To decipher how the synthesis of energy-transducing enzymes responds to environmental cues, the response of three aerobic cytochrome gene promoters was analysed under different conditions. Two of these promoters are upstream of structural genes ( and ) for individual subunits of the cytochrome respiratory complex. The third promoter is that for the operon, which encodes two -type cytochromes of unknown function, cytochrome and CycG. Primer extension analysis identified a single oxygen-responsive transcription start site for each gene. Utilizing operon fusions to as a measure of promoter activity, transcription from the and promoters was approximately twofold higher when cells were grown at high (30%) oxygen tensions than under low (2%) oxygen or anaerobic (photosynthetic) conditions. Analysis of promoter function using specific host mutations indicated that loss of the FNR homologue, FnrL, causes a small, but reproducible, increase in and transcription when cells are grown at 2% oxygen. However, neither the ΔFnrL mutation nor alterations in sequences related to a consensus target site for the FNR protein increased function of any of these three promoters to that seen under aerobic conditions in wild-type cells. From this we conclude that FnrL is not solely responsible for reduced transcription of these three aerobic cytochrome genes under low oxygen or anaerobic conditions. When activity of these three promoters was monitored after cells were shifted from anaerobic (photosynthetic) conditions to a 30% oxygen atmosphere, it took several cell doublings for LacZ levels to increase to those found in steady-state 30% oxygen cultures. From these results, it appears that activity of these promoters is also regulated by a stable molecule whose synthesis or function responds slowly to the presence of high oxygen tensions.


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