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

ABSTRACT: 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 (Lys(179), Met(186)) and the dimerization interface (Val(197), Leu(201)) that are important for SsrB transcriptional activation and DNA binding. An essential cysteine residue in the N-terminal receiver domain was also identified (Cys(45)), and the effect of Cys(203) 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 Cys(203) or Cys(45) may be an important mode of SsrB inactivation inside the host.

SUBMITTER: Carroll RK

PROVIDER: S-EPMC2673270 | biostudies-literature | 2009 May

REPOSITORIES: biostudies-literature

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

Carroll Ronan K RK Liao Xiubei X Morgan Leslie K LK Cicirelli Elisha M EM Li Yuanhe Y Sheng Wanyun W Feng Xiuhong X Kenney Linda J LJ

The Journal of biological chemistry 20090105 18

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 memb ...[more]

PMID: 19126546

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Project description:The cystic fibrosis transmembrane conductance regulator (CFTR), encoded by the gene mutated in cystic fibrosis patients, belongs to the family of ATP-binding cassette (ABC) proteins, but, unlike other members, functions as a chloride channel. CFTR is activated by protein kinase A (PKA)-mediated phosphorylation of multiple sites in its regulatory domain, and gated by binding and hydrolysis of ATP at its two nucleotide binding domains (NBD1, NBD2). The recent crystal structure of NBD1 from mouse CFTR (Lewis, H.A., S.G. Buchanan, S.K. Burley, K. Conners, M. Dickey, M. Dorwart, R. Fowler, X. Gao, W.B. Guggino, W.A. Hendrickson, et al. 2004. EMBO J. 23:282-293) identified two regions absent from structures of all other NBDs determined so far, a "regulatory insertion" (residues 404-435) and a "regulatory extension" (residues 639-670), both positioned to impede formation of the putative NBD1-NBD2 dimer anticipated to occur during channel gating; as both segments appeared highly mobile and both contained consensus PKA sites (serine 422, and serines 660 and 670, respectively), it was suggested that their phosphorylation-linked conformational changes might underlie CFTR channel regulation. To test that suggestion, we coexpressed in Xenopus oocytes CFTR residues 1-414 with residues 433-1480, or residues 1-633 with 668-1480, to yield split CFTR channels (called 414+433 and 633+668) that lack most of the insertion, or extension, respectively. In excised patches, regulation of the resulting CFTR channels by PKA and by ATP was largely normal. Both 414+433 channels and 633+668 channels, as well as 633(S422A)+668 channels (lacking both the extension and the sole PKA consensus site in the insertion), were all shut during exposure to MgATP before addition of PKA, but activated like wild type (WT) upon phosphorylation; this indicates that inhibitory regulation of nonphosphorylated WT channels depends upon neither segment. Detailed kinetic analysis of 414+433 channels revealed intact ATP dependence of single-channel gating kinetics, but slightly shortened open bursts and faster closing from the locked-open state (elicited by ATP plus pyrophosphate or ATP plus AMPPNP). In contrast, 633+668 channel function was indistinguishable from WT at both macroscopic and microscopic levels. We conclude that neither nonconserved segment is an essential element of PKA- or nucleotide-dependent regulation.

Project description:We performed chromatin immunoprecipitation on microarray (ChIP-chip) transcriptional profiling of the SsrB regulator of Salmonella enterica Typhimurium to better characterize its regulon and to identify the DNA-recognition element coordinating its specific interaction at cis-regulatory sites. SsrB, the response regulator of the two component regulatory system SsrA-SsrB encoded within the Salmonella Pathogenicity Island (SPI-2), directs transcriptional activation of the closely associated type three secretion system (T3SS) also encoded at this locus. Immunoprecipitaiton of SsrB translationally fused to a C-terminal FLAG-tag from formaldehyde cross-linked genomic DNA was performed under SPI-2 activating and non-activating conditions. Downstream analysis and experimental work identified specific interactions within SPI-2 at the previously identified ssrA, ssaB, sseA, ssaG, ssaM promoters and identified an additional cryptic promoter driving expression upstream of ssaR. Additionally, a previously unknown SsrB interaction site within ssaE was shown to be the major contributor driving the expression of the downstream genes and not the previously identified interaction site within the intergenic region of ssaE-sseA. Interactions were also identified outside of SPI-2 upstream of other T3SS-associated genes encoded within other genomic islands, and for other uncharacterized genes. Integration of this data with transcriptional microarray work, previously published DNase I footprinting data, bacteria 1-hybrid investigations and comparative genomics analyses of cis-regulatory regions within the orthologous Sodalis symbiosis region 3 (SSR-3) of Sodalis glossinidius enabled identification of a conserved 18bp palindrome which was experimentally validated as being required for transcriptional activation of SsrB dependant genes. SsrB-FLAG immunoprecipitations were performed under SsrB activating and non-activating conditions from formaldehyde cross-linked bacterial lysates. Nine immunoprecipitation reactions were pooled into three replicate samples for the activating condition and three reactions were pooled into one sample for the non-activating control condition. The immunoprecipitated DNA in addition to the non-immunoprecipitated control DNA for each of the four samples were labeled with Cy3 or Cy5 fluorophores respectively and were concurrently hybridized to four arrays on a single slide.

Dataset Information

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

Publications

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

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