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Schuster M, Li C, Smith P, Kuttler C. Parameters, architecture and emergent properties of the Pseudomonas aeruginosa LasI/LasR quorum-sensing circuit. J R Soc Interface 2023; 20:20220825. [PMID: 36919437 PMCID: PMC10015328 DOI: 10.1098/rsif.2022.0825] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2022] [Accepted: 02/22/2023] [Indexed: 03/16/2023] Open
Abstract
Quorum sensing is a widespread process in bacteria that controls collective behaviours in response to cell density. Populations of cells coordinate gene expression through the perception of self-produced chemical signals. Although this process is well-characterized genetically and biochemically, quantitative information about network properties, including induction dynamics and steady-state behaviour, is scarce. Here we integrate experiments with mathematical modelling to quantitatively analyse the LasI/LasR quorum sensing pathway in the opportunistic pathogen Pseudomonas aeruginosa. We determine key kinetic parameters of the pathway and, using the parametrized model, show that quorum sensing behaves as a bistable hysteretic switch, with stable on and off states. We investigate the significance of feedback architecture and find that positive feedback on signal production is critical for induction dynamics and bistability, whereas positive feedback on receptor expression and negative feedback on signal production play a minor role. Taken together, our data-based modelling approach reveals fundamental and emergent properties of a bacterial quorum sensing circuit, and provides evidence that native quorum sensing can indeed function as the gene expression switch it is commonly perceived to be.
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Affiliation(s)
- Martin Schuster
- Department of Microbiology, Oregon State University, Corvallis, OR 97331, USA
| | - Christina Li
- Department of Microbiology, Oregon State University, Corvallis, OR 97331, USA
| | - Parker Smith
- Department of Microbiology, Oregon State University, Corvallis, OR 97331, USA
| | - Christina Kuttler
- Department of Mathematics, Technische Universität München, 85748 Garching, Germany
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2
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Physiological Advantage of Phenotypic Heterogeneity in a Quorum-Sensing Population. J Indian Inst Sci 2020. [DOI: 10.1007/s41745-020-00175-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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3
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Miller C, Gilmore J. Detection of Quorum-Sensing Molecules for Pathogenic Molecules Using Cell-Based and Cell-Free Biosensors. Antibiotics (Basel) 2020; 9:antibiotics9050259. [PMID: 32429345 PMCID: PMC7277912 DOI: 10.3390/antibiotics9050259] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2020] [Revised: 05/13/2020] [Accepted: 05/14/2020] [Indexed: 11/18/2022] Open
Abstract
Since the discovery and subsequent use of penicillin, antibiotics have been used to treat most bacterial infections in the U.S. Over time, the repeated prescription of many antibiotics has given rise to many antibiotic-resistant microbes. A bacterial strain becomes resistant by horizontal gene transfer, where surviving microbes acquire genetic material or DNA fragments from adjacent bacteria that encode for resistance. In order to avoid significant bacterial resistance, novel and target therapeutics are needed. Further advancement of diagnostic technologies could be used to develop novel treatment strategies. The use of biosensors to detect quorum-sensing signaling molecules has the potential to provide timely diagnostic information toward mitigating the multidrug-resistant bacteria epidemic. Resistance and pathogenesis are controlled by quorum-sensing (QS) circuits. QS systems secrete or passively release signaling molecules when the bacterial concentration reaches a certain threshold. Signaling molecules give an early indication of virulence. Detection of these compounds in vitro or in vivo can be used to identify the onset of infection. Whole-cell and cell-free biosensors have been developed to detect quorum-sensing signaling molecules. This review will give an overview of quorum networks in the most common pathogens found in chronic and acute infections. Additionally, the current state of research surrounding the detection of quorum-sensing molecules will be reviewed. Followed by a discussion of future works toward the advancement of technologies to quantify quorum signaling molecules in chronic and acute infections.
