University at Buffalo
The Witebsky Center

The Witebsky Center
University at Buffalo
Bacteriology Host-Microbe Interactions Immunology Parasitology Virology
The Witebsky Center The Witebsky Center
Mark R. O'Brian, Ph.D.

Mark R. O’Brian, Ph.D. Professor
Department of Biochemistry
Director of the Witebsky Center for Microbial Pathogenesis & Immunology
Phone: (716) 829-3200
Fax: (716) 829-2725



Regulation of heme metabolism, iron homeostasis and oxidative stress

The adaptive success of bacteria depends, in part, on the ability to sense and respond to external cues at the level of gene expression. Bradyrhizobium japonicum resides as a free-living soil bacterium or as the endosymbiont of soybean with root nodules. In addition to its agricultural significance, B. japonicum also serves as a model to study bacteria-eukaryote interactions more generally, including related pathogens that are refractive to genetic and biochemical study.

We are interested in the regulation of heme metabolism and its integration with iron homeostasis in B. japonicum. Hemes are needed for many cellular processes, and we have shown that heme plays important regulatory roles in cells. We are interested in the molecular basis of this novel regulatory mechanism. We identified the iron response regulator (Irr) protein as a regulator that couples heme biosynthesis with global iron metabolism. Irr responds to the iron through the status of heme to positively and negatively control the expression of genes within the Irr regulon.

Control of metal homeostasis is integrated with oxidative stress responses in cells. We have initiated a new project to characterize how manganese contributes to the oxidative stress response in B. japonicum, and the roles of Irr and other metalloregulatory proteins in this process.

Professional Activities

Chair, Gordon Research Conference, Chemistry and Biology of Tetrapyrroles, 2004.

Instuctor, EMBO-sponsored short course on Functional Analysis of Microbial Genomes,

2006, Montevideo, Uruguay.

Recipient, University at Buffalo Exceptional Scholar Award for Sustained Achievement, 2006.Commendation for Excellence in Teaching, Siegel Award Commitee, School of Medicine and Biomedical Sciences, State University of New York at Buffalo, 2008.

Editorial Board, Applied and Environmental Microbiology, 2001-2009.

Monitoring Editor, Plant Physiology, 2005-2010.


Selected Publications

Qi, Z. and M.R. O’Brian. 2002. Interaction between the bacterial iron response regulator (Irr) and ferrochelatase mediates genetic control of heme biosynthesis. Mol. Cell,  9:155-162.

Sangwan, I. and M.R. O’Brian. 2002. Identification of a soybean protein that interacts with GAGA element dinucleotide repeat DNA. Plant Physiol. 129: 1788-1794.

Panek, H. and M.R. O’Brian. 2002. A genomic view of prokaryotic heme biosynthesis. Microbiol. 148: 2273-2282.

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.


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.


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.

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., Panek, H.R. and O’Brian, M.R. 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. 2007. Control of DegP-dependent degradation of c-type

cytochromes by heme and the cytochrome c maturation system in Escherichia coli. J.

Bacteriol.  189: 6253-6259.

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.

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.

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.

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. 72: 399-409.

Puri, S., T.H. Hohle and M.R. O’Brian. 2010. Control of bacterial iron homeostasis by      manganese. Proc. Natl. Acad. Sci. U.S.A. 107: 10691-10695.

Hohle, T.H. and M.R. O’Brian. 2010. Transcriptional control of the Bradyrhizobium japonicum irr gene requires repression by Fur and anti-repression by  Irr. J. Biol. Chem. 285: 36074-36080.