Signal-sensing mechanisms of the putative osmosensor KdpD in Escherichia coli

What do we know about osmoadaptation of Yersinia pestis?

The plague agent Yersinia pestis mainly spreads among mammalian hosts and their associated fleas. Production of a successful mammal-flea-mammal life cycle implies that Y. pestis senses and responds to distinct cues in both host and vector. Among these cues, osmolarity is a fundamental parameter. The plague bacillus lives in a tightly regulated environment in the mammalian host, while osmolarity fluctuates in the flea gut (300-550 mOsM). Here, we review the mechanisms that enable Y. pestis to perceive fluctuations in osmolarity, as well as genomic plasticity and physiological adaptation of the bacterium to this stress.

The Broad-Spectrum Antimicrobial Potential of [Mn(CO)4(S2CNMe(CH2CO2H))], a Water-Soluble CO-Releasing Molecule (CORM-401): Intracellular Accumulation, Transcriptomic and Statistical Analyses, and Membrane Polarization

AIMS

Carbon monoxide (CO)-releasing molecules (CORMs) are candidates for animal and antimicrobial therapeutics. We aimed to probe the antimicrobial potential of a novel manganese CORM.

RESULTS

[Mn(CO)₄S₂CNMe(CH₂CO₂H)], CORM-401, inhibits growth of Escherichia coli and several antibiotic-resistant clinical pathogens. CORM-401 releases CO that binds oxidases in vivo, but is an ineffective respiratory inhibitor. Extensive CORM accumulation (assayed as intracellular manganese) accompanies antimicrobial activity. CORM-401 stimulates respiration, polarizes the cytoplasmic membrane in an uncoupler-like manner, and elicits loss of intracellular potassium and zinc. Transcriptomics and mathematical modeling of transcription factor activities reveal a multifaceted response characterized by elevated expression of genes encoding potassium uptake, efflux pumps, and envelope stress responses. Regulators implicated in stress responses (CpxR), respiration (Arc, Fnr), methionine biosynthesis (MetJ), and iron homeostasis (Fur) are significantly disturbed. Although CORM-401 reduces bacterial growth in combination with cefotaxime and trimethoprim, fractional inhibition studies reveal no interaction.

INNOVATION

We present the most detailed microbiological analysis yet of a CORM that is not a ruthenium carbonyl. We demonstrate CO-independent striking effects on the bacterial membrane and global transcriptomic responses.

CONCLUSIONS

CORM-401, contrary to our expectations of a CO delivery vehicle, does not inhibit respiration. It accumulates in the cytoplasm, acts like an uncoupler in disrupting cytoplasmic ion balance, and triggers multiple effects, including osmotic stress and futile respiration. Rebound Track: This work was rejected during standard peer review and rescued by rebound peer review (Antioxid Redox Signal 16: 293-296, 2012) with the following serving as open reviewers: Miguel Aon, Giancarlo Biagini, James Imlay, and Nigel Robinson. Antioxid. Redox Signal. 28, 1286-1308.

Osmotic Stress

Escherichia coli and Salmonella encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. E. coli and Salmonella limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K+ accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K+ glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as E. coli and Salmonella survive or thrive in their natural environments.

Metabolic Regulation and Coordination of the Metabolism in Bacteria in Response to a Variety of Growth Conditions

Living organisms have sophisticated but well-organized regulation system. It is important to understand the metabolic regulation mechanisms in relation to growth environment for the efficient design of cell factories for biofuels and biochemicals production. Here, an overview is given for carbon catabolite regulation, nitrogen regulation, ion, sulfur, and phosphate regulations, stringent response under nutrient starvation as well as oxidative stress regulation, redox state regulation, acid-shock, heat- and cold-shock regulations, solvent stress regulation, osmoregulation, and biofilm formation, and quorum sensing focusing on Escherichia coli metabolism and others. The coordinated regulation mechanisms are of particular interest in getting insight into the principle which governs the cell metabolism. The metabolism is controlled by both enzyme-level regulation and transcriptional regulation via transcription factors such as cAMP-Crp, Cra, Csr, Fis, P(II)(GlnB), NtrBC, CysB, PhoR/B, SoxR/S, Fur, MarR, ArcA/B, Fnr, NarX/L, RpoS, and (p)ppGpp for stringent response, where the timescales for enzyme-level and gene-level regulations are different. Moreover, multiple regulations are coordinated by the intracellular metabolites, where fructose 1,6-bisphosphate (FBP), phosphoenolpyruvate (PEP), and acetyl-CoA (AcCoA) play important roles for enzyme-level regulation as well as transcriptional control, while α-ketoacids such as α-ketoglutaric acid (αKG), pyruvate (PYR), and oxaloacetate (OAA) play important roles for the coordinated regulation between carbon source uptake rate and other nutrient uptake rate such as nitrogen or sulfur uptake rate by modulation of cAMP via Cya.

Rv1027c-Rv1028c encode functional KdpDE two--component system in Mycobacterium tuberculosis

In Mycobacteriumtuberculosis Rv1027c-Rv1028c genes are predicted to encode KdpDE two component system, which is highly conserved across all bacterial species. Here, we show that the system is functionally active and KdpD sensor kinase undergoes autophosphorylation and transfers phosphoryl group to KdpE, response regulator protein. We identified His(642) and Asp(52) as conserved phosphorylation sites in KdpD and KdpE respectively and by SPR analysis confirmed the physical interaction between them. KdpD was purified with prebound divalent ions and their importance in phosphorylation was established using protein refolding and ion chelation approaches. Genetically a single transcript encoded both KdpD and KdpE proteins. Overall, we report that M. tuberculosis KdpDE system operates like a canonical two component system.

The interacting Cra and KdpE regulators are involved in the expression of multiple virulence factors in enterohemorrhagic Escherichia coli

The human pathogen enterohemorrhagic Escherichia coli (EHEC) O157:H7 codes for two interacting DNA binding proteins, Cra and KdpE, that coregulate expression of the locus of enterocyte effacement (LEE) genes in a metabolite-dependent manner. Cra is a transcription factor that uses fluctuations in the concentration of carbon metabolism intermediates to positively regulate virulence of EHEC. KdpE is a response regulator that activates the transcription of homeostasis genes in response to salt-induced osmolarity and virulence genes in response to changes in metabolite concentrations. Here, we probed the transcriptional profiles of the Δcra, ΔkdpE, and Δcra ΔkdpE mutant strains and show that Cra and KdpE share several targets besides the LEE, but both Cra and KdpE also have independent targets. Several genes within O-islands (genomic islands present in EHEC but absent from E. coli K-12), such as Z0639, Z0640, Z3388, Z4267, and espFu (encoding an effector necessary for formation of attaching and effacing lesions on epithelial cells), were directly regulated by both Cra and KdpE, while Z2077 was only regulated by Cra. These studies identified and confirmed new direct targets for Cra and KdpE that included putative virulence factors as well as characterized virulence factors, such as EspFu and EspG. These results map out the role of the two interacting regulators, Cra and KdpE, in EHEC pathogenesis and global gene regulation.

