title: Multiple histidines in the periplasmic domain of the Salmonella enterica sensor kinase SsrA enhance signaling in response to extracellular acidification.
abstract: The two-component regulatory system SsrA-SsrB in Salmonella enterica controls expression of a virulence gene program required for intracellular survival in host cells. SsrA signaling is induced within the acidic host vacuole in which the bacteria reside; however, the mechanism by which SsrA senses this intracellular environment is unknown. Here, we show that the periplasmic sensor domain of SsrA is enriched in histidine residues that increase SsrA signaling below external pH of 6. While no single histidine accounted for the full acid-responsiveness of SsrA, we localized the acid-responsiveness principally to five histidines in the C-terminal end of the periplasmic sensor domain, with input from additional histidines in the N-terminal end of the senor. A sensor mutant lacking critical pH-responsive histidines was defective for acid-promoted activity, yet retained basal activity similar to wild type at neutral pH, indicating that the role of these histidines is to enhance signaling in response to acidification. In support of this, a pH-blind mutant was insensitive to the vacuole acidification blocking activity of bafilomycin, and was attenuated for competitive fitness during infection of mice. Our data demonstrate that SsrA contains a histidine-rich periplasmic sensor that enhances signaling in response to the innate host defense of vacuolar acidification.
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, L8N 3Z5, Canada.
authors.forename: David T
authors.initials: DT
authors.lastname: Mulder
authors.forename: Joseph B
authors.initials: JB
authors.lastname: McPhee
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Reid-Yu
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Stogios
authors.forename: Alexei
authors.initials: A
authors.lastname: Savchenko
authors.forename: Brian K
authors.initials: BK
authors.lastname: Coombes
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015964:Gene Expression Regulation, Bacterial; D056084:Genetic Fitness; D006639:Histidine / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D018942:Macrolides / Q000494:pharmacology; D008264:Macrophages / Q000382:microbiology; D051379:Mice; D008810:Mice, Inbred C57BL; D016296:Mutagenesis; D019897:Periplasm; D011494:Protein Kinases / Q000737:chemistry* / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D012481:Salmonella Infections, Animal / Q000382:microbiology; D012486:Salmonella typhimurium / Q000201:enzymology / Q000378:metabolism* / Q000472:pathogenicity; D015398:Signal Transduction; D014617:Vacuoles / Q000378:metabolism; D014774:Virulence
chemical_list: D001426:Bacterial Proteins; D004791:Enzyme Inhibitors; D018942:Macrolides; D006639:Histidine; C040929:bafilomycin A1; D011494:Protein Kinases