Osmosensing by the bacterial PhoQ/PhoP two-component system Jing Yuana,b,1, Fan Jina,b, Timo Glattera, and Victor Sourjika,b,1 aMax Planck Institute for Terrestrial Microbiology, 35043 Marburg, Germany; and bLOEWE Center for Synthetic Microbiology (SYNMIKRO), 35043 Marburg, Germany Edited by Susan Gottesman, National Institutes of Health, Bethesda, MD, and approved November 6, 2017 (received for review October 5, 2017) The PhoQ/PhoP two-component system plays an essential role in domain and the cytoplasmic domains (10, 11), with the latter the response of enterobacteria to the environment of their mam- detecting the cytoplasmic pH change. The PhoQ TM domain is a malian hosts. It is known to sense several stimuli that are poten- dimer of two TM helices that form a four-helix bundle (12). tially associated with the host, including extracellular magnesium Upon activation of the sensory domain, the periplasmic ends of limitation, low pH, and the presence of cationic antimicrobial the two TM1 helices move closer to each other and push the two peptides. Here, we show that the PhoQ/PhoP two-component TM2 helices farther apart, accompanied by a TM2 rotation, systems of Escherichia coli and Salmonella can also perceive an which requires the solvation of a semichannel in the four-helix osmotic upshift, another key stimulus to which bacteria become bundle (12). Asn202, located near the center of the TM2 helix exposed within the host. In contrast to most previously estab- and proximal to this semichannel, affects the solvation of the lished stimuli of PhoQ, the detection of osmotic upshift does not cavity, and mutations of Asn202 to a nonpolar residue lock PhoQ require its periplasmic sensor domain. Instead, we show that the in the kinase-inactive state (13). Thus, a combination of scis- activity of PhoQ is affected by the length of the transmembrane soring and rotation of the helices was proposed as the mecha- (TM) helix as well as by membrane lateral pressure. We therefore nism of PhoQ activation, mediating signal transmission through propose that osmosensing relies on a conformational change the TM domain (12–14). within the TM domain of PhoQ induced by a perturbation in cell The phosphorylated response regulator PhoP activates the membrane thickness and lateral pressure under hyperosmotic con- expression of a set of genes, which varies among different bac- ditions. Furthermore, the response mediated by the PhoQ/PhoP teria (15, 16). E. coli and Salmonella share several conserved two-component system was found to improve bacterial growth recovery under hyperosmotic stress, partly through stabilization ancestral genes of the PhoP regulon, including a magnesium of the sigma factor RpoS. Our findings directly link the PhoQ/PhoP transporter (mgtA in E. coli), the phoPQ operon itself, a negative two-component system to bacterial osmosensing, suggesting that regulator of PhoQ/PhoP (mgrB), and several other genes (17, this system can mediate a concerted response to most of the 18). Notably, unlike most other genes in the PhoP regulon, the expression of mgtA in Salmonella was shown to be controlled by established host-related cues. + two additional regulatory factors: a Mg2 -binding riboswitch in ′ enterobacteria | osmolarity | virulence | signal transduction | the 5 -UTR of mgtA (19) and a proline-rich leader peptide MgtL, stress response which may up-regulate mgtA transcription under proline limita- tion, such as that resulting from osmotic stress (20). A virulence gene mgtC in the PhoP regulon of Salmonella Typhimurium he two-component systems are widely employed by bacteria ′ Tto sense and respond to environmental changes (1). A also encodes a proline-rich leader peptide in its 5 -UTR, which prototypical two-component system consists of a membrane- imbedded sensor kinase and a cytosolic response regulator. Significance When the sensor kinase is activated by an environmental stim- ulus, it autophosphorylates and then transfers the phosphate Whether residing in or invading the host, enterobacteria have group to its cognate response regulator (2). The activated re- to deal with host-related stress conditions. These stress factors sponse regulator subsequently binds to specific gene promoters also serve as sensory cues, informing bacteria that they are to regulate their expression, thus initiating cellular responses to present inside the host. Here, we report that the PhoQ/PhoP environmental changes (3). two-component system, which was known to sense several Out of about 30 two-component systems present in Escherichia host-related environmental changes, responds to osmotic up- coli or Salmonella species (1), the PhoQ/PhoP system is the shift, another key stimulus associated with the host. This sens- major sensor for various host-associated environmental cues, ing is proposed to rely on a mechanism that detects changes regulating magnesium homeostasis, cell envelope composition, in the physical