Virulence meets metabolism: Cra and KdpE gene regulation in enterohemorrhagic Escherichia coli

Gastrointestinal (GI) bacteria sense diverse environmental signals as cues for differential gene regulation and niche adaptation. Pathogens such as enterohemorrhagic Escherichia coli (EHEC), which causes bloody diarrhea, use these signals for the temporal and energy-efficient regulation of their virulence factors. One of the main virulence strategies employed by EHEC is the formation of attaching and effacing (AE) lesions on enterocytes. Most of the genes necessary for the formation of these lesions are grouped within a pathogenicity island, the locus of enterocyte effacement (LEE), whose expression requires the LEE-encoded regulator Ler. Here we show that growth of EHEC in glycolytic environments inhibits the expression of ler and consequently all other LEE genes. Conversely, growth within a gluconeogenic environment activates expression of these genes. This sugar-dependent regulation is achieved through two transcription factors: KdpE and Cra. Both Cra and KdpE directly bind to the ler promoter, and Cra’s affinity to this promoter is catabolite dependent. Moreover, we show that the Cra and KdpE proteins interact in vitro and that KdpE’s ability to bind DNA is enhanced by the presence of Cra. Cra is important for AE lesion formation, and KdpE contributes to this Cra-dependent regulation. The deletion of cra and kdpE resulted in the ablation of AE lesions. One of the many challenges that bacteria face within the GI tract is to successfully compete for carbon sources. Linking carbon metabolism to the precise coordination of virulence expression is a key step in the adaptation of pathogens to the GI environment.

An appropriate and prompt response to environmental cues is crucial for bacterial survival. Cra and KdpE are two proteins found in both nonpathogenic and pathogenic bacteria that regulate genes in response to differences in metabolite concentration. In this work, we show that, in the deadly pathogen enterohemorrhagic Escherichia coli (EHEC) O157:H7, which causes bloody diarrhea, these two proteins influence important virulence traits. We also propose that their control of one or more of these virulence traits is due to the direct interaction of the Cra and KdpE proteins with each other, as well as with their DNA targets. This work shows how EHEC coopts established mechanisms for sensing the metabolites and stress cues in the environment, to induce virulence factors in a temporal and energy-efficient manner, culminating in disease. Understanding how pathogens commandeer nonpathogenic systems can help us develop measures to control them.

Expression and activity of Kdp under acidic conditions in Escherichia coli

Escherichia coli has three major K(+) uptake systems, Trk, Kup, and Kdp, which have been studied extensively at near neutral pH. However, the function of these transporters under acidic conditions is not well understood, although growth and survival under acidic conditions are important for bacterial pathogenesis. In this study, we examined the expression and activity of Kdp under acidic conditions and found that the transport activity of Kdp is decreased at low pH and that the expression of kdp is regulated by the internal K(+) concentration in a pH-independent manner. Consequently, the low activity of Kdp was compensated for by the induction of its elevated expression by low K(+) accumulation via Kdp at acidic pH.

Bacterial stimulus perception and signal transduction: response to osmotic stress

When exposed to osmotic stress from the environment, bacteria act to maintain cell turgor and hydration by responding both on the level of gene transcription and protein activity. Upon a sudden decrease in external osmolality, internal solutes are released by the action of membrane embedded mechanosensitive channels. In response to an osmotic upshift, the concentration of osmolytes in the cytoplasm is increased both by de novo synthesis and by active uptake. In order to coordinate these processes of osmoregulation, cells are equipped with systems and mechanisms of sensing physical stimuli correlated to changes in the external osmolality (osmosensing), with pathways to transduce these stimuli into useful signals which can be processed in the cell (signal transduction), and mechanisms of regulating proper responses in the cell to recover from the environmental stress and to maintain all necessary physiological functions (osmoregulation). These processes will be described by a number of representative examples, mainly of osmoreactive transport systems with a focus on available data of their molecular mechanism.

The complexity of the 'simple' two-component system KdpD/KdpE in Escherichia coli

The KdpD/KdpE two-component system of Escherichia coli activates the expression of the kdpFABC operon encoding the high-affinity K(+) uptake system KdpFABC in response to K(+) limitation or salt stress. Earlier, it was proposed that the histidine kinase KdpD is a turgor sensor; recent studies suggest that KdpD integrates three chemical stimuli from the cytoplasm. The histidine kinase KdpD contains several structural features and subdomains that are important for stimulus perception, modulation of the kinase to phosphatase ratio, and signaling. The response regulator KdpE receives the phosphoryl group from KdpD and induces kdpFABC transcription. The three-dimensional structure of the receiver domain was resolved, providing insights into the activation mechanism of this transcriptional regulator. Two accessory components, the universal stress protein UspC and the phosphotransferase system component IIA(Ntr), are known to interact with KdpD, allowing the modulation of kdpFABC expression under certain physiological conditions. Here, we will discuss the complexity of a 'simple' two-component system and its interconnectivity with metabolism and the general stress response.

Domain swapping reveals that the N-terminal domain of the sensor kinase KdpD in Escherichia coli is important for signaling

BACKGROUND

The KdpD/KdpE two-component system of Escherichia coli regulates expression of the kdpFABC operon encoding the high affinity K+ transport system KdpFABC. The input domain of KdpD comprises a domain that belongs to the family of universal stress proteins (Usp). It has been previously demonstrated that UspC binds to this domain, resulting in KdpD/KdpE scaffolding under salt stress. However the mechanistic significance of this domain for signaling remains unclear. Here, we employed a "domain swapping" approach to replace the KdpD-Usp domain with four homologous domains or with the six soluble Usp proteins of E. coli.

RESULTS

Full response to salt stress was only achieved with a chimera that contains UspC, probably due to unaffected scaffolding of the KdpD/KdpE signaling cascade by soluble UspC. Unexpectedly, chimeras containing either UspF or UspG not only prevented kdpFABC expression under salt stress but also under K+ limiting conditions, although these hybrid proteins exhibited kinase and phosphotransferase activities in vitro. These are the first KdpD derivatives that do not respond to K+ limitation due to alterations in the N-terminal domain. Analysis of the KdpD-Usp tertiary structure revealed that this domain has a net positively charged surface, while UspF and UspG are characterized by net negative surface charges.

CONCLUSION

The Usp domain within KdpD not only functions as a binding surface for the scaffold UspC, but it is also important for KdpD signaling. We propose that KdpD sensing/signaling involves alterations of electrostatic interactions between the large N- and C-terminal cytoplasmic domains.

