Structural and functional analysis of the C-terminal DNA binding domain of the Salmonella typhimurium SPI-2 response regulator SsrB

In bacterial pathogenesis, virulence gene regulation is controlled by two-component regulatory systems. In Escherichia coli, the EnvZ/OmpR two-component system is best understood as regulating expression of outer membrane proteins, but in Salmonella enterica, OmpR activates transcription of the SsrA/B two-component system located on Salmonella pathogenicity island 2 (SPI-2). The response regulator SsrB controls expression of a type III secretory system in which effectors modify the vacuolar membrane and prevent its degradation via the endocytic pathway. Vacuolar modification enables Salmonella to survive and replicate in the macrophage phagosome and disseminate to the liver and spleen to cause systemic infection. The signals that activate EnvZ and SsrA are unknown but are related to the acidic pH encountered in the vacuole. Our previous work established that SsrB binds to regions of DNA that are AT-rich, with poor sequence conservation. Although SsrB is a major virulence regulator in Salmonella, very little is known regarding how it binds DNA and activates transcription. In the present work, we solved the structure of the C-terminal DNA binding domain of SsrB (SsrB_C) by NMR and analyzed the effect of amino acid substitutions on function. We identified residues in the DNA recognition helix (Lys₁₇₉, Met₁₈₆) and the dimerization interface (Val₁₉₇, Leu₂₀₁) that are important for SsrB transcriptional activation and DNA binding. An essential cysteine residue in the N-terminal receiver domain was also identified (Cys₄₅), and the effect of Cys₂₀₃ on dimerization was evaluated. Our results suggest that although disulfide bond formation is not required for dimerization, dimerization occurs upon DNA binding and is required for subsequent activation of transcription. Disruption of the dimer interface by a C203E substitution reduces SsrB activity. Modification of Cys₂₀₃ or Cys₄₅ may be an important mode of SsrB inactivation inside the host.