As discussed in the Project Description some of Erwinia amylovora´s virulence factors depend on the quorum sensing (QS) phenomenon and, in this specific case, the type of AHL used by this phytopathogen is the 3-oxo-C6-HSL. For simplicity, they will continue to be named as AHLs along the whole model. Despite the fact that E. amylovora is not a very studied bacteria and therefore its QS either, we were able to simulate its behavior comparing it to the Vibrio fischeri´s one. The main reason was because of its similarities in the gene regulation. V. fischeri works with the LuxR/LuxI regulation while E. amylovora was found to work under a similar system with a highly homologous regulation, EamR/EamI.
EamI: REGULATED GENE RESPONSIBLE FOR QUORUM SENSING ACTIVATION
The EamI is a regulated gene and more precisely an auto activator one. Its activator is formed by a dimer of a complex made of a LuxR protein bonded to an AHL molecule. Thus two molecules of each are needed. The EamI protein itself is an AHL synthase, that is why it is called an auto activator, but how is it involved in QS virulence regulation? In Vibrio fischeri, when the activation of the LuxI gene is held, it also triggers the expression of the entire lux operon, where the genes responsible for producing the characteristic luminescence of this bacteria are found. Homologously, this occurs the same way in E. amylovora, when the EamI activation is held, the genes that will be triggered are the virulence responsible ones: the exopolysaccharides (ams and lsc operons) and the T3SS. When this regulation is interrupted (our purpose), two virulence factors will be inhibited at once and maybe even three because of the correlation between the exopolysaccharides and biofilm formation. Given the high impact that inhibiting the QS has, we chose to focus on modeling the behavior of this phenomenon in our project. The model, for simplicity, only models the EamI expression but it assumes the trigger situation described before. The next image tries to demonstrate the autoregulation of the EamI gene a little bit more.
When a certain cell density is reached, bacteria start communicating between each other and therefore the quorum sensing is carried out. The interaction starts when a first bacteria produces its own AHLs (purple triangles), some of those will exit the cell wall and eventually will become external AHLs. When these molecules manage to enter the second cell, they meet an EamR protein (blue circle), creating an activator complex that will find a second activator complex to create a dimerized complex which will bind to the operator box of EamI. This dimer-complex is responsible for activating the expression of the EamI gene (purple rectangle), resulting in an AHL synthase (purple rhombus). Now that the cell is synthesizing its own AHLs molecules, it will continue to autoactivate itself on an on, and at the same time it will start to externalize more AHLs, repeating the activation cascade between cells.
MATHEMATICAL MODELING FOR WILD TYPE Erwinia amylovora
Once we knew how the wild system works genetically speaking, we represented it with a system of mathematical equations. Simbology and Parameteres can be found in the Results tab.
Barnard, A. and Salmond, G. (2006) Quorum sensing in Erwinia species. Analytical and Bioanalytical Chemistry, 387(2), pp.415-423.
Frederick K. Balagaddé et al. (2008) A synthetic Escherichia coli predator–prey ecosystem, EMBO, 187, pp. 1-26.
Ingalls, B. (2013) Modeling of Chemical Reaction Networks & Gene Regulatory Networks. From Mathematical Modeling in Systems Biology(pp. 21-314 ). England: MIT press.
James, S. et al. (2000) Luminescence Control in the Marine Bacterium Vibrio fischeri: An Analysis of the Dynamics of lux Regulation., JMB, 296, pp. 1127-1137.
Koczan JM, McGrath MJ, Zhao Y, Sundin GW (2009) Contribution of Erwinia amylovora exopolysaccharides amylovoran and levan to biofilm formation: implications in pathogenicity. Phytopathology 99:1237-1244
Tzvia Iljon, Jenna Stirling and Robert J.Smith (2012) A mathematical model describing an outbreak of fire blight. En Understanding the Dynamics of Emerging and Re-Emergign Infectious Diseases Using Mathematical Models(91-104). University of Ottawa, Canada: Transworld Research Network.
Venturi, V., Venuti, C., Devescovi, G., Lucchese, C., Friscina, A., Degrassi, G., Aguilar, C. and Mazzucchi, U. (2017) The plant pathogen Erwinia amylovora produces acyl-homoserine lactone signal molecules in vitro and in planta.