title: Fnr Is required for NifL-dependent oxygen control of nif gene expression in Klebsiella pneumoniae.
abstract: In Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by NifL in response to molecular oxygen and combined 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 Escherichia 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 indirectly, in NifL-dependent oxygen regulation of nif gene expression in K. pneumoniae. The data indicate that in the absence of Fnr, NifL apparently does not receive the signal for anaerobiosis. We therefore hypothesize that in the absence of oxygen, Fnr, as the primary oxygen sensor, activates transcription 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.
authors.affiliation: Institut für Mikrobiologie und Genetik, Universität Göttingen, 37077 Göttingen, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Grabbe
authors.forename: K
authors.initials: K
authors.lastname: Klopprogge
authors.forename: R A
authors.initials: RA
authors.lastname: Schmitz
mesh_terms: D000693:Anaerobiosis; D001426:Bacterial Proteins / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007711:Klebsiella pneumoniae / Q000187:drug effects / Q000235:genetics*; D008954:Models, Biological; D016254:Mutagenesis, Insertional; D009586:Nitrogen Fixation / Q000235:genetics; D010100:Oxygen / Q000494:pharmacology*; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C060704:FNR protein, E coli; D007506:Iron-Sulfur Proteins; C061338:NifA protein, Bacteria; D011994:Recombinant Proteins; D014157:Transcription Factors; C084296:nifL protein, Bacteria; D010100:Oxygen