Dr. Michael Dunn

Biography

I began my Bachelors in Science degree in the Biology department at Delaware Valley College (Doylestown, Pennsylvania) in 1980. In 1982, I interviewed for and was accepted into the Plant Tissue Culture Laboratory of Dr. Robert Zacharias at the U. S. Department of Agriculture's Eastern Regional Research Center (ERRC) in Philadelphia, where I worked full-time for two semesters and one summer on the phytopathogenic induction of hypersensitive response in soybean and tobacco suspension tissue cultures. During a second summer I assayed cutinase activity in the phytopathogenic fungus Phytopthora infestans in the laboratory of Dr. Robert Moreau. After obtaining my B. S. in Biology in 1985, I joined the ERRC Plant Science group and worked for a very fruitful two years under Dr. William Fett in the characterization of exopolysaccharide, lipopolysaccharide, bacteriocin and phytohorome production by phytopathogenic species of Xanthomonas and Pseudomonas. Dr. Fett was generous to allow me (as a research technician) co-authorship on four publications during this period, two of which (on in planta exopolysaccharide production and in vitro phytohormone production) are highly cited articles. In 1985, I joined the Interdiciplinary Plant Group's Plant Pathology department at the University of Missouri-Columbia and completed my Ph. D. under the direction of Dr. Arthur L. Karr by discovering and characterizing a exopolysaccharide depolymerase in the nitrogen-fixing soybean symbiont Bradyrhizobium japonicum. In addition, I collaborated with Drs. Steve Pueppke and Hari Krishnan to characterize alterations in the exopolysaccharide structure in Sinorhizobium fredii caused by the soybean isoflavone genestein. After receiving my degree in 1992, I began my postdoctoral fellowship under Dr. Jaime Mora in the Molecular Ecology department of the Mexican National Universities' Nitrogen Fixation Research Center (CIFN) in Cuernavaca, Mexico. During my postdoctoral period I measured and analyzed changes in Kreb's cycle enzyme activities in the common bean symbionts Rhizobium etli and Rhizobium tropici under a variety of growth conditions in vitro, with an emphasis of characterizing the metabolic changes provoked by the B-vitamin biotin. During this period I worked in close collaboration with Dr. Mora's Ph. D student Sergio Encarnación, whose thesis project centered on characterizing the physiological response of these rhizobia to biotin supplementation. I accepted a position as an Assistant Professor in Dr. Mora's group at CIFN in 1994 and was integrated into the group as an Associate Professor in 1996, by which time I was engaged on characterizing biotin-dependent carboxylase activities in R. etli and R. tropici. We were the first to clone and characterize a prokaryotic pyruvate carboxylase (from R. etli, in which it has an essential anaplerotic role), which has since been used by other research groups for the metabolic engineering of Escherichia coli for enhancing the production of specific amino acids. I followed up this work by cloning and characterizing the pyruvate carboxylase from the alfalfa symbiont Sinorhizobium meliloti in collaboration with Dr. Turlough Finan (McMaster University, Hamilton, Canada), which demonstrated drastic differences in its regulatory characteristics as compared to the enzyme from R. etli despite their significant sequence similarity. We then focused on characterizing a second biotin-dependent carboxylase produced by R. etli, the acyl-CoA carboxylase, whose kinetic characteristics were indicative of an enzyme involved in degradative anaplorotic pathways rather than fatty acid synthesis. Upon receiving his Ph. D., Sergio Encarnación in collaboration with Dr. Mora established a Proteome Characterization facility in the department, and the first publication from this effort allowed us to detail, on a global scale, the effects of biotin on the metabolism of R. etli in response to biotin supplementation. Given the profound metabolic effects of biotin on R. etli metabolism and global protein expression, the question then became as to how R. etli obtained the vitamin during growth in culture or in symbiosis with bean plants. My Ph. D. student, Karina Navarro, undertook the studies necessary to address this question and focused her research on how biotin was obtained from the environment by R. etli growing in culture. She achieved the first detailed molecular genetic characterization of biotin transport genes in prokaryotes and obtained evidence supporting the importance of biotin uptake during symbiosis. Presently, I am working towards establishing whether or not enzyme-enzyme interactions exist between Kreb's cycle and anaplerotic enzymes in R. etli, a subject which has been little explored in bacteria but which is known to exist in higher taxons. Gel permeation chromotography studies of R. etli proteins show that potential interactions do exist, and I am working to define these interactions. I have also shown that the R. etli pyruvate carboxylase requires an accessory protein for its correct assembly or enzymatic activity and am working towards the identification of this accessory protein. During the course of my career, I have published 19 articles which have accumulated 206 citations (excluding self-citations or those made by co-authors). My teaching duties have included graduate courses in protein purification and an undergraduate course on microbiology and biochemistry.

Research Interest

Carbon metabolism; Protein-protein interactions