Studies on the role of HtpG in the tetrapyrrole biosynthesis pathway of the cyanobacterium Synechococcus elongatus PCC 7942

DnaK2 Mediates a Negative Feedback Regulation of the Heat Shock Responsive Hik2-Rre1 Two-Component System in the Cyanobacterium Synechococcus Elongatus PCC 7942

The two-component system (TCS) is a conserved signal transduction module in bacteria. The Hik2-Rre1 system is responsible for transcriptional activation upon high-temperature shift as well as plastoquinone-related redox stress in the cyanobacterium Synechococcus elongatus PCC 7942. As heat-induced de novo protein synthesis was previously shown to be required to quench the heat-activated response, we investigated the underlying mechanism in this study. We found that the heat-inducible transcription activation was alleviated by the overexpression of dnaK2, which is an essential homolog of the highly conserved HSP70 chaperone and whose expression is induced under the control of the Hik2-Rre1 TCS. Phosphorylation of Rre1 correlated with transcription of the regulatory target hspA. The redox stress response was found to be similarly repressed by dnaK2 overexpression. Considered together with the previous information, we propose a negative feedback mechanism of the Hik2-Rre1-dependent stress response that maintains the cellular homeostasis mediated by DnaK2.

HtpG Is a Metal-Dependent Chaperone Which Assists the DnaK/DnaJ/GrpE Chaperone System of Mycobacterium tuberculosis via Direct Association with DnaJ2

Protein folding is a crucial process in maintaining protein homeostasis, also known as proteostasis, in the cell. The requirement for the assistance of molecular chaperones in the appropriate folding of several proteins has already called into question the previously held view of spontaneous protein folding. These chaperones are highly ubiquitous cellular proteins, which not only help in mediating the proper folding of other nascent polypeptides but are also involved in refolding of the misfolded or the aggregated proteins. Hsp90 family proteins such as high-temperature protein G (HtpG) are abundant and ubiquitously expressed in both eukaryotic and prokaryotic cells. Although HtpG is known as an ATP-dependent chaperone protein in most organisms, function of this protein remains obscured in mycobacterial pathogens. Here, we aim to investigate significance of HtpG as a chaperone in the physiology of Mycobacterium tuberculosis. We report that M. tuberculosis HtpG (mHtpG) is a metal-dependent ATPase which exhibits chaperonin activity towards denatured proteins in coordination with the DnaK/DnaJ/GrpE chaperone system via direct association with DnaJ2. Increased expression of DnaJ1, DnaJ2, ClpX, and ClpC1 in a ΔhtpG mutant strain further suggests cooperativity of mHtpG with various chaperones and proteostasis machinery in M. tuberculosis. IMPORTANCE M. tuberculosis is exposed to variety of extracellular stressful conditions and has evolved mechanisms to endure and adapt to the adverse conditions for survival. mHtpG, despite being dispensable for M. tuberculosis growth under in vitro conditions, exhibits a strong and direct association with DnaJ2 cochaperone and assists the mycobacterial DnaK/DnaJ/GrpE (KJE) chaperone system. These findings suggest the potential role of mHtpG in stress management of the pathogen. Mycobacterial chaperones are responsible for folding of nascent protein as well as reactivation of protein aggregates. M. tuberculosis shows differential adaptive response subject to the availability of mHtpG. While its presence facilitates improved protein refolding via stimulation of the KJE chaperone activity, in the absence of mHtpG, M. tuberculosis enhances expression of DnaJ1/J2 cochaperones as well as Clp protease machinery for maintenance of proteostasis. Overall, this study provides a framework for future investigation to better decipher the mycobacterial proteostasis network in the light of stress adaptability and/or survival.

Genes regulating oxidative-inflammatory response in circulating monocytes and neutrophils in septic syndrome

Despite significant progress in the past decades, sepsis still lacks a specific treatment. Under normal conditions, leucocytes play a critical role in controlling infection and it is suggested that their activity is impaired during sepsis which contribute to the dysregulation of immune reactions. Indeed, in response to infection, several intracellular pathways are affected mainly those regulating the oxidative- inflammatory axis. Herein, we focused on the contribution of NF-kB, iNOS, Nrf2, HO-1 and MPO genes in the pathophysiology of septic syndrome, by analyzing the differential expression of their transcripts in circulating monocytes and neutrophils, and monitoring the nitrosative/oxidative status in septic syndrome patients. Circulating neutrophils of septic patients displayed a significant overexpression of NF-kB compared to other groups. In monocytes, patients with septic shock expressed the highest levels of iNOS and NF-kB mRNA. However, genes involved in cytoprotective response had increased expression in patients with sepsis, in particular, the Nrf2 and its target gene HO-1. Moreover, patient monitoring indicates that the iNOS enzyme expression and NO plasma levels may play a role in assessing the severity of septic conditions. Overall, in either monocytes or neutrophils, we pointed out the major role of NF-κB and Nrf2 in the pathophysiological process. Therefore, therapies targeted to redox abnormalities may be useful for better management of septic patients.