An intracellularly inducible gene involved in virulence and polyphosphate production in Francisella

Francisella tularensis is an intracellular pathogen capable of multiplying to high levels in macrophages. By protein analysis, only a few proteins have been shown previously to be expressed at high levels in macrophages relative to bacteria grown in culture media. To identify additional genes that show increased expression during intracellular growth, we developed a plasmid for use in Francisella based on the induction of expression of green fluorescent protein. Clones of F. tularensis subsp. novicida were identified that were fluorescent only intracellularly and not when grown in vitro. Sequencing identified a range of genes comprising some such as dnaK that are already known to be expressed intracellularly and some novel targets. One of these newly identified regulated genes, FTN1472/FTT1564, was selected for further study. Isogenic mutants were generated in F. tularensis subsp. novicida and subsp. tularensis by allelic replacement. Inactivation of the gene resulted in abolition of polyphosphate production by F. novicida, strongly supporting the bioinformatic analysis, which had suggested that the gene may encode a polyphosphate kinase. The mutants exhibited defects for intracellular growth in macrophages and were attenuated in mice, indicating a key role for the putative polyphosphate kinase in the virulence of Francisella.

The molecular basis of salt adaptation in Methanosarcina mazei Gö1

The study on the molecular basis of salt adaptation and its regulation in archaea is still in its infancy, but genomics and functional genome analyses combined with classical biochemistry shed light on the processes conferring salt adaptation in the methanogenic archaeon Methanosarcina mazei Gö1. In this article, we will review discoveries made within the last years that will culminate in the description of the overall cellular response of M. mazei Gö1 to elevated salinities. This response includes accumulation of solutes and export of Na+ as well as potential uptake/export of K+ but also a restructuring of the cell surface.

Reduction of turgor is not the stimulus for the sensor kinase KdpD of Escherichia coli

Stimulus perception by the KdpD/KdpE two-component system of Escherichia coli is still controversial with respect to the nature of the stimulus that is perceived by the sensor kinase KdpD. Limiting potassium concentrations in the medium or high osmolality leads to KdpD/KdpE signal transduction, resulting in kdpFABC expression. It has been hypothesized that changes in turgor are sensed by KdpD through alterations in the physical state of the cytoplasmic membrane. However, in this study the quantitative determination of expression levels of the kdpFABC operon revealed that the system responds very effectively to K(+)-limiting conditions in the medium but barely and to various degrees to salt and sugar stress. Since the current view of stimulus perception calls for mainly intracellular parameters, which might be sensed by KdpD, we set out to test the cytoplasmic concentrations of ATP, K(+), Na(+), glutamate, proline, glycine, trehalose, putrescine, and spermidine under K(+)-limiting conditions. As a first result, the determination of the cytoplasmic volume, which is a prerequisite for such measurements, revealed that a transient shrinkage of the cytoplasmic volume, which is indicative of a reduction in turgor, occurred only under osmotic upshift but not under K(+)-limiting conditions. Furthermore, the intracellular ATP concentration significantly increased under osmotic upshift, whereas only a slight increase occurred after a potassium downshift. Finally, the cytoplasmic K(+) concentration rose severalfold only after an osmotic upshock. For the first time, these data indicate that stimulus perception by KdpD correlates neither with changes in the cytoplasmic volume nor with changes in the intracellular ATP or K(+) concentration or those of the other solutes tested. In conclusion, we propose that a reduction in turgor cannot be the stimulus for KdpD.

The Kdp-ATPase system and its regulation

K+, the dominant intracellular cation, is required for various physiological processes like turgor homeostasis, pH regulation etc. Bacterial cells have evolved many diverse K+ transporters to maintain the desired concentration of internal K+. In E.coli, the KdpATPase (comprising of the KdpFABC complex), encoded by the kdpFABC operon, is an inducible high-affinity K+ transporter that is synthesised under conditions of severe K+ limitation or osmotic upshift. The E.coli kdp expression is transcriptionally regulated by the KdpD and KdpE proteins, which together constitute a typical bacterial two-component signal transduction system. The Kdp system is widely dispersed among the different classes of bacteria including the cyanobacteria. The ordering of the kdpA, kdpB and kdpC is relatively fixed but the kdpD/E genes show different arrangements in distantly related bacteria. Our studies have shown that the cyanobacterium Anabaena sp. strain L-31 possesses two kdp operons, kdp1 and kdp2, of which, the later is expressed under K+ deficiency and desiccation. Among the regulatory genes,the kdpD ORF of Anabaena L-31 is truncated when compared to the kdpD of other bacteria, while a kdpE -like gene is absent. The extremely radio-resistant bacterium, Deinococcus radiodurans strain R1, also shows the presence of a naturally short kdpD ORF similar to Anabaena in its kdp operon. The review elaborates the expression of bacterial kdp operons in response to various environmental stress conditions, with special emphasis on Anabaena. The possible mechanism(s)of regulation of the unique kdp operons from Anabaena and Deinococcus are also discussed.

The extension of the fourth transmembrane helix of the sensor kinase KdpD of Escherichia coli is involved in sensing

The KdpD sensor kinase and the KdpE response regulator control expression of the kdpFABC operon coding for the KdpFABC high-affinity K+ transport system of Escherichia coli. In search of a distinct part of the input domain of KdpD which is solely responsible for K+ sensing, sequences of kdpD encoding the transmembrane region and adjacent N-terminal and C-terminal extensions were subjected to random mutagenesis. Nine KdpD derivatives were identified that had lost tight regulation of kdpFABC expression. They all carried single amino acid replacements located in a region encompassing the fourth transmembrane helix and the adjacent arginine cluster of KdpD. All mutants exhibited high levels of kdpFABC expression regardless of the external K+ concentration. However, 3- to 14-fold induction was observed under extreme K+-limiting conditions and in response to an osmotic upshift when sucrose was used as an osmolyte. These KdpD derivatives were characterized by a reduced phosphatase activity in comparison to the autokinase activity in vitro, which explains constitutive expression. Whereas for wild-type KdpD the autokinase activity and also, in turn, the phosphotransfer activity to KdpE were inhibited by increasing concentrations of K+, both activities were unaffected in the KdpD derivatives. These data clearly show that the extension of the fourth transmembrane helix encompassing the arginine cluster is mainly involved in sensing both K+ limitation and osmotic upshift, which may not be separated mechanistically.

