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J. Biol. Chem., Vol. 282, Issue 18, 13429-13437, May 4, 2007

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Role of the H Domain of the Histidine Kinase-like Protein NifL in Signal Transmission^*

From the Department of Molecular Microbiology, John Innes Centre, Norwich NR4 7UH, United Kingdom

The NifL protein from Azotobacter vinelandii senses both the redox and fixed nitrogen status to regulate nitrogen fixation by controlling the activity of the transcriptional activator NifA. NifL has a domain architecture similar to that of the cytoplasmic histidine protein kinases. It contains two N-terminal PAS domains and a C-terminal transmitter region containing a conserved histidine residue (H domain) and a nucleotide binding GHKL domain corresponding to the catalytic core of the histidine kinases. Despite these similarities, NifL does not exhibit kinase activity and regulates its partner NifA by direct protein-protein interactions rather than phosphorylation. NifL senses the redox status via a FAD co-factor located within the PAS1 domain and responds to the nitrogen status by interaction with the signal transduction protein GlnK, which binds to the GHKL domain. The ability of NifL to inhibit NifA is antagonized by the binding of 2-oxoglutarate to the N-terminal GAF domain of NifA. In this study we have performed site-directed mutagenesis of the H domain of NifL to examine its role in signal transmission. Our results suggest that this domain plays a major role in transmission of signals perceived by the PAS1 and GHKL domains to ensure that NifL achieves the required conformation necessary to inhibit the 2-oxoglutarate-bound form of NifA. Some of the substitutions discriminate the redox and fixed nitrogen sensing functions of NifL implying that the conformational requirements and/or domain interactions necessary for NifA inhibition differ with respect to the signal input.

Received for publication, November 22, 2006 , and in revised form, February 12, 2007.

^* This work was supported by a grant from the Biotechnology and Biological Sciences Research Council (UK). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Sponsored by a fellowship from the Spanish Ministerio de Educacion y Ciencia. Current address: Dept. of Biochemistry, University of Bristol, Bristol BS8 1TD, UK.

² To whom correspondence should be addressed. Tel.: 44-1603-450747; Fax: 44-1603-450778; E-mail: ray.dixon{at}bbsrc.ac.uk.

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