Heme and nitric oxide binding by the transcriptional regulator DnrF from the marine bacterium increases promoter affinity.

Under oxygen-limiting conditions, the marine bacterium DFL12 generates energy via denitrification, a respiratory process in which nitric oxide (NO) is an intermediate. Accumulation of NO may cause cytotoxic effects. The response to this nitrosative (NO-triggered) stress is controlled by the Crp/Fnr-type transcriptional regulator DnrF. We analyzed the response to NO and the mechanism of NO sensing by the DnrF regulator. Using reporter gene fusions and transcriptomics, here we report that DnrF selectively repressed nitrate reductase () genes, preventing further NO formation. In addition, DnrF induced the expression of the NO reductase genes (), which promote NO consumption. We used UV-visible and EPR spectroscopy to characterize heme binding to DnrF and subsequent NO coordination. DnrF detects NO via its bound heme cofactor. We found that the dimeric DnrF bound one molecule of heme per subunit. Purified recombinant apo-DnrF bound its target promoter sequences (, , , and ) in electromobility shift assays, and we identified a specific palindromic DNA-binding site 5'-TTGATNATCAA-3' in these target sequences via mutagenesis studies. Most importantly, successive addition of heme as well as heme and NO to purified recombinant apo-DnrF protein increased affinity of the holo-DnrF for its specific binding motif in the promoter. On the basis of these results, we propose a model for the DnrF-mediated NO stress response of this marine bacterium.