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Haque S, Yadav DK, Bisht SC, Yadav N, Singh V, Dubey KK, Jawed A, Wahid M, Dar SA. Quorum sensing pathways in Gram-positive and -negative bacteria: potential of their interruption in abating drug resistance. J Chemother 2019; 31:161-187. [DOI: 10.1080/1120009x.2019.1599175] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- Shafiul Haque
- Research and Scientific Studies Unit, College of Nursing and Allied Health Sciences, Jazan University, Jazan, Saudi Arabia
- Gene Expression Laboratory, Department of Biosciences, Faculty of Natural Sciences, Jamia Millia Islamia, New Delhi, India
| | - Dinesh K. Yadav
- Department of Botany, University of Allahabad, Allahabad, Uttar Pradesh, India
| | - Shekhar C. Bisht
- Department of Biotechnology, H.N.B Garhwal University, Srinagar, Uttarakhand, India
| | - Neelam Yadav
- Department of Botany, University of Allahabad, Allahabad, Uttar Pradesh, India
| | - Vineeta Singh
- Microbiology Division, CSIR-Central Drug Research Institute, Lucknow, Uttar Pradesh, India
| | - Kashyap Kumar Dubey
- Industrial Biotechnology Laboratory, University Institute of Engineering and Technology, M.D. University, Rohtak, Haryana, India
| | - Arshad Jawed
- Research and Scientific Studies Unit, College of Nursing and Allied Health Sciences, Jazan University, Jazan, Saudi Arabia
| | - Mohd Wahid
- Research and Scientific Studies Unit, College of Nursing and Allied Health Sciences, Jazan University, Jazan, Saudi Arabia
| | - Sajad Ahmad Dar
- Research and Scientific Studies Unit, College of Nursing and Allied Health Sciences, Jazan University, Jazan, Saudi Arabia
- Departments of Microbiology, University College of Medical Sciences (University of Delhi), Delhi, India
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Haque S, Ahmad F, Dar SA, Jawed A, Mandal RK, Wahid M, Lohani M, Khan S, Singh V, Akhter N. Developments in strategies for Quorum Sensing virulence factor inhibition to combat bacterial drug resistance. Microb Pathog 2018; 121:293-302. [PMID: 29857121 DOI: 10.1016/j.micpath.2018.05.046] [Citation(s) in RCA: 57] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2018] [Revised: 05/27/2018] [Accepted: 05/28/2018] [Indexed: 12/22/2022]
Abstract
Quorum sensing (QS) is a complex bacterial intercellular communication system. It is mediated by molecules called auto-inducers (AIs) and allows coordinated responses to a variety of environmental signals by inducing alterations in gene expression. Communication through QS can tremendously stimulate the pathogenicity and virulence via multiple mechanisms in pathogenic bacteria. The present review explores the major types of multitudinous QS systems known in Gram-positive and Gram-negative bacteria and their roles in bacterial pathogenesis and drug resistance. Because bacterial resistance to antibiotics is increasingly becoming a significant clinical challenge to human health; alternate strategies to combat drug resistance are warranted. Targeting bacterial pathogenicity by interruptions in QS using natural QS inhibitors and synthetic quorum-quenching analogs are being increasingly considered for development of next generation antimicrobials. The review highlights the recent advancements in discovery of promising new QS modulators and their efficiency in controlling infections caused by multidrug-resistant bacterial pathogens.
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Affiliation(s)
- Shafiul Haque
- Research and Scientific Studies Unit, College of Nursing & Allied Health Sciences, Jazan University, Jazan, 45142, Saudi Arabia.
