MBI PhD Qualifying Exam

Time: 5pm
Date: 22 May 2019, Wednesday
Venue: MBI, Level 5 Meeting Room.

Supervisor: Dr Linda J. Kenney

The Role of SsrB in Salmonella Typhimurium Lifestyle

by Dasvit SHETTY, Kenney Group

Abstract

SsrB is a response regulator that controls the induction of SPI-2 genes during the intracellular phase of Salmonella Typhimurium infection. SsrB is a NarL/FixJ family response regulator and is phosphorylated by its associated sensor kinase SsrA in order to activate SPI-2 genes. An unphosphorylated SsrB mutant, D56A, was discovered to regulate the master biofilm regulator csgD, thus functioning as a lifestyle switch between the infectious and carrier states of the pathogen. This discovery led to an RNA-seq analysis to investigate more genes that might be under SsrB control during the non-virulent lifestyle. Analysis of the RNA-seq data of wild-type and DssrB strains in biofilm-inducing conditions showed down-regulation of many stress response genes in the DssrB strain, most notably dksA, dsrA and hfq genes. These genes are known regulators of the ancestral stress sigma factor RpoS; and were unaffected in the D56AssrB strain. This study aims to explore the role of SsrB in its non-canonical regulation of RpoS through DksA, DsrA and Hfq.

Although D56A SsrB has been shown to bind to DNA using AFM, it is unknown whether its binding affinity is comparable to wild-type phosphorylated SsrB (SsrB~P). Measuring the dissociation constants of D56A SsrB and SsrB~P to its known cognate promoters using a single molecule unzipping assay will assist in understanding the role of phosphorylation in DNA affinity and promoter-specificity of SsrB. Another unique characteristic of SsrB among NarL/FixJ regulators is its increased DNA binding at low pH. Residues in SsrB that influence this property and the molecular mechanism are not known. Hence, this study aims to use NMR and a single molecule unzipping assay to elucidate the basis of the increased DNA affinity of SsrB at low pH.

**Please note the examination following the seminar is closed-door**