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Journal of Bacteriology, December 1998, p. 6661-6667, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Physiological Role for the GlnK Protein of Enteric Bacteria: Relief of NifL Inhibition under Nitrogen-Limiting Conditions

Luhong He,1 Eric Soupene,1 Alexander Ninfa,2 and Sydney Kustu1,*

Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102,1 and Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109-06062

Received 28 July 1998/Accepted 9 October 1998

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.

* Corresponding author. Mailing address: 111 Koshland Hall, U. C. Berkeley, Berkeley, CA 94720-3102. Phone: (510) 643-9308. Fax: (510) 642-4995. E-mail: kustu{at}nature.berkeley.edu.

Journal of Bacteriology, December 1998, p. 6661-6667, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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