| | - Faraz Ahmad
- Department of Public Health, College of Public Health, Imam Abdulrahman Bin Faisal University, Dammam, 31441, Saudi Arabia
| | - Sajad A Dar
- Research and Scientific Studies Unit, College of Nursing & Allied Health Sciences, Jazan University, Jazan, 45142, Saudi Arabia
| | - Arshad Jawed
- Research and Scientific Studies Unit, College of Nursing & Allied Health Sciences, Jazan University, Jazan, 45142, Saudi Arabia
| | - Raju K Mandal
- Research and Scientific Studies Unit, College of Nursing & Allied Health Sciences, Jazan University, Jazan, 45142, Saudi Arabia
| | - Mohd Wahid
- Research and Scientific Studies Unit, College of Nursing & Allied Health Sciences, Jazan University, Jazan, 45142, Saudi Arabia
| | - Mohtashim Lohani
- Department of Emergency Medical Services, College of Applied Medical Sciences, Jazan University, Jazan, 45142, Saudi Arabia
| | - Saif Khan
- Department of Clinical Laboratory Science, College of Applied Medical Sciences, University of Ha'il, Ha'il, 2440, Saudi Arabia
| | - Vineeta Singh
- Department of Biotechnology, Institute of Engineering & Technology, Lucknow, 226021, Uttar Pradesh, India
| | - Naseem Akhter
- Department of Laboratory Medicine, Faculty of Applied Medical Sciences, Albaha University, Albaha, 65431, Saudi Arabia
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6
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Maleki F, Becskei A. An open-loop approach to calculate noise-induced transitions. J Theor Biol 2017; 415:145-157. [PMID: 27993627 DOI: 10.1016/j.jtbi.2016.12.012] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2016] [Revised: 11/03/2016] [Accepted: 12/13/2016] [Indexed: 12/20/2022]
Abstract
Bistability permits the co-existence of two distinct cell fates in a population of genetically identical cells. Noise induced transitions between two fates of a bistable system are difficult to calculate due to the intricate interplay between nonlinear dynamics and noise in bistable positive feedback loops. Here we opened multivariable feedback loops at the slowest variable to obtain the open-loop function and the fluctuations in the open-loop output. By the subsequent reclosing of the loop, we calculated the mean first passage time (MFPT) using the Fokker-Planck equation in good agreement with the exact stochastic simulation. When an external component interacts with a feedback component, it amplifies the extrinsic noise in the loop. Consequently, the open-loop function is shifted and the transition rates between the two states in the closed loop are increased. Despite this shift, the open-loop output reflects the system faithfully to predict the MFPT in the feedback loop. Therefore, the open-loop approach can help theoretical analysis. Furthermore, the measurement of the mean value, variance, and the reaction time-scale of the open-loop output permits the prediction of MFPT simply from experimental data, which underscores the practical value of the stochastic open-loop approach.
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Affiliation(s)
- Farzaneh Maleki
- Biozentrum, Computational and systems biology, University of Basel, 4056 Basel, Switzerland
| | - Attila Becskei
- Biozentrum, Computational and systems biology, University of Basel, 4056 Basel, Switzerland.
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Hsu C, Jaquet V, Maleki F, Becskei A. Contribution of Bistability and Noise to Cell Fate Transitions Determined by Feedback Opening. J Mol Biol 2016; 428:4115-4128. [PMID: 27498164 DOI: 10.1016/j.jmb.2016.07.024] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2016] [Revised: 07/26/2016] [Accepted: 07/29/2016] [Indexed: 01/06/2023]
Abstract
Alternative cell fates represent a form of non-genetic diversity, which can promote adaptation and functional specialization. It is difficult to predict the rate of the transition between two cell fates due to the strong effect of noise on feedback loops and missing parameters. We opened synthetic positive feedback loops experimentally to obtain open-loop functions. These functions allowed us to identify a deterministic model of bistability by bypassing noise and the requirement to resolve individual processes in the loop. Combining the open-loop function with kinetic measurements and reintroducing the measured noise, we were able to predict the transition rates for the feedback systems without parameter tuning. Noise in gene expression was the key determinant of the transition rates inside the bistable range. Transitions between two cell fates were also observed outside of the bistable range, evidenced by bimodality and hysteresis. In this case, a slow transient process was the rate-limiting step in the transitions. Thus, feedback opening is an effective approach to identify the determinants of cell fate transitions and to predict their rates.
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Affiliation(s)
- Chieh Hsu
- Biozentrum, University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland; School of Biosciences, University of Kent, Canterbury, Kent CT2 7NJ, UK
| | - Vincent Jaquet
- Biozentrum, University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland
| | - Farzaneh Maleki
- Biozentrum, University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland
| | - Attila Becskei
- Biozentrum, University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland.