Nutrient sensing and signaling: NPKS

Plants often grow in soils that contain very low concentrations of the macronutrients nitrogen, phosphorus, potassium, and sulfur. To adapt and grow in nutrient-deprived environments plants must sense changes in external and internal mineral nutrient concentrations and adjust growth to match resource availability. The sensing and signal transduction networks that control plant responses to nutrient deprivation are not well characterized for nitrogen, potassium, and sulfur deprivation. One branch of the signal transduction cascade related to phosphorus-deprivation response has been defined through the identification of a transcription factor that is regulated by sumoylation. Two different microRNAs play roles in regulating gene expression under phosphorus and sulfur deprivation. Reactive oxygen species increase rapidly after mineral nutrient deprivation and may be one upstream mediator of nutrient signaling. A number of molecular analyses suggest that both short-term and longer-term responses will be important in understanding the progression of signaling events when the external, then internal, supplies of nutrients become depleted.

New type of kdp region with a split sensor-kinase kdpD gene located within two divergent kdp operons from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius

The kdp region from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius consists of two divergent operons: kdpZFABCN, which is tenfold induced at low K+ concentrations and encodes the K+-translocating P-type ATPase KdpZFABC as well as KdpN, a novel covalent homo-dimer of the cytoplasmic N-terminal part from sensor kinase KdpD; and secondly, the constitutively expressed kdpHE operon, encoding the remainder of KdpD and the response regulator KdpE.

The cytoplasmic C-terminal domain of the Escherichia coli KdpD protein functions as a K+ sensor

The KdpD protein is a K(+) sensor kinase located in the cytoplasmic membrane of Escherichia coli. It contains four transmembrane stretches and two short periplasmic loops of 4 and 10 amino acid residues, respectively. To determine which part of KdpD functions as a K(+) sensor, genetic variants were constructed with truncations or altered arrangements of the transmembrane segments. All KdpD constructs were tested by complementation of an E. coli kdpD deletion strain for their ability to grow at a K(+) concentration of 0.1 mM in the medium. A soluble protein composed of the C-terminal cytoplasmic domain was able to complement the kdpD deletion strain. In addition, analysis of the beta-galactosidase activity of an E. coli strain which carries a transcriptional fusion of the upstream region of the kdpFABC operon and a promoterless lacZ gene revealed that this soluble KdpD mutant responds to changes in the K(+) concentration in the extracellular medium. The results suggest that the sensing and response functions are both located in the C-terminal domain and might be modulated by the N-terminal domain as well as by membrane anchoring.

Timing of induction of osmotically controlled genes in Salmonella enterica Serovar Typhimurium, determined with quantitative real-time reverse transcription-PCR

The signals that control the transcription of osmoregulated genes are not understood satisfactorily. The "turgor control model" suggested that the primary osmoregulatory signal in Enterobacteriaceae is turgor loss, which induces the kdp K+ transport operon and activates the Trk K+ permease. The ensuing increase in cytoplasmic K+ concentration was proposed to be the signal that turns on all secondary responses, including the induction of the proU (proline-glycine betaine transport) operon. The "ionic strength model" proposed that the regulatory signal for all osmotically controlled responses is the increase in the cytoplasmic ionic strength or macromolecular crowding after an osmotic upshift. The assumption in the turgor control model that the induction of kdp is a primary response to osmotic shock predicts that this response should precede all secondary responses. Both models predict that the induction of all osmotically activated responses should be independent of the chemical nature of the solute used to impose osmotic stress. We tested these predictions by quantitative real-time reverse transcription-PCR analysis of the expression of six osmotically regulated genes in Salmonella enterica serovar Typhimurium. After shock with 0.3 M NaCl, proU was induced at 4 min, proP and rpoS were induced at 4 to 6 min, kdp was induced at 8 to 9 min, and otsB and ompC were induced at 10 to 12 min. After an equivalent osmotic shock with 0.6 M sucrose, proU was induced with kinetics similar to those seen with NaCl, but induction of kdp was reduced 150-fold in comparison to induction by NaCl. Our results are inconsistent with both the turgor control and the ionic strength control models.

Differential expression of the two kdp operons in the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31

In several types of bacteria, the Kdp ATPase (comprising of the KdpABC complex) is an inducible, high-affinity potassium transporter that scavenges K+ from the environment. The cyanobacterium Anabaena sp. strain L-31 showed the presence of not one but two distinct kdp operons in its genome. The kdp1 consisted of kdpA1B1G1C1D genes, whereas the kdp2 contained the kdpA2B2G2C2 genes. Among the regulatory genes, the kdpD open reading frame of Anabaena sp. strain L-31 was truncated compared to the kdpD of other bacteria, whereas a kdpE-like gene was absent in the vicinity of the two kdp operons. In response to K+ limitation (<0.05 mM external K+), only kdp2 (and not kdp1) expression could be detected as a 5.3-kb transcript on Northern blots, indicating that kdpA2B2G2C2 genes constitute a polycystronic operon. Unlike E. coli, addition of osmolytes like NaCl, or a change in pH of the medium did not enhance the kdp expression in Anabaena sp. strain L-31. Interestingly, the Anabaena sp. strain L-31 kdp2 operon was strongly induced in response to desiccation stress. The addition of K+ to K+ -starved cultures resulted in repression and degradation of kdp2 transcripts. Our results clearly show that kdp2 is the major kdp operon expressed in Anabaena sp. strain L-31 and may play an important role in adaptation to K+ limitation and desiccation stress.

An atypical KdpD homologue from the cyanobacterium Anabaena sp. strain L-31: cloning, in vivo expression, and interaction with Escherichia coli KdpD-CTD

The kdpFABC operon of Escherichia coli, coding for the high-affinity K(+) transport system KdpFABC, is transcriptionally regulated by the products of the adjacently located kdpDE genes. The KdpD protein is a membrane-bound sensor kinase consisting of a large N-terminal domain and a C-terminal transmitter domain interconnected by four transmembrane segments (the transmembrane segments together with the C-terminal transmitter domain of KdpD are referred to as CTD), while KdpE is a cytosolic response regulator. We have cloned and sequenced the kdp operon from a nitrogen-fixing, filamentous cyanobacterium, Anabaena sp. strain L-31 (GenBank accession. number AF213466). The kdpABC genes are similar in size to those of E. coli, but the kdpD gene is short (coding only for 365 amino acids), showing homology only to the N-terminal domain of E. coli KdpD. A kdpE-like gene is absent in the vicinity of this operon. Anabaena KdpD with six C-terminal histidines was overproduced in E. coli and purified by Ni(2+)-nitrilotriacetic acid affinity chromatography. With antisera raised against the purified Anabaena KdpD, the protein was detected in Anabaena sp. strain L-31 membranes. The membrane-associated or soluble form of the Anabaena KdpD(6His) could be photoaffinity labeled with the ATP analog 8-azido-ATP, indicating the presence of an ATP binding site. The coproduction of Anabaena KdpD with E. coli KdpD-CTD decreased E. coli kdpFABC expression in response to K(+) limitation in vivo relative to the wild-type KdpD-CTD protein. In vitro experiments revealed that the kinase activity of the E. coli KdpD-CTD was unaffected, but its phosphatase activity increased in the presence of Anabaena KdpD(6His). To our knowledge this is the first report where a heterologous N-terminal domain (Anabaena KdpD) is shown to affect in trans KdpD-CTD (E. coli) activity, which is just opposite to that observed for the KdpD-N-terminal domain of E. coli.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

