Fnr is required for NifL-dependent oxygen control of nif gene expression in Klebsiella pneumoniae

In Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by NifL, in response to molecular oxygen and comb ined nitrogen. We recently showed that K. pneumoniae NifL is a flavoprotein , which apparently senses oxygen through a redox-sensitive, conformational change. We have now studied the oxygen regulation of NifL activity in Esche richia coli and K. pneumoniae strains by monitoring its inhibition of NifA- mediated expression of K. pneumoniae phi (nifH'-'lacZ) fusions in different genetic backgrounds. Strains of both organisms carrying fnr null mutations failed to release NifL, inhibition of NifA transcriptional activity under oxygen limitation: nif induction was similar to the induction under aerobic conditions. When the transcriptional regulator Fnr was synthesized from a plasmid, it was able to complement, i.e., to relieve NifL inhibition in the fnr mutant backgrounds. Hence, Fnr appears to be involved, directly or ind irectly, in NifL-dependent oxygen regulation of nif gene expression in K. p neumoniae. The data indicate that in the absence of Fnr, NifL, apparently d oes not receive the signal for anaerobiosis. We therefore hypothesize that in the absence of oxygen, Fnr, as the primary oxygen sensor, activates tran scription of a gene or genes whose product or products function to relieve NifL, inhibition by reducing the flavin adenine dinucleotide cofactor under oxygen-limiting conditions.