טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBen-Barak Zelas Zohar
SubjectRegulation of avrA from Salmonella Enterica and its Role
in the Bacterial Virulence
DepartmentDepartment of Biotechnology and Food Engineering
Supervisors Professor Sima Yaron
Dr. Shai Cohen
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The effector protein AvrA of Salmonella enterica turns the inflammatory response of the host and enhances apoptosis of the host cell. Consequently it contributes to the complexity of the network of Salmonella’s virulence factors.

The present study outlines the expression of avrA gene among S. enterica serovars with the aim of disclosing the molecular nature of the AvrA regulation and its involvement in the virulence of S. enterica.  Twenty five S. enterica strains were chosen in order to study the molecular nature of AvrA expression in more detail in different environmental signals.

AvrA protein expression takes place either constitutively (class 1) or after acid induction only (class 2) or not at all under any conditions tested thus far (class 3). Class 0 strains do not contain the avrA gene. In contrast, the transcription was found positive and in similar levels in all S. enterica strains. These data point to a kind of post-transcriptional control of the AvrA protein. Moreover, such diversity in the induction of the AvrA protein could be either due to nucleotide-sequence differences of the promoter region of avrA gene or to differences in a regulatory system.  Sequence analysis of the avrA gene and its promoter shows that it is rather conserved, reveals that regulatory proteins play a role in such diversity.

In this study we have identified, for the first time, regulatory proteins that affect the AvrA expression at different levels. By screening of specific mutants we found that deletions of csrA and csrB abolished the expression but not the transcription of AvrA, suggesting that AvrA is regulated in a post-transcriptional manner by the co-operative action of the ribonucleic acid binding protein CsrA and its sequestering through the ncRNA, CsrB, in order to achieve a critical and effective concentration of CsrA that activates translation (see-saw regulation). 

In addition, two regulators, SlyA and PhoP, which probably bind the avrA promoter and repress the AvrA transcription, were identified.  

Several reports showed that the presence of AvrA accelerates apoptosis in the infected macrophage cell culture, while others did not observe this effect. Our results solve this contradiction by showing that the reason is the use of different Salmonella strains, from different AvrA-expression classes. Infection with class 1 strains indeed induces apoptosis.

The results summarized in this work underline that the main regulation level of the effector protein AvrA is post-transcriptional regulation, and it can be regarded as a critical step in fine tuning the virulence network in different Salmonella strains.