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8
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Drees B, Reiger M, Jung K, Bischofs IB. A modular view of the diversity of cell-density-encoding schemes in bacterial quorum-sensing systems. Biophys J 2015; 107:266-77. [PMID: 24988360 PMCID: PMC4119280 DOI: 10.1016/j.bpj.2014.05.031] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2014] [Revised: 05/20/2014] [Accepted: 05/23/2014] [Indexed: 11/29/2022] Open
Abstract
Certain environmental parameters are accessible to cells only indirectly and require an encoding step for cells to retrieve the relevant information. A prominent example is the phenomenon of quorum sensing by microorganisms, where information about cell density is encoded by means of secreted signaling molecules. The mapping of cell density to signal molecule concentration and the corresponding network modules involved have been at least partially characterized in many bacteria, and vary markedly between different systems. In this study, we investigate theoretically how differences in signal transport, signal modification, and site of signal detection shape the encoding function and affect the sensitivity and the noise characteristics of the cell-density-encoding process. We find that different modules are capable of implementing both fairly basic as well as more complex encoding schemes, whose qualitative characteristics vary with cell density and are linked to network architecture, providing the basis for a hierarchical classification scheme. We exploit the tight relationship between encoding behavior and network architecture to constrain the network topology of partially characterized natural systems, and verify one such prediction by showing experimentally that Vibrio harveyi is capable of importing Autoinducer 2. The framework developed in this research can serve not only to guide reverse engineering of natural systems but also to stimulate the design of synthetic systems and generally facilitate a better understanding of the complexities arising in the quorum-sensing process because of variations in the physical organization of the encoder network module.
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Affiliation(s)
- Bastian Drees
- Center for Molecular Biology (ZMBH), University of Heidelberg, Germany; BioQuant, University of Heidelberg, Germany
| | - Matthias Reiger
- Munich Center for Integrated Protein Science (CIPSM) at the Department of Microbiology, Ludwig-Maximilians-Universität München, Germany
| | - Kirsten Jung
- Munich Center for Integrated Protein Science (CIPSM) at the Department of Microbiology, Ludwig-Maximilians-Universität München, Germany.
| | - Ilka B Bischofs
- Center for Molecular Biology (ZMBH), University of Heidelberg, Germany; BioQuant, University of Heidelberg, Germany.
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Heilmann S, Krishna S, Kerr B. Why do bacteria regulate public goods by quorum sensing?-How the shapes of cost and benefit functions determine the form of optimal regulation. Front Microbiol 2015; 6:767. [PMID: 26284049 PMCID: PMC4517451 DOI: 10.3389/fmicb.2015.00767] [Citation(s) in RCA: 48] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2015] [Accepted: 07/14/2015] [Indexed: 11/29/2022] Open
Abstract
Many bacteria secrete compounds which act as public goods. Such compounds are often under quorum sensing (QS) regulation, yet it is not understood exactly when bacteria may gain from having a public good under QS regulation. Here, we show that the optimal public good production rate per cell as a function of population size (the optimal production curve, OPC) depends crucially on the cost and benefit functions of the public good and that the OPC will fall into one of two categories: Either it is continuous or it jumps from zero discontinuously at a critical population size. If, e.g., the public good has accelerating returns and linear cost, then the OPC is discontinuous and the best strategy thus to ramp up production sharply at a precise population size. By using the example of public goods with accelerating and diminishing returns (and linear cost) we are able to determine how the two different categories of OPSs can best be matched by production regulated through a QS signal feeding back on its own production. We find that the optimal QS parameters are different for the two categories and specifically that public goods which provide accelerating returns, call for stronger positive signal feedback.
