title: Physiological role for the GlnK protein of enteric bacteria: relief of NifL inhibition under nitrogen-limiting conditions.
abstract: In Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by a flavoprotein, NifL, in the presence of molecular oxygen and/or combined nitrogen. We recently demonstrated that the general nitrogen regulator NtrC is required to relieve NifL inhibition under nitrogen (N)-limiting conditions. We provide evidence that the sole basis for the NtrC requirement is its role as an activator of transcription for glnK, which encodes a PII-like allosteric effector. Relief of NifL inhibition is a unique physiological function for GlnK in that the structurally related GlnB protein of enteric bacteria-apparently a paralogue of GlnK-cannot substitute. Unexpectedly, although covalent modification of GlnK by uridylylation normally occurs under N-limiting conditions, several lines of evidence indicate that uridylylation is not required for relief of NifL inhibition. When GlnK was synthesized constitutively from non-NtrC-dependent promoters, it was able to relieve NifL inhibition in the absence of uridylyltransferase, the product of the glnD gene, and under N excess conditions. Moreover, an altered form of GlnK, GlnKY51N, which cannot be uridylylated due to the absence of the requisite tyrosine, was still able to relieve NifL inhibition.
authors.affiliation: Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102, USA.
authors.forename: L
authors.initials: L
authors.lastname: He
authors.forename: E
authors.initials: E
authors.lastname: Soupene
authors.forename: A
authors.initials: A
authors.lastname: Ninfa
authors.forename: S
authors.initials: S
authors.lastname: Kustu
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D009713:Nucleotidyltransferases; D051922:PII Nitrogen Regulatory Proteins; D015534:Trans-Activators; D014157:Transcription Factors / Q000378:metabolism; D006599:UDPglucose-Hexose-1-Phosphate Uridylyltransferase / Q000378:metabolism; D014541:Uridine Kinase / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C113488:GlnK protein, Azorhizobium caulinodans; C061338:NifA protein, Bacteria; D051922:PII Nitrogen Regulatory Proteins; D015534:Trans-Activators; D014157:Transcription Factors; C497876:glnG protein, E coli; C084296:nifL protein, Bacteria; C404143:PIID regulatory protein, Bacteria; D014541:Uridine Kinase; C497878:glnK protein, E coli; D009713:Nucleotidyltransferases; D006599:UDPglucose-Hexose-1-Phosphate Uridylyltransferase; D009584:Nitrogen