ASU SoLS Faculty: Shelley E. Haydel

Mycobacterium tuberculosis is still the leading bacterial agent as a global cause of death accounting for an estimated 1.9 million deaths annually. In addition, the World Health Organization estimates that one-third of the world’s population is infected with M. tuberculosis and 8 million new cases occur annually worldwide. With the proliferation of multidrug-resistant strains and the association with AIDS, this adverse global impact of tuberculosis will inevitably continue. As an intracellular pathogen, M. tuberculosis must be able to survive and adapt to various environmental conditions encountered during host aerosolization, macrophage phagocytosis, and latency. In response to changing environmental conditions, bacteria commonly utilize two-component sensor histidine kinase/response regulator networks to modulate appropriate gene expression. Since it is widely accepted that these regulatory systems constitute a conserved strategy important in regulating virulence determinants in many bacteria, the presence of two-component systems in M. tuberculosis suggests that tubercle bacilli also utilize this mechanism in its pathogenic lifestyle. Inhibitors of two-component signal transduction systems are attractive candidates for the development of novel antibacterial agents. While recent reports have identified a series of compounds that inhibit two-component system proteins, much research remains to validate these inhibitors as potential antimicrobial targets. Incorporating genetics, biochemistry, and microbial pathogenesis, my research focuses on understanding the regulation of two-component systems and the importance of this regulatory control during M. tuberculosis in vivo growth, survival, and long-term persistence.

The underlying hypothesis of my overall research objective is that two-component systems of M. tuberculosis play an important role in regulating gene expression in response to environmental cues encountered within the host during infection, and that this regulation is used by M. tuberculosis to persist and cause disease in humans. The primary focus of my research is to identify the regulon of the M. tuberculosis trcRS, prrAB, rv1626, and rv3143 two-component systems using genomics and proteomics approaches and to determine the role of these regulatory systems in mycobacterial virulence.

Another focus of my research is to determine the function of Rv1057 in M. tuberculosis pathogenesis and/or physiology. Using various genetic and biochemical approaches, I have demonstrated that the TrcR response regulator represses expression of the rv1057 gene and that rv1057 is expressed after 18 and 48 h of M. tuberculosis growth in human macrophages. Additionally, it has been determined that rv1057 is induced during M. tuberculosis intraphagosomal growth in resting and activated murine macrophages from wild-type and nitric oxide synthase 2-deficient mice and during M. tuberculosis growth in the presence of palmitic acid. I have recently constructed an M. tuberculosis rv1057 deletion mutant which will be used to perform functional assays and determine its role in virulence. Since rv1057 is expressed during M. tuberculosis growth in human macrophages and repressed by trcR or trcRS during M. tuberculosis in vitro growth, the preliminary hypothesis is that the trcRS system functions to repress genes necessary for M. tuberculosis pathogenesis. In addition, a trcS mutant displayed a hypervirulent phenotype in SCID mice with a significant decrease in the time to death of infected mice, thus supporting this hypothesis. Additional experiments are required to further support this hypothesis and to determine if the trcRS system represents a unique regulatory control mechanism in M. tuberculosis pathogenesis.

Based on PSI-BLAST analyses, motif alignments, and secondary structure, I have determined that the Rv1057 protein belongs to a family of proteins with a repeated domain, which adopt a three-dimensional organization known as a β-propeller. The Rv1057 β-propeller predicted structure is based on seven blades, with each blade consisting of a four-stranded, antiparallel β-sheet, that form a pseudo-symmetrical axis around the central tunnel. Similar to other bacteria, M. tuberculosis appears to encode few β-propeller proteins. Since rv1057 is expressed during M. tuberculosis growth in human macrophages and the protein represents a unique predicted structure in M. tuberculosis, a better understanding of its structure is warranted. Resolution of the Rv1057 β-propeller structure will allow for probing of its function and/or catalytic mechanism in M. tuberculosis pathogenesis and for possible drug discovery studies based upon structure-activity relationships.

Selected Publications

Haydel, S. E. and J. E. Clark-Curtiss. 2006. The Mycobacterium tuberculosis TrcR response regulator represses transcription of the intracellularly-expressed Rv1057 gene, encoding a seven-bladed β-propeller. J. Bacteriol. 188:150-159.

Haydel, S. E. and J. E. Clark-Curtiss. 2004. Global expression analysis of two-component system regulators during Mycobacterium tuberculosis growth in human macrophages. FEMS Microbiol. Lett. 236:341-347.

Clark-Curtiss, J. E. and S. E. Haydel. 2003. Molecular genetics of Mycobacterium tuberculosis pathogenesis. Ann. Rev. Microbiol. 57:517-549.

Haydel, S. E., W. H. Benjamin, Jr., N. E. Dunlap, J. E. Clark-Curtiss. 2002. Expression, autoregulation, and DNA binding properties of the Mycobacterium tuberculosis TrcR response regulator. J. Bacteriol. 184:2192-2203.

Haydel, S. E., N. E. Dunlap, W. H. Benjamin, Jr. 1999. In vitro evidence of two-component system phosphorylation between the Mycobacterium tuberculosis TrcR/TrcS proteins. Microb. Pathog. 26:195-206.