Department of Biochemistry
School of Medicine & Biomedical Sciences
Faculty
Department of Biochemistry
School
of Medicine & Biomedical Sciences
Faculty
MARK R. O'BRIAN, Ph.D.
Professor
Regulation of Heme Biosynthesis, Iron Homeostasis and Oxidative StressHeme is involved in many cellular processes as the active moiety of proteins involved in respiration, oxidative stress response, redox reactions and signal transduction. Heme is also a regulatory molecule that controls gene expression at the level of transcription, translation, protein targeting and protein stability. My lab is interested in the genetic control of heme biosynthesis, and in understanding the role of heme as regulatory molecule.
The bacterium Bradyrhizobium japonicum is a good model for studying the control of heme synthesis and iron metabolism because it can readily adapt to changes in its environment and to external stresses. It is also related to animal pathogens that are more experimentally refractive than B. japonicum. Our current work addresses the following questions: 1) What physiological and environmental factors control heme biosynthesis? 2) How are heme biosynthesis genes regulated at the molecular level? 3) How can heme serve as a signaling molecule to mediate the regulation of gene expression? 4) How is heme metabolism integrated with iron homeostasis at a global level? 5) How do cells sense and respond to oxidative stress?
We identified the Iron Response Regulator (Irr), a protein that belongs to the Fur family of transcriptional regulators that mediates iron-dependent regulation of the heme biosynthesis. Irr is also regulated by iron at the level of protein degradation, a process mediated by heme. Irr interacts directly with ferrochelatase, the enzyme which catalyzes the final step of the heme pathway. We are addressing the hypothesis that control of heme biosynthesis involves a novel regulatory function of a biosynthetic enzyme to affect gene expression. Using whole genome microarray analysis, we found that Irr regulates many genes within the iron regulon, and is not restricted to heme biosynthesis. This has led to the hypothesis that B. japonicum senses iron through the status of heme to regulate iron homeostasis and metabolism. Finally, we show that Irr mediates a cellular response to oxidative stress.
Selected Publications
Hohle, T.H. and M.R. O’Brian. 2009. The mntH gene encodes the major Mn2+ transporter in Bradyrhizobium japonicum and is regulated by manganese via the Fur protein. Mol. Microbiol. In press.
Small, S.K., S. Puri, I. Sangwan and M.R. O’Brian. 2009. Positive control of ferric siderophore receptor gene expression by the Irr protein in Bradyrhizobium japonicum. J. Bacteriol. 191: 1361-1368.
Small, S.K., S. Puri and M.R. O’Brian. 2009. Heme-dependent metalloregulation by the iron response regulator (Irr) protein in Rhizobium and other Alpha-Proteobacteria. Biometals, 22: 89-97.
Sangwan, I., S.K. Small and M.R. O’Brian. (2008) The Bradyrhizobium japonicum Irr protein is a transcriptional repressor with high affinity DNA binding activity. J. Bacteriol. 190: 5172-5177.
Amarelle, V., M.R. O’Brian, and E. Fabiano. (2008) ShmR is essential for utilization of heme as a nutritional iron source in Sinorhizobium meliloti. Appl. Envrion. Microbiol. 74: 6473-6475.
Gao, T. and M.R. O'Brian, (2007) Control of DegP-dependant degradation of c-type cytochromes by heme and the cytochrome c maturation system in Escherichia coli. J. Bacteriol. 189: 6253-6259.
Yang, J., Panek, H.R. and M.R. O’Brian,. (2006) Oxidative stress promotes degradation of the Irr protein to regulate heme biosynthesis in Bradyrhizobium japonicum. Mol. Microbiol. 60: 209-218.Yang, J., I. Sangwan, A. Lindemann, F. Hauser, H. Hennecke, H.-M. Fischer, and M.R. O’Brian. (2006) Bradyrhizobium japonicum senses iron through the status of heme to regulate iron homeostasis and metabolism. Mol. Microbiol. 60: 427-437.
Puri, S. and M.R. O’Brian. (2006) The hmuQ and hmuD genes from Bradyrhizobium japonicum encode heme-degrading enzymes. J. Bacteriol. 188: 6476-6482.
Yang, J., I. Sangwan, and M.R. O’Brian. (2006) The Bradyrhizobium japonicum Fur protein is an iron-responsive regulator in vivo. Mol. Gen. Genomics 276: 555-564.
Gao, T. and M.R. O’Brian. (2005) Iron-dependent cytochrome c1 expression is mediated by the status of heme in Bradyrhizobium japonicum. J. Bacteriol. 187: 5084-5089.
Yang, J., K. Ishimori and M.R. O’Brian. (2005) Two heme binding sites are involved in the regulated degradation of the bacterial iron response regulator (Irr) protein. J. Biol. Chem. 280: 7671-7676.
Friedman, Y.E. and M.R. O’Brian. (2004) The ferric uptake regulator (Fur) protein from Bradyrhizobium japonicum is an iron-responsive transcriptional repressor in vitro. J. Biol. Chem. 279: 32100-32105.
Platero, R., L. Peixoto, M.R. O’Brian and E. Fabiano (2004) Fur is involved in manganese-dependent regulation of mntA (sitA) expression in Sinorhizobium meliloti. Appl. Environ. Microbiol. 70: 4349-4355.
Panek, H.R. and M.R. O’Brian. (2004) KatG is the primary detoxifier of hydrogen peroxide produced by aerobic metabolism in Bradyrhizobium japonicum. J. Bacteriol. 186: 7874-7880.
Friedman, Y.E. and M.R. O’Brian. (2003) A novel DNA-binding site for the ferric uptake regulator (Fur) protein from Bradyrhizobium japonicum. J. Biol. Chem. 278: 38395- 38401.