Kahn, Michael
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Research Interests
My laboratory is trying 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 regulatory systems that mediate plant-bacterial communications about metabolism 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.

Publications
Galibert, F, TM Finan, SR Long, A Pühler, P Abola, F Ampe, F Barloy-Hubler, MJ Barnett, A Becker, P Boistard, G Bothe, M Boutry, L Bowser, J Buhrmester, E Cadieu, D Capela, P Chain, A Cowie, RW Davis, S Dréano,  NA Federspiel, RF Fisher, S Gloux, T Godrie, A Goffeau, B Golding, J Gouzy, M Gurjal, I Hernandez-Lucas, A Hong, L Huizar, RW Hyman, T Jones, D Kahn, ML Kahn, S Kalman, D Keating, E Kiss, C Komp, V Lelaure, D Masuy, C Palm, MC. Peck, T Pohl, D Portetelle, B Purnelle, U Ramsperger, R Surzycki, P Thébault, M Vandenbol, F-J Vorhölter, S Weidner, DH Wells, K Wong, K-C Yeh, J Batut  (2001) The complete genome sequence of the legume symbiont Sinorhizobium meliloti. Science, 293:668-72

Kahn, ML, A Parra, CL Ford, F Kaser, D McCaskill, RE Ketchum, 2002 A mass spectrometry method for measuring 15N incorporation into pheophytin: Application to quantifying symbiotic nitrogen fixation Analytical Biochemistry 307:219-225

Walt, A and M. L. Kahn. 2002. The fixA and fixB genes are necessary for anaerobic carnitine reduction in Escherichia coli J Bacteriol. 184:4044-4047

Denison, R. F., C. Bledsoe, M. Kahn, F. O'Gara, E. L. Simms, and L. S. Thomashow. 2003. Cooperation in the rhizosphere and the "free rider" problem. Ecology, 84:838-845

House, B. L., Mortimer, M. W. and M. L. Kahn. 2004. New recombination methods for Sinorhizobium meliloti genetics. Applied Environ Microbiol 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.

Yurgel, S and M. L. Kahn. 2004. Dicarboxylate transport by rhizobia  FEMS Microbiol Revs 28: 485-501

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.

Schroeder BK, BL House, MW Mortimer, SN Yurgel, SC Maloney, KL. 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

Trainer MA, SN Yurgel, and ML Kahn 2007. Role of a conserved membrane glycine residue in a dicarboxylate transporter from Sinorhizobium meliloti. Journal of Bacteriology. 189:2160-3