Introduction 
Free-living organisms often encounter wide variations in the pH of their surroundings.
Thus, pH may act as a signal that triggers cellular responses designed to cope with a new environment.
The Gram-negative bacterium Salmonella enterica serovar Typhimurium, for example, experiences a number of acidic environments both inside and outside animal hosts.
During infection of a mammalian host, Salmonella is exposed to severe acidity in the stomach (Rychlik and Barrow, 2005) and mild acidification in the endocytic vacuoles of intestinal epithelia and macrophages (Brumell and Grinstein, 2004).
Moreover, Salmonella has been recovered from soil and water (Winfield and Groisman, 2003) where the pH can be significantly low.
While growth in acidic conditions has been shown to promote changes in the gene expression profiles of several bacterial species (Tucker et al., 2002; McGowan et al., 2003; Weinrick et al., 2004; Leaphart et al., 2006), less is known about the identity of the molecule(s) that sense extracytoplasmic fluctuations in pH and the mechanisms by which such sensors promote changes in gene expression.
Previous studies have revealed that Salmonella responds to acidic challenges through an adaptive system called the acid tolerance response in which adaptation to mild acid conditions enables the organism to survive periods of severe acid stress (Foster and Hall, 1990; Foster, 1995).
The acid tolerance response of Salmonella results in the synthesis of over 50 acid shock proteins (Bearson et al., 1998) that are likely to function primarily when variations in internal pH occur, i.e. when Salmonella experiences severe acidic conditions (pH approximately3) (Foster, 2004).
Growth of Salmonella in mild acid (pH 5.8) also promotes transcription of genes regulated by the response regulator PmrA (Soncini and Groisman, 1996).
The expression of these genes has been shown to be dispensable for the acid tolerance response (Bearson et al., 1998) which suggests that there are still uncharacterized cellular function(s) that Salmonella needs to regulate in acidic environments.
The PmrA protein and its cognate sensor kinase PmrB form a two-component regulatory system that is required for virulence in mice (Gunn et al., 2000), infection of chicken macrophages (Zhao et al., 2002), growth in soil (Chamnongpol et al., 2002), resistance to the cationic peptide antibiotic polymyxin B (Roland et al., 1993) and resistance to Fe3+- (Wosten et al., 2000) and Al3+-mediated killing (Nishino et al., 2006).
The PmrA-regulated products characterized thus far mediate modifications to the various components of the lipopolysacharide (LPS) structure including the lipid A (Gunn et al., 1998; Trent et al., 2001; Zhou et al., 2001; Breazeale et al., 2003; Lee et al., 2004), the core region (Nishino et al., 2006) and the O-antigen (Delgado et al., 2006).
While other PmrA-regulated genes have been identified (Marchal et al., 2004; Tamayo et al., 2005), their biochemical activities and the role(s) that they play in Salmonella's life remain unknown.
Besides mild acid pH, two other stimuli are known to promote expression of PmrA-activated genes: (i) submillimolar levels of extracellular Fe3+ or Al3+, which are directly sensed by the PmrB protein (Wosten et al., 2000), and (ii) low concentrations of extracellular Mg2+ (Soncini and Groisman, 1996) (Fig. 1).
The low Mg2+ activation of the PmrA protein requires PhoQ, a protein that senses extracellular Mg2+ levels (Vescovi et al., 1996), PhoQ's cognate regulator PhoP, and the PhoP-activated protein PmrD (Kox et al., 2000; Kato and Groisman, 2004).
PmrD binds to the phosphorylated form of PmrA protecting it from dephosphorylation by PmrB (Kato and Groisman, 2004).
Here we show that PmrA's cognate sensor kinase PmrB is required for responding to external changes in pH through a mechanism that requires a histidine and several glutamic acid residues located in its periplasmic domain, as well as the post-translational activator PmrD protein.
