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Washington State University Molecular Plant Sciences

509-335-8327
kahn@wsu.edu
Professor and Fellow of the Institute of Biological Chemistry. Associate Director of the Agricultural Research Center. Ph.D.1976, Stanford University.

kahn

Research

My laboratory works to understand the symbiosis between nitrogen-fixing Rhizobium bacteria and legumes at a molecular and a metabolic level. In the symbiosis, nitrogen reduced to ammonium by the bacteria is exchanged for carbon compounds produced by the plant. This allows each organism to benefit from the unique metabolic abilities of the other and provides a considerable selective advantage to both.

We work primarily in two areas of metabolism, the synthesis and degradation of carbon compounds through the TCA cycle and the synthesis and degradation of amino acids that are derived from this cycle. One of the outstanding problems in the field is to understand why the bacteria release reduced nitrogen to their plant host. Nitrogen fixation is an extraordinarily expensive reaction and is usually under tight control. But in symbiosis the bacteria reduce much more nitrogen than they need and the reasons for this are not clear.

We have been studying how the plant feeds the bacteria and how the bacteria generate the reductant and ATP needed for fixation. Projects currently being worked on in the laboratory give some of the flavor of our approach: Using site-specific and nutritionally selected mutants we are trying to distinguish which of several potential substrates and products are important in fixation; we are characterizing proteins we believe are part of a nitrogenase-specific electron transport system and are developing temperature-sensitive mutants to probe how particular enzymes are coupled to symbiotic nitrogen fixation. In addition, we are developing new methods for detecting the contribution of associative or symbiotic nitrogen fixation to host plant nutrition.

Selected Publications

Yurgel SN, Rice J, Kahn ML. (2012) Nitrogen Metabolism in Sinorhizobium meliloti – Alfalfa Symbiosis: Dissecting the Role of GlnD and PII Proteins.  Mol Plant Microbe Interact. 25:355-62

Yurgel SN, Rice J, Mulder M, Kahn ML. GlnB/GlnK PII proteins and regulation of the Sinorhizobium meliloti Rm1021 nitrogen stress response and symbiotic function. Journal of Bacteriology. 2010 May;192(10):2473-81.

Koziol U, Hannibal L, Rodríguez MC, Fabiano E, Kahn ML, Noya F. Deletion of citrate synthase restores growth of Sinorhizobium meliloti 1021 aconitase mutants. Journal of Bacteriology. 2009 Dec;191(24):7581-6.

Humann JL, Ziemkiewicz HT, Yurgel SN, Kahn ML. Regulatory and DNA repair genes contribute to the desiccation resistance of Sinorhizobium meliloti Rm1021. Applied and Environmental Microbiology. 2009 Jan;75(2):446-53.

Yurgel SN, Kahn ML. A mutant GlnD nitrogen sensor protein leads to a nitrogen-fixing but ineffective Sinorhizobium meliloti symbiosis with alfalfa. Proc Natl Acad Sci U S A. 2008 Dec 2;105(48):18958-63.

Trainer M.A., S.N. Yurgel, and M.L. Kahn 2007. Role of a conserved membrane glycine residue in a dicarboxylate transporter from Sinorhizobium meliloti. Journal of Bacteriology 189:2160-3.

Yurgel, S.N. and M.L. Kahn. 2007. Pleiotropic effects of mutations that alter the S. meliloti cytochrome c respiratory system. Microbiology 153:399-410.

Yurgel S.N., B.K. Schroeder, B.L. House, M.W. Mortimer, S.C. Maloney, C.A. Taylor, K.L. Ward, H.T. Ziemkiewicz, J.J.Bovitz, H. Jin, and M.L. Kahn. 2006. Genomic and Genetic approaches to understanding the physiology of Sinorhizobium meliloti. In “Biology of Molecular Plant-Microbe Interactions”, Volume 5, Eds: F. Sanchez, C. Quinto, I. Lopez-Lara, and O. Geiger pp. 126-131. IS-MPMI Press St Paul.

Schroeder, BK, BL. House, MW. Mortimer, SC. Maloney, CA. Taylor, KL. Ward, HT. Ziemkiewicz, S Clark, JJ. Bovitz, H Jin, S Yurgel, and ML. Kahn. 2006. Analyzing a Sinorhizobium meliloti 1021 ORFeome in a functional genomic platform. In Y-P Wang et al. (eds) Biological Nitrogen Fixation, Sustainable Agriculture and the Environment. International Nitrogen Fixation Meeting, Springer pp. 127-128.

Schroeder B.K., B.L. House, M.W. Mortimer, S.N. Yurgel, S.C. Maloney, K.L. Ward and M.L. Kahn. 2005. Developing a Functional Genomics Platform for Sinorhizobium meliloti: Construction of an ORFeome. Applied and Environmental Microbiology 71:5858-64

Parra-Colmenares, A. and M.L. Kahn. 2005. Determination of nitrogen fixation effectiveness in selected Medicago truncatula isolates by measuring nitrogen isotope incorporation into pheophytin. Plant and Soil. 270:159-168

Grzemski, W., J.P. Akowski and M.L. Kahn. 2005. Altering Bacterial Metabolism in a Nitrogen Fixing Symbiosis Using Conditional and Impaired Mutations in Sinorhizobium meliloti Citrate Synthase. Molecular Plant Microbe Interactions 18:134-141

Yurgel, S.N. and M.L. Kahn. Sinorhizobium meliloti dctA mutants with partial ability to transport dicarboxylic acids. 2005 Journal of Bacteriology 187:1161-72 Article includes cover illustration.

House, B.L., Mortimer, M.W. and M.L. Kahn. 2004. New recombination methods for Sinorhizobium meliloti genetics. Applied and Environmental Microbiology 70:2806-15.

Barnett, M.J. and M.L. Kahn. 2004. Sinorhizobium meliloti pSymA: Nitrogen fixation and more. In “Genomes and Genomics of Nitrogen Fixing Organisms”, R. Palacios and W. Newton, eds. pp.113-132.

Kahn, M.L., Schroeder, B.K., House, B.L., Mortimer, M.M., Yurgel S.N., Maloney, S.C., Warren, K.L., Fishe,r R.F., Barnett M.J., Toman C., Long, S.R. 2004. Foraging for Meaning-Postgenome Approaches to Sinorhizobium meliloti. In ” Biology of Molecular Plant-Microbe Interactions, Volume 4, Eds: B Lugtenberg, I Tikhonovich, and N Provorov pp 416-422. IS-MPMI Press, St Paul.

Yurgel, S. and M.L. Kahn. 2004. Dicarboxylate Transport by Rhizobia. FEMS Microbiology Reviews 28:485-501.