THE ACTIVITY OF THE ESCHERICHIA-COLI TRANSCRIPTION FACTOR FNR IS REGULATED BY A CHANGE IN OLIGOMERIC STATE

The transcription factor FNR globally regulates gene expression in res ponse to oxygen deprivation in Escharichia coli. To understand how oxy gen deprivation activates FNR, a constitutively active FNR mutant pro tein, DA154, was studied to determine how this mutant bypassed the nor mal regulation pathway. When purified from aerobically grown cells, th e DA154 protein had a larger apparent native molecular mass and an inc reased affinity for a consensus FNR target site as compared with wild- type FNR prepared under identical conditions. These results suggested that FNR DA154 may bypass the normal regulation pathway by converting FNR from an inactive monomer to an active dimer under aerobic conditi ons. To determine whether wild-type FNR is active as a dimer under ana erobic conditions, FNR mutants were isolated that inhibit the activity of wild-type FNR by forming mixed dimers (i.e., dominant-negative mut ants). These dominant-negative FNR mutants were shown to have substita tions in the putative DNA-binding domain and to be defective in bindin g to a consensus FNR DNA target site in vitro. One representative domi nant-negative mutant, EK209, was also shown to be unable to form mixed oligomers in vivo under aerobic conditions, suggesting that FNR may b e monomeric in the inactive state. Taken together, these data have led us to propose that under anaerobic conditions FNR is a dimer that is active for DNA binding, and under aerobic conditions, FNR is inactivat ed by conversion to a monomer.