Evolution of a Bacterial Regulon Controlling Virulence and Mg2+ Homeostasis

Abstract

Related organisms typically rely on orthologous regulatory proteins to respond to a given signal. However, the extent to which (or even if) the targets of shared regulatory proteins are maintained across species has remained largely unknown. This question is of particular significance in bacteria due to the widespread effects of horizontal gene transfer. Here, we address this question by investigating the regulons controlled by the DNA-binding PhoP protein, which governs virulence and Mg2+ homeostasis in several bacterial species. We establish that the ancestral PhoP protein directs largely different gene sets in ten analyzed species of the family Enterobacteriaceae, reflecting both regulation of species-specific targets and transcriptional rewiring of shared genes. The two targets directly activated by PhoP in all ten species (the most distant of which diverged >200 million years ago), and coding for the most conserved proteins are the phoPQ operon itself and the lipoprotein-encoding slyB gene, which decreases PhoP protein activity. The Mg2+-responsive PhoP protein dictates expression of Mg2+ transporters and of enzymes that modify Mg2+-binding sites in the cell envelope in most analyzed species. In contrast to the core PhoP regulon, which determines the amount of active PhoP and copes with the low Mg2+ stress, the variable members of the regulon contribute species-specific traits, a property shared with regulons controlled by dissimilar regulatory proteins and responding to different signals.

The PhoP/PhoQ System and Its Role in Serratia marcescens Pathogenesis

“…The presence of PhoP-binding motifs in the promoter region of the S. marcescens phoPQ operon allowed us to predict that the system would be autoregulated, as was previously determined for other bacteria, such as E. coli, S. enterica, and Y. pestis (39,62,68). To analyze the regulatory features of the PhoP/PhoQ system in Serratia, we obtained a phoP mutant using the suicide plasmid pKNOCK-Gm, and we also constructed a phoP::lacZ reporter transcriptional fusion as described in Materials and Methods.…”

“…In the Serratia genus, the phoPQ ortholog has been previously identified in the Serratia proteomaculans 568 genome (62), and a search in the S. marcescens Db11 genome (http://www.sanger.ac.uk) also reveals a conserved phoPQ operon. S. marcescens PhoP exhibits 82% identity and 95% homology, while PhoQ exhibits 62% identity and 80% homology, to the Salmonella enterica PhoP and PhoQ proteins, respectively.…”

“…Evolutionary analysis has shown that in most of these organisms the PhoP/PhoQ system preserves an ancient function in the control of the magnesium homeostasis, while in pathogenic bacteria it is also dedicated to governing the expression of critical virulence phenotypes (62). Despite the fact that in pathogens such as Salmonella enterica, Mycobacterium tuberculosis, and Yersinia pestis (8,20,35,44,60) the PhoP/PhoQ system converges in conferring survival capacity inside the host, the set of regulated genes recruited under its control, and therefore the mechanism engaged, varies in a speciesspecific manner.…”

Fur Negatively Regulates hns and Is Required for the Expression of HilA and Virulence in Salmonella enterica Serovar Typhimurium

“…Indeed, we found that growth in the presence of the ferrous iron chelator 2,2Ј-dipyridyl resulted in reduced expression of hilA-lacZ (data not shown). To rule out the possibility that 2,2Ј-dipyridyl may scavenge Mg 2ϩ from the cell, we assessed the effect of the chelator on the expression of a PhoP activated gene (pagP) using a pagP-lacZ fusion, where PhoP binds pagP and is essential for induction of this gene (4,52). Unlike with the hilA fusion, the expression of pagP-lacZ was not differentially affected by the addition of 200 M 2, 2Јdipyri-dyl to the growth media (see Fig.…”

The importance of the magnesium transporter MgtB for virulence of Yersinia pseudotuberculosis and Yersinia pestis

“…Mg 2+ has been shown to be an important signal controlling gene regulation via the PhoPQ two-component regulatory system for a range of Gram-negative bacteria, including Y. pestis and Y. pseudotuberculosis (Grabenstein et al, 2004;Oyston et al, 2000). The PhoPQ regulon has been evaluated in Y. pestis and shown to involve over 400 differentially regulated genes (Grabenstein et al, 2006;Li et al, 2008;Perez et al, 2009;Zhou et al, 2005) including mgtB. However, the specific role of this transporter in these species and the contribution of MgtB to the phenotypes observed following inactivation of the PhoPQ regulon has not been studied.…”