Membrane fluidity and the perception of environmental signals in cyanobacteria and plants

Photosynthetic organisms, namely, plants and cyanobacteria, are directly exposed to changes in their environment and their survival depends on their ability to acclimate to such changes. Several lines of evidence suggest that temperature stress, such as unusually low or high temperatures, and osmotic stress might be perceived by plants and cyanobacteria via changes in the fluidity of their cell membranes. The availability of techniques for gene-targeted mutagenesis and gene transfer, as well as for the analysis of genomes and transcripts, has allowed us to examine and evaluate this hypothesis and its implications. In this review, we summarize recent studies of the regulation of gene expression by changes in the extent of unsaturation of fatty acids and membrane fluidity, and we present a discussion of the induction of gene expression by environmental stress and of sensors of environmental conditions and relationships between their activity and the fluidity of membranes in cyanobacteria and plants.

The N-terminal input domain of the sensor kinase KdpD of Escherichia coli stabilizes the interaction between the cognate response regulator KdpE and the corresponding DNA-binding site

The sensor kinase/response regulator system KdpD/KdpE of Escherichia coli regulates expression of the kdpFABC operon, which encodes the high affinity K+ transport system KdpFABC. The membrane-bound sensor kinase KdpD consists of an N-terminal input domain (comprising a large cytoplasmic domain and four transmembrane domains) and a cytoplasmic C-terminal transmitter domain. Here we show that the cytoplasmic N-terminal domain of KdpD (KdpD/1-395) alone supports semi-constitutive kdpFABC expression, which becomes dependent on the extracellular K+ concentration under K+-limiting growth conditions. However, it should be noted that the non-phosphorylatable derivative KdpD/H673Q or the absence of KdpD abolishes kdpFABC expression completely. KdpD/1-395 mediated kdpFABC expression requires the corresponding response regulator KdpE with an intact phosphorylation site. Experiments with an Escherichia coli mutant unable to synthesize acetyl phosphate as well as transposon mutagenesis suggest that KdpE is phosphorylated in vivo by low molecular weight phosphodonors in the absence of the full-length sensor kinase. Various biochemical approaches provide first evidence that kdpFABC expression mediated by KdpD/1-395 is due to a stabilizing effect of this domain on the binding of KdpE approximately P to its corresponding DNA-binding site. Such a stabilizing effect of a sensor kinase domain on the DNA-protein interaction of the cognate response regulator has never been observed before for any other sensor kinase. It describes a new mechanism in bacterial two-component signal transduction.

Osmoregulation in Lactococcus lactis: BusR, a transcriptional repressor of the glycine betaine uptake system BusA

The busA (opuA) locus of Lactococcus lactis encodes a glycine betaine uptake system. Transcription of busA is osmotically inducible and its induction after an osmotic stress is reduced in the presence of glycine betaine. Using a genetic screen in CLG802, an Escherichia coli strain carrying a lacZ transcriptional fusion expressed under the control of the busA promoter, we isolated a genomic fragment from the L. lactis subsp. cremoris strain MG1363, which represses transcription from busAp. The cloned locus responsible for this repression was identified as a gene present upstream from the busA operon, encoding a putative DNA binding protein. This gene was named busR. Electrophoretic mobility shift and footprinting experiments showed that BusR is able to bind a site that overlaps the busA promoter. Overexpression of busR in L. lactis reduced expression of busA. Its disruption led to increased and essentially constitutive transcription of busA at low osmolarity. Therefore, BusR is a major actor of the osmotic regulation of busA in L. lactis.

The transmembrane domains of the sensor kinase KdpD of Escherichia coli are not essential for sensing K+ limitation

The sensor kinase/response regulator system KdpD/KdpE of Escherichia coli regulates the expression of the kdpFABC operon, which encodes the high affinity K+ transport system KdpFABC. The membrane-bound sensor kinase KdpD consists of four transmembrane domains, a large cytoplasmic N-terminal domain and a cytoplasmic C-terminal transmitter domain. To elucidate the role of the four transmembrane domains, various deletions were introduced in kdpD and the activities of the resulting truncated derivatives of KdpD were determined. A KdpD protein lacking all four transmembrane domains was able to sense low K+ concentrations, whereas at higher K+ concentrations kdpFABC expression was constitutive. These and further results with various truncated KdpD proteins lacking distinct parts of the transmembrane domains or derivatives in which a linker peptide or two transmembrane domains of PutP, the Na+/proline transporter of Escherichia coli, replaced the missing part indicated that the transmembrane domains are not essential for sensing of K+ limitation, but may be important for the correct positioning of the large N- and C-terminal cytoplasmic domains to each other.

The roles and regulation of potassium in bacteria

Potassium is the major intracellular cation in bacteria as well as in eucaryotic cells. Bacteria accumulate K+ by a number of different transport systems that vary in kinetics, energy coupling, and regulation. The Trk and Kdp systems of enteric organisms have been well studied and are found in many distantly related species. The Ktr system, resembling Trk in many ways, is also found in many bacteria. In most species two or more independent saturable K(+)-transport systems are present. The KefB and KefC type of system that is activated by treatment of cells with toxic electrophiles is the only specific K(+)-efflux system that has been well characterized. Pressure-activated channels of at least three types are found in bacteria; these represent nonspecific paths of efflux when turgor pressure is dangerously high. A close homolog of eucaryotic K+ channels is found in many bacteria, but its role remains obscure. K+ transporters are regulated both by ion concentrations and turgor. A very general property is activation of K+ uptake by an increase in medium osmolarity. This response is modulated by both internal and external concentrations of K+. Kdp is the only K(+)-transport system whose expression is regulated by environmental conditions. Decrease in turgor pressure and/or reduction in external K+ rapidly increase expression of Kdp. The signal created by these changes, inferred to be reduced turgor, is transmitted by the KdpD sensor kinase to the KdpE-response regulator that in turn stimulates transcription of the kdp genes. K+ acts as a cytoplasmic-signaling molecule, activating and/or inducing enzymes and transport systems that allow the cell to adapt to elevated osmolarity. The signal could be ionic strength or specifically K+. This signaling response is probably mediated by a direct sensing of internal ionic strength by each particular system and not by a component or system that coordinates this response by different systems to elevated K+.

