Division of Nephrology, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA
Received date: August 20, 2014; Accepted date: August 22, 2014; Published date: August 26, 2014
Citation: Kogenaru S (2014) HrpX Transcription Factor - A Potential Common Target for Several Bacterial Diseases from a Synthetic Biology Perspective. J Bacteriol Parasitol 5:e123. doi: 10.4172/2155-9597.1000e123
Copyright: © 2014 Sunitha. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Understanding the design principles of bacterial pathogenesis is central to combat several infectious diseases in humans, animals and plants . This includes identification of common bacterial pathogenicity mechanisms employed by a wide variety of bacteria, which infect diverse hosts . Recent studies have identified one such common virulence mechanism: Type III Secretion System (T3SS), employed by several animal and plant pathogenic bacteria [3-6]. Classically, the components of T3SS physically assemble to form a complex needle-like structure that enables the bacteria to inject virulence factors directly into their host cell's cytoplasm. These factors in turn specifically interfere with their host cellular processes to elicit pathogenicity [7,8]. However, despite the common injection mechanism, each bacterial species injects unique set of species-specific virulence factors that define the host specificity [3,4].
The proteobacterial groups that primarily employ T3SS to infect variety of plants are Xanthomonas, Ralstonia and Burkholderia to mention a few [7-9]. Among them, the species of Xanthomonas genus are well studied and are known to infect wide spectrum of host plants, including several economically important plants like wheat, rice, beans, tomato, cotton, paper and citrus . Currently around 27 plant-associated Xanthomonas species are known . T3SS in these species are encoded by a cluster of hypersensitive response and pathogenicity (hrp) genes [9,12]. In most of these species, hrp cluster expression is regulated by a key transcription factor HrpX via the two-component response regulator HrpG . HrpX is an AraC-type transcriptional activator that specifically recognizes the plant-inducible promoter (PIP-box) motifs present in the cis-regulatory regions of the regulated target genes [14,15]. The consensus sequence of PIP-box consists of a direct repeats of "TTCGC" with a spacer of 8-26 base pairs between the repeats .
Over the past three decades, the cataloguing of the HrpX target genes is underway by genetic, biochemical and high-throughput methods [10,17-23]. Recent genome-wide transcriptomic studies have reconfirmed many of these target genes, and additionally identified new ones, which lead to the discovery of a more comprehensive picture of the HrpX regulome [13,24]. Despite the detailed knowledge about HrpX regulome, the factor controlling the HrpX transcriptional activity is currently lacking.
The knowledge of protein sequence and domain composition is vital to the discovery of a factor controlling the HrpX function. Bioinformatics analysis of the HrpX sequence from Xanthomonas citri subsp. citri (Xcc) revealed the existence of tetratrico-peptide-repeats (TPRs) domain, and a jelly roll like domain, in addition to the already known AraC-type DNA-binding domain . TPRs are known to mediate protein-protein interactions; hence, it can be hypothesized that the presence of this domain may facilitate HrpX to recruit the transcription machinery to the core promoter, thereby initiate the transcription of the target gene. On the other hand, the jelly roll like domain is known to bind to a host environment molecule in other pathogenic bacteria . This tentatively suggests that jelly roll like domain in HrpX might also have a role to bind to a factor present in the host plant environment that ultimately controls the target gene regulation.
In order to demonstrate that exogenous factor controls the HrpX functioning, synthetic biology approaches would be advantageous to employ [27,28]. By this approach, a synthetic reporter system can be constructed by placing the regulation of a fluorescent reporter gene under the control of HrpX, using a known regulatory sequence motif of PIP-box. Further, the expression of hrpX can be controlled by an inducible transcription factor like AraC or LacI . This synthetic transcriptional cascade will enable to determine, whether both HrpX and an environmental factor are required in order to regulate the target gene expression that can be assessed by directly measuring the fluorescence from a reporter. Establishment of this synthetic functional assay system will further allow to disentangle the hrpX from its native organism context, by implementing it in a standard model organisms like Escherichia coli . This synthetic system will directly allow us to test whether a factor from the host environment controls the HrpX transcriptional activity or not. Conversely, this system will also be useful to screen for synthetic small-molecule inhibitor for the HrpX in a high throughput fashion.
This proof-of-concept experiment will provide a direct link to the host environment controlled regulation of virulence gene expression, hence helps in mechanistic understanding of HrpX functioning. Other interesting questions useful to be explored include the identification of the active molecule from the host environment responsible, and how similar is the identified molecule structurally across different Xanthomonas species. Based on the structural similarity of the molecule, one might be able to possibly design a master small-molecule inhibitor to control the diseases caused by Xanthomonas species. Future studies exploring on these questions will directly enable the development of a master strategy to fight many bacterial infections.