FnrP interactions with the Pasteurella haemolytica leukotoxin promoter

Pasteurella haemolytica FnrP is homologous to Fnr, the global transcriptional regulator of anaerobic respiration in Escherichia coli. To investigate the role of O(2) in the expression of P. haemolytica leukotoxin, we tested a lktC::lacZ fusion constructed in E. coli for a FnrP-mediated regulatory effect under aerobic and anaerobic growth conditions. Both E. coli Fnr and FnrP suppressed leukotoxin transcription under aerobic conditions. Under anaerobic conditions, Fnr suppressed transcription, while FnrP increased transcription. These results were confirmed using FnrP*, a mutant form of FnrP that activates anaerobically inducible genes under aerobic conditions. In mobility shift assays, partially purified FnrP bound to a potential regulatory site in a P. haemolytica lktC promoter fragment.