Chloride, a new environmental signal molecule involved in gene regulation in a moderately halophilic bacterium, Halobacillus halophilus

The gram-positive, aerobic, moderately halophilic bacterium Halobacillus halophilus is challenged in its environment by frequently changing salt (NaCl) concentrations. Recently, H. halophilus was shown to be the first prokaryote that is dependent on Cl(-) for growth. In a search for the biological function of Cl(-) in this prokaryote, we identified different Cl(-)-dependent processes, which suggests a more general role for Cl(-) in the metabolism of H. halophilus. To analyze the effect of Cl(-) in more detail, we concentrated on one model system, the Cl(-)-dependent production of flagella, and aimed to identify the molecular basis for the Cl(-) dependence of flagellum production. Here, we report that synthesis of the major subunit of the flagellum, FliC, is dependent on the Cl(-) concentration of the medium, as determined by Western blot analyses. The gene encoding FliC was cloned and sequenced, and Northern blot as well as reverse transcriptase PCR analyses revealed that expression of fliC is Cl(-) dependent. FliC is the first protein of known function demonstrated to be synthesized in a Cl(-)-dependent manner in a prokaryote. Two-dimensional gel electrophoresis of cells grown under different conditions revealed five more Cl(-)-induced proteins; these were identified by N-terminal sequencing and database searches to be orthologs of proteins involved in stress response in Bacillus subtilis. The data indicate that Cl(-) is an important environmental signal in this moderate halophile and regulates protein synthesis and gene expression. Furthermore, the data may suggest that Cl(-) plays a role in the signal transduction involved in salt perception by this bacterium.

Ionic regulation of MscK, a mechanosensitive channel from Escherichia coli

Three gene products that form independent mechanosensitive channel activities have been identified in Escherichia coli. Two of these, MscL and MscS, play a vital role in allowing the cell to survive acute hypotonic stress. Much less is known of the third protein, MscK (KefA). Here, we characterize the MscK channel activity and compare it with the activity of its structural and functional homologue, MscS. While both show a slight anionic preference, MscK appears to be more sensitive to membrane tension. In addition, MscK, but not MscS activity appears to be regulated by external ionic environment, requiring not only membrane tension but also high concentrations of external K(+), NH(4)(+), Rb(+) or Cs(+) to gate; no activity is observed with Na(+), Li(+) or N-methyl-D-glucamine (NMDG). An MscK gain-of-function mutant gates spontaneously in the presence of K(+) or similar ions, and will gate in the presence of Na(+), Li(+) and NMDG, but only when stimulated by membrane tension. Increased sensitivity and the highly regulated nature of MscK suggest a more specialized physiological role than other bacterial mechanosensitive channels.

A signal transduction system in Streptomyces coelicolor that activates the expression of a putative cell wall glycan operon in response to vancomycin and other cell wall-specific antibiotics

We have investigated a signal transduction system proposed to allow Streptomyces coelicolor to sense and respond to changes in the integrity of its cell envelope. The system consists of four proteins, encoded in an operon: sigmaE, an RNA polymerase factor; CseA (formerly ORF202), a protein of unknown function; CseB, a response regulator; and CseC, a sensor histidine protein kinase with two predicted transmembrane helices (Cse stands for control of sigma E). To develop a sensitive bioassay for inducers of the sigE system, the promoter of the sigE operon (sigEp) was fused to a reporter gene conferring resistance to kanamycin. Antibiotics that acted as inducers of the sigE signal transduction system were all inhibitors of intermediate and late steps in peptidoglycan biosynthesis, including ramoplanin, moenomycin A, bacitracin, several glycopeptides and some beta-lactams. The cell wall hydrolytic enzyme lysozyme also acted as an inducer. These data suggest that the CseB-CseC signal transduction system may be activated by the accumulation of an intermediate in peptidoglycan biosynthesis or degradationa. A computer-based searching method was used to identify a sigmaE target operon of 12 genes (the cwg operon), predicted to specify the biosynthesis of a cell wall glycan. In low-Mg(2+) medium, transcription of the cwg operon was induced by vancomycin in a sigE-dependent manner but, in high-Mg(2+) medium, there was substantial cwg transcription in a sigE null mutant, and this sigE-independent activity was also induced by vancomycin. Based on these data, we propose a model for the regulation and function of the sigmaE signal transduction system.

The RcsCB His-Asp phosphorelay system is essential to overcome chlorpromazine-induced stress in Escherichia coli

The RcsCB His-Asp phosphorelay system regulates the expression of several genes of Escherichia coli, but the molecular nature of the inducing signal is still unknown. We show here that treatment of an exponentially growing culture of E. coli with the cationic amphipathic compound chlorpromazine (CPZ) stimulates expression of a set of genes positively regulated by the RcsCB system. This induction is abolished in rcsB or rcsC mutant strains. In addition, treatment with CPZ inhibits growth. The wild-type strain is able to recover from this inhibition and resume growth after a period of adaptation. In contrast, strains deficient in the RcsCB His-Asp phosphorelay system are hypersensitive to CPZ. These results suggest that cells must express specific RcsCB-regulated genes in order to cope with the CPZ-induced stress. This is the first report of the essential role of the RcsCB system in a stress situation. These results also strengthen the notion that alterations of the cell envelope induce a signal recognized by the RcsC sensor.

How do membrane proteins sense water stress?

Maintenance of cell turgor is a prerequisite for almost any form of life as it provides a mechanical force for the expansion of the cell envelope. As changes in extracellular osmolality will have similar physicochemical effects on cells from all biological kingdoms, the responses to osmotic stress may be alike in all organisms. The primary response of bacteria to osmotic upshifts involves the activation of transporters, to effect the rapid accumulation of osmoprotectants, and sensor kinases, to increase the transport and/or biosynthetic capacity for these solutes. Upon osmotic downshift, the excess of cytoplasmic solutes is released via mechanosensitive channel proteins. A number of breakthroughs in the last one or two years have led to tremendous advances in our understanding of the molecular mechanisms of osmosensing in bacteria. The possible mechanisms of osmosensing, and the actual evidence for a particular mechanism, are presented for well studied, osmoregulated transport systems, sensor kinases and mechanosensitive channel proteins. The emerging picture is that intracellular ionic solutes (or ionic strength) serve as a signal for the activation of the upshift-activated transporters and sensor kinases. For at least one system, there is strong evidence that the signal is transduced to the protein complex via alterations in the protein-lipid interactions rather than direct sensing of ion concentration or ionic strength by the proteins. The osmotic downshift-activated mechanosensitive channels, on the other hand, sense tension in the membrane but other factors such as hydration state of the protein may affect the equilibrium between open and closed states of the proteins.