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Affiliation(s)
- Silja Heilmann
- Computational Biology, Xavier lab, Memorial Sloan Kettering Cancer Center New York, NY, USA
| | - Sandeep Krishna
- National Centre for Biological Sciences, Simons Centre for the Study of Living Machines Bangalore, India
| | - Benjamin Kerr
- Department of Biology, University of Washington Seattle, WA, USA
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Fujimoto K, Sawai S. A design principle of group-level decision making in cell populations. PLoS Comput Biol 2013; 9:e1003110. [PMID: 23825937 PMCID: PMC3694814 DOI: 10.1371/journal.pcbi.1003110] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2012] [Accepted: 05/05/2013] [Indexed: 11/19/2022] Open
Abstract
Populations of cells often switch states as a group to cope with environmental changes such as nutrient availability and cell density. Although the gene circuits that underlie the switches are well understood at the level of single cells, the ways in which such circuits work in concert among many cells to support group-level switches are not fully explored. Experimental studies of microbial quorum sensing show that group-level changes in cellular states occur in either a graded or an all-or-none fashion. Here, we show through numerical simulations and mathematical analysis that these behaviors generally originate from two distinct forms of bistability. The choice of bistability is uniquely determined by a dimensionless parameter that compares the synthesis and the transport of the inducing molecules. The role of the parameter is universal, such that it not only applies to the autoinducing circuits typically found in bacteria but also to the more complex gene circuits involved in transmembrane receptor signaling. Furthermore, in gene circuits with negative feedback, the same dimensionless parameter determines the coherence of group-level transitions from quiescence to a rhythmic state. The set of biochemical parameters in bacterial quorum-sensing circuits appear to be tuned so that the cells can use either type of transition. The design principle identified here serves as the basis for the analysis and control of cellular collective decision making. Although the genetic circuits underlying state switching at the single-cell level are well understood, how such circuits work in concert among many cells to support the population-level switching of cellular behaviors is not fully explored. Experiments using microbial signaling systems show that group-level changes in cellular state occur in either a graded or an all-or-none fashion. We show that the type of group-level decision making used by populations is uniquely determined by a single dimensionless parameter that compares the quorum-signaling molecules accumulated within the cells with those secreted by the population. Bacterial quorum-sensing circuits appear to be tuned so that the cells can convert between the two types of decision-making in response to slight biochemical variations. Furthermore, the role of the parameter is universal such that it not only applies to the autoinducing circuits typically found in bacteria but also to the more complex gene circuits involved in transmembrane receptor signaling and negative feedback. The design principle that we describe thus serves as the basis for the analysis and control of collective cellular decision making in general.
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Affiliation(s)
- Koichi Fujimoto
- Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan.
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11
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Abstract
Many pathogenic bacteria use quorum sensing (QS) systems to regulate the expression of virulence genes in a density-dependent manner. In one widespread QS paradigm the enzyme LuxI generates a small diffusible molecule of the acyl-homoserine lactone (AHL) family; high cell densities lead to high AHL levels; AHL binds the transcription factor LuxR, triggering it to activate gene expression at a virulence promoter. The emergence of antibiotic resistance has generated interest in alternative anti-microbial therapies that target QS. Inhibitors of LuxI and LuxR have been developed and tested in vivo, and can act at various levels: inhibiting the synthesis of AHL by LuxI, competitively or non-competitively inhibiting LuxR, or increasing the turnover of LuxI, LuxR, or AHL. Here use an experimentally validated computational model of LuxI/LuxR QS to study the effects of using inhibitors individually and in combination. The model includes the effect of transcriptional feedback, which generates highly non-linear responses as inhibitor levels are increased. For the ubiquitous LuxI-feedback virulence systems, inhibitors of LuxI are more effective than those of LuxR when used individually. Paradoxically, we find that LuxR competitive inhibitors, either individually or in combination with other inhibitors, can sometimes increase virulence by weakly activating LuxR. For both LuxI-feedback and LuxR-feedback systems, a combination of LuxR non-competitive inhibitors and LuxI inhibitors act multiplicatively over a broad parameter range. In our analysis, this final strategy emerges as the only robust therapeutic option.
