli to ensure the optimal use o f oxygen in the cell environment.