Bacterial osmoadaptation: the role of osmolytes in bacterial stress and virulence

Two general strategies exist for the growth and survival of prokaryotes in environments of elevated osmolarity. The 'salt in cytoplasm' approach, which requires extensive structural modifications, is restricted mainly to members of the Halobacteriaceae. All other species have convergently evolved to cope with environments of elevated osmolarity by the accumulation of a restricted range of low molecular mass molecules, termed compatible solutes owing to their compatibility with cellular processes at high internal concentrations. Herein we review the molecular mechanisms governing the accumulation of these compounds, both in Gram-positive and Gram-negative bacteria, focusing specifically on the regulation of their transport/synthesis systems and the ability of these systems to sense and respond to changes in the osmolarity of the extracellular environment. Finally, we examine the current knowledge on the role of these osmostress responsive systems in contributing to the virulence potential of a number of pathogenic bacteria.

Osmoadaptation in bacteria and archaea: common principles and differences

The availability of water is the most important prerequisite for life of any living cell, and exposure of cells to hypersaline conditions always threatens the cells with a drastic loss of water. To re-establish the essential turgor pressure, cells increase the water activity of their cytoplasm by accumulation of compatible solutes, either by synthesis or by uptake. The ability to respond to increasing osmolality is well conserved in all three lines of descent and, here, we compare the osmoadaptive strategies of Bacteria and Archaea. The temporal sequence of events after an osmotic upshock will be discussed, with a focus on the most rapid response, notably the mechanisms of transport activation at the protein level, and different signals for osmolality will be compared. The spectrum of compatible solutes used by different organisms is rather diverse and a comparison of 'bacterial' and 'archaeal' compatible solutes will be given.

Regulation of potassium-dependent Kdp-ATPase expression in the nitrogen-fixing cyanobacterium Anabaena torulosa

The KdpB polypeptides in the cyanobacterium Anabaena torulosa were shown to be two membrane-bound proteins of about 78 kDa, expressed strictly under K(+) deficiency and repressed or degraded upon readdition of K(+). In both Anabaena and Escherichia coli strain MC4100, osmotic and ionic stresses caused no significant induction of steady-state KdpB levels during extreme potassium starvation.

Glutamine and cell signaling in liver

In the liver, glutamine plays an important role in ammonia detoxication and the regulation of pH homeostasis ("intercellular glutamine cycle"). In addition, this amino acid regulates liver metabolism and transport by mechanisms that cannot be attributed to its metabolism. Examples include the stimulation of protein and glycogen synthesis and bile acid secretion and the inhibition of proteolysis in liver. The major trigger for such effects is an increased hepatocyte hydration due to the cumulative uptake of glutamine into the cells, which activates osmosignaling pathways involving mitogen-activated protein kinases (MAPK). Glutamine- and hypoosmolarity-induced cell swelling activates extracellular signal-regulated kinases (ERK) and p38(MAPK). Activation of these MAPK results in an increased capacity of bile acid excretion into bile due to a rapid translocation of canalicular transport ATPases from a subcanalicular storage compartment to the canalicular membrane. Similarly, glutamine augments biliary excretion of cysteinyl leukotrienes in endotoxin-treated rat livers. Also, the antiproteolytic effect of glutamine is largely due to glutamine-induced cell swelling, which activates osmosignaling pathways. Here, the glutamine-induced p38(MAPK) activation mediates the inhibition of autophagic proteolysis at the level of autophagosome formation.

Cs(+) induces the kdp operon of Escherichia coli by lowering the intracellular K(+) concentration

Cs(+) was found to induce expression of the kdpFABC operon, encoding a high-affinity K(+) uptake system of Escherichia coli. Quantitative expression analyses at the transcriptional and translational levels reveal that CsCl causes much higher induction of kdpFABC than does NaCl. A decrease of the intracellular K(+) concentration is found in cells exposed to CsCl. The results indicate that kdpFABC expression is induced when the intracellular K(+) concentration is lowered. Moreover, the results imply that the signal transduction cascade mediated by KdpD and KdpE is able to integrate multiple signals.

trans-acting mutations in loci other than kdpDE that affect kdp operon regulation in Escherichia coli: effects of cytoplasmic thiol oxidation status and nucleoid protein H-NS on kdp expression

Transcription of the K(+) transport operon kdp in Escherichia coli is induced during K(+)-limited growth by the action of a dual-component phosphorelay regulatory system comprised of a sensor kinase (integral membrane protein), KdpD, and a DNA-binding response regulator (cytoplasmic protein), KdpE. In this study, we screened for new dke (named dke for decreased kdp expression) mutations (in loci other than kdpDE) that led to substantially decreased kdp expression. One dke mutation was shown to be in hns, encoding the nucleoid protein H-NS. Another dke mutation was mapped to trxB (encoding thioredoxin reductase), and an equivalent reduction in kdp expression was demonstrated also for trxA mutants that are deficient in thioredoxin 1. Exogenously provided dithiothreitol rescued the kdp expression defect in trxB but not trxA mutants. Neither trxB nor trxA affected gene regulation mediated by another dual-component system tested, EnvZ-OmpR. Mutations in genes dsbC and dsbD did not affect kdp expression, suggesting that the trx effects on kdp are not mediated by alterations in protein disulfide bond status in the periplasm. Reduced kdp expression was observed even in a trxB strain that harbored a variant KdpD polypeptide bearing no Cys residues. A trxB hns double mutant was even more severely affected for kdp expression than either single mutant. The dke mutations themselves had no effect on strength of the signal controlling kdp expression, and constitutive mutations in kdpDE were epistatic to hns and trxB. These results indicate that perturbations in cytoplasmic thiol oxidation status and in levels of the H-NS protein exert additive effects, direct or indirect, at a step(s) upstream of KdpD in the signal transduction pathway, which significantly influence the magnitude of KdpD kinase activity obtained for a given strength of the inducing signal for kdp transcription.

K+ and ionic strength directly influence the autophosphorylation activity of the putative turgor sensor KdpD of Escherichia coli

The membrane-bound histidine kinase KdpD is a putative turgor sensor that regulates, together with the response regulator KdpE, the expression of the kdpFABC operon coding for the high affinity K(+)-uptake system KdpFABC of Escherichia coli. To elucidate the nature of the primary stimulus for KdpD, we developed an in vitro assay based on right-side-out membrane vesicles. Conditions were varied inside and outside of the vesicles, and KdpD autophosphorylation activity was tested. It was shown that an increase of the ionic strength inside the vesicles was accompanied by an increase of the autophosphorylation activity of KdpD with ATP. However, K(+) at concentrations higher than 1 mm inhibited KdpD autophosphorylation activity. This K(+)-specific effect was not observed with KdpD-Arg-511 --> Gln, a KdpD derivative, which causes K(+)-independent kdpFABC expression. When the osmolality outside the vesicles was increased, autophosphorylation activity of KdpD was stimulated, whereby salts were more effective than sugars. Treatment of the vesicles with amphipathic compounds did not affect KdpD autophosphorylation activity. Based on these results it is proposed that changes of intracellular parameters elicited by K(+) limitation or osmotic upshock directly influence KdpD autophosphorylation activity, whereby K(+) has an inhibitory and ionic strength a stimulatory effect.