properties of the membrane. Thus, a single en- stress resistance, and virulence. It is known to respond to such terobacterial kinase, PhoQ, senses a major part of host- stimuli as low divalent cations (4, 5), low pH (6), and the pres- associated stimuli. The PhoQ-mediated osmosensing increases ence of cationic antimicrobial peptides (AMPs) (7). PhoQ, the bacterial fitness under hyperosmotic conditions found inside the dimeric sensor kinase of this system, consists of five domains, host, and it is likely to play an important role in the regulation including the periplasmic sensor domain, the transmembrane of virulence. (TM) domain, and three cytosolic domains: the HAMP domain Author contributions: J.Y., F.J., T.G., and V.S. designed research; J.Y., F.J., and T.G. per- involved in signal transmission, the dHp domain required for formed research; J.Y., F.J., T.G., and V.S. analyzed data; and J.Y. and V.S. wrote the paper. dimerization, and the catalytic domain. The dimeric sensor do- The authors declare no conflict of interest. main is reported to adopt different conformations when any of This article is a PNAS Direct Submission. the three types of stimuli mentioned above is present (4–7). The This open access article is distributed under Creative Commons Attribution-NonCommercial- sensing mechanisms for low magnesium and AMPs partially NoDerivatives License 4.0 (CC BY-NC-ND). overlap, involving an acidic surface facing the membrane in the 1To whom correspondence may be addressed. Email: [email protected]. periplasmic sensor domain. Mutations in this region prevent mpg.de or [email protected]. 2+ PhoQ interactions with Mg or AMPs and abrogate the re- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. sponse (7–9). Sensing of mild acidic pH involves both the sensor 1073/pnas.1717272114/-/DCSupplemental. E10792–E10798 | PNAS | Published online November 28, 2017 www.pnas.org/cgi/doi/10.1073/pnas.1717272114 Downloaded by guest on September 29, 2021 reportedly senses the proline level in vivo and promotes the A B PNAS PLUS translation of mgtC under hyperosmotic stress (21). However, 6 mgtA 0 min 30 min mgrB in MG1655 15 min 60 min another study suggested that mgtL translation might not respond ) 5 3 2 to cytoplasmic proline level (22). Thus, the effects of osmolarity 4 phoP on the PhoP regulon remained unclear, despite the potential 3 mgtA in ∆phoP mgtA in ∆phoQ 2 importance of hyperosmotic stress for the induction of virulence 2 genes as a cue that E. coli or Salmonella encounters when en- 1 tering the eukaryotic host. 1 In this study, we demonstrate that hyperosmotic stress is sensed 0 -1 by the PhoQ/PhoP system. We propose that the underlying (log induction Fold osmosensing by PhoQ relies on a mechanism, whereby changes -2 0 0 1020304050 Normalized reporter activity in the membrane properties under hyperosmotic conditions are PmgrB-GFP PmgtLA-GFP sensed via conformational rearrangement within the TM domain Time (min) of PhoQ. Finally, we show that the PhoQ/PhoP-mediated response C DE NaCl: 0 mM 300 mM to high osmolarity accelerates the recovery of bacterial cells from 6 mgtA S ) σ 2 in MG1655 osmotic stress, which is partly due to the induction of -mediated 5 phoP PhoP-P ∆phoQ stress response. MG1655 4 mgtA PhoP EF-G in ∆mgrB Results MgtA 3 phoP 30 BorD 2 PhoQ/PhoP System Is Activated by Osmotic Upshift. When testing 20 transcription of E. coli PhoQ/PhoP-regulated genes upon stim- PhoP 1 ulation with 300 mM NaCl, we observed a drastic (32-fold) but SlyB Fold induction (log induction Fold 0 10 short-lived (15 min) up-regulation of mgtA (Fig. 1A). Other PhoP-P% -1 genes in the PhoP regulon, including mgrB, phoP, slyB, and borD, -2.5 0 2.5 01020304050 Log (fold change) 0 showed similar, but less prominent, transient transcriptional ac- 2 Time (min) tivation (SI Appendix, Fig. S1A). The maximum transcriptional up-regulation occurred around 5–10 min after such hyperosmotic Fig. 1. PhoQ senses osmotic upshift. (A) Changes in transcription of in- stimulation, followed by an adaptation phase, when the tran- dicated PhoP-regulated genes after stimulation of exponentially growing Δ Δ scription returned back to the prestimulation level. Since the E. coli wild-type, phoQ, and phoP strains with 300 mM NaCl at time point 0 (corresponding to OD600 = 0.4; see Materials and Methods for details). EnvZ/OmpR two-component system is known to sense osmotic Transcription was monitored by RT-qPCR using primers within the coding changes of the environment, we next investigated whether a regions of respective genes (SI Appendix, Table S2). The up-regulation of cross-regulation from EnvZ/OmpR might be involved in the up- transcription is significant for mgtA, mgrB,andphoP in the wild-type cells regulation of PhoP regulon during osmotic upshift. Among four (P < 0.001 according to t test), whereas it is not significant in ΔphoQ and single deletion strains (ΔphoQ, ΔphoP, ΔenvZ, and ΔompR), ΔphoP strains (P > 0.3).