O-2 AS THE REGULATORY SIGNAL FOR FNR-DEPENDENT GENE-REGULATION IN ESCHERICHIA-COLI

With an oxystat, changes in the pattern of expression of FNR-dependent genes from Escherichia coli were studied as a function of the O-2 ten sion (pO(2)) in the medium, Expression of all four tested genes was de creased by increasing O-2. However, the pO(2) values that gave rise to half-maximal repression (pO(0.5)) were dependent on the particular pr omoter and varied between 1 and 5 millibars (1 bar = 10(5) Pa). The pO (0.5), value for the ArcA-regulated succinate dehydrogenase genes was in the same range (pO(0.5) = 4.6 millibars), At these pO(2) values, th e cytoplasm can be calculated to be well supplied with O-2 by diffusio n. Therefore, intracellular O-2 could provide the signal to FNR, sugge sting that there is no need for a signal transfer chain, Genetic inact ivation of the enzymes and coenzymes of aerobic respiration had no or limited effects on the pO(0.5) of FNR-regulated genes. Thus, neither t he components of aerobic respiration nor their redox state are the pri mary sites for O-2 sensing, supporting the significance of intracellul ar O-2. Non-redox-active, structural O-2 analogs like CO, CN-, and N-3 (-) could not mimic the effect of O-2 on FNR-regulated genes under ana erobic conditions and did not decrease the inhibitory effect of O-2 un der aerobic conditions.