Regulation of enzymatic activity and gene expression by membrane fluidity

Changes in the cellular environment can lead to alterations in the fluidity of the membranes of prokaryotes and eukaryotes. Changes in temperature and osmotic conditions are two of the best-studied stresses that can affect membrane fluidity. Los and Murata discuss the types of sensors that detect these changes in membrane fluidity and the types of signals that are generated.

Modulation of KdpD phosphatase implicated in the physiological expression of the kdp ATPase of Escherichia coli

The KdpD sensor kinase and the KdpE response regulator control the expression of the kdpFABC operon, encoding the KdpFABC high-affinity K+ transport system of Escherichia coli. Low turgor pressure has been postulated to be the environmental stimulus to express KdpFABC. KdpD has autokinase, phosphotransferase and, like many sensor kinases, response regulator (phospho-KdpE) specific phosphatase activity. To determine which of these activities are altered in response to the environmental stimulus, we isolated and analysed six kdpD mutants that cause constitutive expression of KdpFABC. In three of the mutants, phosphatase activity was undetectable and, in two, phosphatase was reduced. Kinase activity was unaffected in four of the mutants, but elevated in one. In one mutant, a pseudorevertant of a kdpD null mutation, kinase and phosphatase were both reduced to 20% of the wild-type level. These findings suggest that initiation of signal transduction by KdpD is mediated by the inhibition of the phospho-KdpE-specific phosphatase activity of KdpD, leading to an accumulation of phospho-KdpE, which in turn activates the expression of the KdpFABC system. The data also suggest that levels of activity in vitro may differ from what occurs in vivo, because in vitro conditions cannot replicate those in vivo.

Tuning of the porin expression under anaerobic growth conditions by his-to-Asp cross-phosphorelay through both the EnvZ-osmosensor and ArcB-anaerosensor in Escherichia coli

BACKGROUND

Widespread bacterial signal transduction circuits are generally referred to as 'two-component systems' or 'histidine (His)-to-aspartate (Asp) phosphorelays.' In Escherichia coli, as many as 30 distinct His-to-Asp phosphorelay signalling pathways operate in response to a wide variety of environmental stimuli, such as medium osmolarity and anaerobiosis. In this regard, it is of interest whether or not some of them together constitute a network of signalling pathways through a physiologically relevant mechanism (often referred to as 'cross-regulation'). We have addressed this issue, with special reference to the osmo-responsive EnvZ and anaero-responsive ArcB phosphorelay signalling pathways in E. coli.

RESULTS

Under standard aerobic growth conditions, it is well known that the osmoregulatory profile of the outer membrane porins (OmpC and OmpF) is mainly regulated by the EnvZ-OmpR phosphorelay system in response to medium osmolarity. In this study, it was found that, under anaerobic growth conditions, E. coli cells exhibit a markedly altered expression profile of OmpC and OmpF This profile was significantly different from that observed for the cells grown aerobically. Results from extensive genetic studies showed that, under such anaerobic growth conditions, the arcB gene encoding the anaero-sensory His-kinase appears to be an auxiliary genetic determinant that regulates the expression profile of porins. We then provided several lines of in vivo and in vitro evidence, which taken together, supported the following conclusions.

CONCLUSIONS

Under anaerobic growth conditions, porin expression is tuned not only by the authentic osmo-resposive EnvZ sensor, but also by the anaero-responsive ArcB sensor, in an OmpR-dependent manner. It is suggested that such ArcB-mediated cross-regulation plays a physiological role by integrating anaerobic respiratory signals into the porin regulation in E. coli anaerobiosis. The proposed model is a clear example of the interplay of two distinct His-to-Asp phosphorelay signalling pathways.

The hydrophilic N-terminal domain complements the membrane-anchored C-terminal domain of the sensor kinase KdpD of Escherichia coli

The putative turgor sensor KdpD is characterized by a large, N-terminal domain of about 400 amino acids, which is not found in any other known sensor kinase. Comparison of 12 KdpD sequences from various microorganisms reveals that this part of the kinase is highly conserved and includes two motifs (Walker A and Walker B) that are very similar to the classical ATP-binding sites of ATP-requiring enzymes. By means of photoaffinity labeling with 8-azido-[alpha-(32)P]ATP, direct evidence was obtained for the existence of an ATP-binding site located in the N-terminal domain of KdpD. The N-terminal domain, KdpD/1-395, was overproduced and purified. Although predicted to be hydrophilic, it was found to be membrane-associated and could be solubilized either by treatment with buffer of low ionic strength or detergent. The membrane-associated form, but not the solubilized one, retained the ability to bind 8-azido-[alpha-(32)P]ATP. Previously, it was shown that the phosphatase activity of a truncated KdpD, KdpD/Delta12-395, is deregulated in vitro (Jung, K., and Altendorf, K. (1998) J. Biol. Chem. 273, 17406-17410). Here, we demonstrated that this effect was reversed in vesicles containing both the truncated KdpD and the N-terminal domain. Furthermore, coexpression of kdpD/Delta12-395 and kdpD/1-395 restored signal transduction in vivo. These results highlight the importance of the N-terminal domain for the function of KdpD and provide evidence for an interaction of this domain and the transmitter domain of the sensor kinase.

Rapid inactivation of the Escherichia coli Kdp K+ uptake system by high potassium concentrations

The Kdp K+ uptake system of Escherichia coli is induced by limitation for K+ and/or high osmolarity. In the present study, the regulation of the activity of the Kdp system has been investigated in E. coli mutants possessing only the Kdp system as the mechanism of K+ accumulation. Cells grown in the presence of low K+ (0.1-1 mM) exhibit normal growth. However, growth inhibition results from exposure of cells to moderate levels of external K+ (> 5 mM). Measurement of the cytoplasmic pH, of K+ pools and of transport via the Kdp system demonstrates that the Kdp system is rapidly and irreversibly inhibited by moderate external K+. Concentrations of K+ greater than 2 mM are sufficient to cause inhibition of Kdp. At pH 6, this results in rapid lowering of the capacity for pH homeostasis, but at pH 7 the intracellular pH is unaffected. Parallel analysis of the expression of the Kdp system in a Kdp+/kdpFABC-lacZ strain shows that levels of K+ that are sufficient to inhibit Kdp activity also repress expression. As a result, growth inhibition of strains solely possessing Kdp arises jointly from inhibition of Kdp activity and repression of Kdp gene expression. These data identify an important aspect of the regulation of potassium transport via the Kdp system and also provide support for a model of regulation of Kdp expression via at least two mechanisms: sensing of both turgor and external K+ concentration.