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Affiliation(s)
- Rajat Anand
- National Centre for Biological Sciences, Tata Institute of Fundamental Research, UAS/GKVK Campus, Bangalore, India
| | - Navneet Rai
- National Centre for Biological Sciences, Tata Institute of Fundamental Research, UAS/GKVK Campus, Bangalore, India
- Department of BioSciences and Bioengineering, Indian Institute of Technology Bombay, Powai, Mumbai, India
| | - Mukund Thattai
- National Centre for Biological Sciences, Tata Institute of Fundamental Research, UAS/GKVK Campus, Bangalore, India
- * E-mail:
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Thattai M. Using topology to tame the complex biochemistry of genetic networks. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2013; 371:20110548. [PMID: 23277605 PMCID: PMC3538440 DOI: 10.1098/rsta.2011.0548] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Living cells are controlled by networks of interacting genes, proteins and biochemicals. Cells use the emergent collective dynamics of these networks to probe their surroundings, perform computations and generate appropriate responses. Here, we consider genetic networks, interacting sets of genes that regulate one another's expression. It is possible to infer the interaction topology of genetic networks from high-throughput experimental measurements. However, such experiments rarely provide information on the detailed nature of each interaction. We show that topological approaches provide powerful means of dealing with the missing biochemical data. We first discuss the biochemical basis of gene regulation, and describe how genes can be connected into networks. We then show that, given weak constraints on the underlying biochemistry, topology alone determines the emergent properties of certain simple networks. Finally, we apply these approaches to the realistic example of quorum-sensing networks: chemical communication systems that coordinate the responses of bacterial populations.
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Affiliation(s)
- Mukund Thattai
- National Centre for Biological Sciences, Tata Institute of Fundamental Research, UAS/GKVK Campus, Bellary Road, Bangalore 560065, India.
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de las Heras A, Fraile S, de Lorenzo V. Increasing signal specificity of the TOL network of Pseudomonas putida mt-2 by rewiring the connectivity of the master regulator XylR. PLoS Genet 2012; 8:e1002963. [PMID: 23071444 PMCID: PMC3469447 DOI: 10.1371/journal.pgen.1002963] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2012] [Accepted: 08/07/2012] [Indexed: 11/28/2022] Open
Abstract
Prokaryotic transcription factors (TFs) that bind small xenobiotic molecules (e.g., TFs that drive genes that respond to environmental pollutants) often display a promiscuous effector profile for analogs of the bona fide chemical signals. XylR, the master TF for expression of the m-xylene biodegradation operons encoded in the TOL plasmid pWW0 of Pseudomonas putida, responds not only to the aromatic compound but also, albeit to a lesser extent, to many other aromatic compounds, such as 3-methylbenzylalcohol (3MBA). We have examined whether such a relaxed regulatory scenario can be reshaped into a high-capacity/high-specificity regime by changing the connectivity of this effector-sensing TF within the rest of the circuit rather than modifying XylR structure itself. To this end, the natural negative feedback loop that operates on xylR transcription was modified with a translational attenuator that brings down the response to 3MBA while maintaining the transcriptional output induced by m-xylene (as measured with a luxCDABE reporter system). XylR expression was then subject to a positive feedback loop in which the TF was transcribed from its own target promoters, each known to hold different input/output transfer functions. In the first case (xylR under the strong promoter of the upper TOL operon, Pu), the reporter system displayed an increased transcriptional capacity in the resulting network for both the optimal and the suboptimal XylR effectors. In contrast, when xylR was expressed under the weaker Ps promoter, the resulting circuit unmistakably discriminated m-xylene from 3MBA. The non-natural connectivity engineered in the network resulted both in a higher promoter activity and also in a much-increased signal-to-background ratio. These results indicate that the working regimes of given genetic circuits can be dramatically altered through simple changes in the way upstream transcription factors are self-regulated by positive or negative feedback loops. It is generally taken for granted that promoters regulated by transcriptional factors (TFs) that respond to small molecules control their specificity to given effectors by tightening or relaxing the intrinsic dual interaction between the TF and the particular inducer. One such promoter is Pu, which drives expression of an operon for the biodegradation of m-xylene by the soil bacterium P. putida mt-2. While XylR, the chief TF of this system, binds this substrate and activates Pu, the same regulator responds, to a lesser extent, to 3-methylbenzylalcohol and thus also activates the promoter. This work provides evidence that such natural effector promiscuity of the system can be altogether suppressed by replacing the naturally occurring negative autoregulation loop that governs XylR expression with an equivalent positive feedback loop. Based on this result, we argue that signal specificity of a given regulatory device depends not only on the TF involved but also on TF connectivity to upstream signals and downstream targets.
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Affiliation(s)
| | | | - Victor de Lorenzo
- Systems Biology Program, Centro Nacional de Biotecnología, Consejo Superior de Investigaciones Cientificas, Madrid, Spain
- * E-mail:
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