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Julie Thibault

Functional characterization and mechanism of inhibition of ExsA, the master regulator of the Pseudomonas aeruginosa Type Three Secretion System

Published on 8 January 2010


Thesis presented January 08, 2010

Abstract:
Expression of the genes encoding the Type Three Secretion System T3SS, major virulence factor of Pseudomonas aeruginosa, is under the positive transcriptional control of ExsA. This activator is a member of the AraC/XylS family of transcriptional regulators characterized by a DNA binding domain containing two helix-turn-helix motifs. The activity of these proteins is generally modulated by a ligand which binds to an additional non-conserved domain. AraC/XylS family members involved in SST3 control can be regulated by protein ligands. The ExsA activi​ty is inhibited by the anti-activator ExsD.
The study of the functional domains of ExsA through in vitro and in vivo approaches showed that the C-terminal of ExsA is the DNA-binding domain and that two monomers bind to the promoter of the exsCEBA regulatory gene operon (pC). However, this domain possesses a lower affinity for pC and a lower transcriptional activity than the entire protein. Indeed, the efficient binding of ExsA to pC requires its dimerization through its N-terminal domain. The last
α-helix of this domain seems to play a crucial role in this dimerization. The second goal of my work was to understand the interaction between ExsA and ExsD and to define the molecular mechanism of inhibition used by the anti-activator. By co-producing the two proteins, we succeeded to purify and characterize the ExsA-ExsD complex. ExsA and ExsD form a heterodimeric complex within which ExsD prevents ExsA from binding to its target sequences.

Keywords:
Pseudomonas aeruginosa, Type III Secretion System, Transcriptional activator, AraC/XylS family of transcriptional regulators, Protein/protein interaction, DNA/protein interaction, Protein Anti-activator

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