The PmrB protein is necessary for the mild acid activation of PmrA 
The PmrB protein is necessary for activation of the PmrA protein in low Mg2+ (Kox et al., 2000; Kato and Groisman, 2004) and in the presence of Fe3+ (Wosten et al., 2000), consistent with the notion that PmrB is the major phosphodonor for PmrA.
We investigated whether PmrB was also required for the pH-dependent induction of pbgP, which was chosen as a prototypical PmrA-activated gene because the PmrA protein binds to the pbgP promoter in vitro (Wosten and Groisman, 1999) and in vivo (Shin and Groisman, 2005).
Thus, we determined the beta-galactosidase activity of isogenic pmrB strains harbouring a chromosomal pbgP-lac transcriptional fusion: expression was approximately sixfold lower in a pmrB mutant than in the pmrB+ strain following growth at pH 5.8 (Fig. 3), indicating that a functional pmrB gene is necessary for a normal response to mild acid pH.
There was residual pbgP expression in the pmrB mutant induced with mild acid pH (Fig. 3), which was in contrast to the absence of pbgP transcription in the pmrA mutant (Fig. 2).
This suggested that PmrA could become phosphorylated from another phosphodonor(s) when PmrB is not present.
We considered the possibility of PmrA being phosphorylated from acetyl phosphate because acetyl phosphate has been shown to serve as phosphoryl donor to several response regulators when their cognate sensors are absent (see Wolfe, 2005 for a review).
Consistent with this notion, pbgP transcription was abrogated in the pmrB mutant upon deletion of the pta and ackA genes (Fig. 3), which encode the two enzymes that are required for the production of acetyl phosphate (Wolfe, 2005) (Fig. 1).
In contrast, a strain lacking the ability to synthesize acetyl phosphate but with a functional pmrB gene exhibited wild-type pbgP expression levels (Fig. 3), implying that under normal conditions (i.e. when a functional pmrB gene is present) acetyl phosphate does not contribute to PmrA phosphorylation.