Regulation of the groESL1 transcription by the HrcA repressor and a novel transcription factor Orf7.5 in the cyanobacterium Synechococcus elongatus PCC7942

The CIRCE/HrcA system is highly conserved in cyanobacterial genomes. We have shown that heat-shock induction of the groESL1 operon in the cyanobacterium Synechocystis sp. PCC6803 is negatively regulated by the CIRCE/HrcA system. In Synechococcus elongatus PCC7942, a novel heat shock protein, Orf7.5, is involved in positive regulation of the groESL1 transcription. However, Orf7.5 is not conserved in some cyanobacteria, including Synechocystis sp. PCC6803. The purpose of this study is to evaluate the functional conservation of the CIRCE/HrcA system in S. elongatus PCC7942 and to understand the interplay between the CIRCE/HrcA system and the Orf7.5 regulatory system. We constructed single and double mutants of S. elongatus orf7.5, hrcA and orf7.5/hrcA and heat induction of the groESL1 transcription in these mutants was analyzed. Unexpectedly, derepression of the groESL1 transcription in an hrcA mutant was not observed. In all these mutants, the transcription was greatly suppressed under both normal and heat stress conditions, indicating that both HrcA and Orf7.5 are involved in regulation of the groESL1 transcription in a positive way. Consistent with the decrease in the groESL1 mRNA level, all the single and double mutants showed a great loss of acquired thermotolerance. Heat induction of the orf7.5 promoter activity was totally diminished in the orf7.5 mutant, indicating that Orf7.5 activates its own transcription. Yeast two hybrid analysis showed that the principle sigma factor RpoD1 interacts with Orf7.5. These results indicate that Orf7.5 enhances the transcription of groESL1 and orf7.5 by interacting with RpoD1.

Constitutive expression of phosphoketolase, a key enzyme for metabolic shift from homo- to heterolactic fermentation in Enterococcus mundtii QU 25

Phosphoketolase (PK) is responsible for heterolactic fermentation; however, the PK gene of Enterococcus mundtii QU 25, xfpA, is transcribed constitutively, even under homolactic fermentation conditions. In order to deduce the regulatory mechanisms of PK activity in QU 25, XfpA levels in QU 25 cells under hetero- and homolactic fermentation conditions were tested using western blotting. The results showed that the XfpA protein expression was similar under both conditions and that the expression products formed complexes, most likely homodimers, indicating that the regulation of PK activity is downstream of translation.

Nostoc entophytum cell response to cadmium exposure: A possible role of chaperon proteins GroEl and HtpG in cadmium-induced stress

The present study is pursuing our previous research, focused on some aspects of Nostoc entophytum ISC32 cell response to the stress caused by exposure to cadmium at the cellular and molecular levels. Variations in the antioxidant system (catalase and ascorbate peroxidase activity) of N. entophytum ISC32 exposed to varying concentrations of Cd (2, and 5 mg/L) resulted in a significant increase in the activity of both catalase and peroxidase. Activity of these enzymes was, however, not significantly changed in the presence of Cd concentrations of 10 and 20 mg/L. Levels of lipid peroxidation, as measured by malondialdehyde (MDA) assay, were observed in response to exposure to Cd (20 mg/L). There was, however, a sharp drop in both antioxidant and lipid peroxidation activities of Cd treated cells after 5 days exposure, likely in consequence of cellular damage. The content of chlorophyll a and phycobiliproteins of living cells were altered under Cd-induced conditions. TEM images of cyanobacterial cells treated with Cd showed cell surface alteration and modification along with altered cellular microcompartments. Cyanobacterial cells treated with Cd at concentrations below the minimum inhibitory concentration (MIC) remained with no apparent structural changes. However, at a higher concentration of Cd (30 mg/L), a clear detachment effect was observed between the mucilage external layer and cell membrane which may be attributed to cell plasmolysis due to toxic effects of Cd. Subsequently, the thickness of the ring-shaped mucilage external layer increased likely as a result of the cell defense mechanisms against toxic concentrations of Cd. Characterization of cells treated with Cd (30 and 150 mg/L) by scanning electron microscopy (SEM) indicated cell shrinkage with varying degrees of distortion and surface wrinkling. Energy-dispersive X-ray spectrometry (EDS) analysis suggested that Cd was not present as nanoparticles within the cell, but in the form of salt or other molecular structures. The up-regulation of chaperons was confirmed for GroEL and HtpG using real-time PCR and northern blot analyses. Interestingly, the expression of GroEL was markedly increased at lower Cd concentration (5 mg/L). However, the ISC32 strain accrued higher levels of HtpG transcript in response to an elevated concentration of Cd (15 mg/L). This pattern seems to be related to the fast and early induction of GroEL, which may be necessary for induction of other factors and heat shock proteins such as HtpG in Cd-treated Nostoc cells. The result of this study paves the way for a more detailed exploration of Cd effects on the defense mechanisms of cyanobacteria. Our research also shed some light on how cyanobacterial cells have evolved to respond to the heavy metal toxicity at the cellular, molecular and ultrastructural levels.

Hsp90 and Hsp70 chaperones: Collaborators in protein remodeling

Heat shock proteins 90 (Hsp90) and 70 (Hsp70) are two families of highly conserved ATP-dependent molecular chaperones that fold and remodel proteins. Both are important components of the cellular machinery involved in protein homeostasis and participate in nearly every cellular process. Although Hsp90 and Hsp70 each carry out some chaperone activities independently, they collaborate in other cellular remodeling reactions. In eukaryotes, both Hsp90 and Hsp70 function with numerous Hsp90 and Hsp70 co-chaperones. In contrast, bacterial Hsp90 and Hsp70 are less complex; Hsp90 acts independently of co-chaperones, and Hsp70 uses two co-chaperones. In this review, we focus on recent progress toward understanding the basic mechanisms of Hsp90-mediated protein remodeling and the collaboration between Hsp90 and Hsp70, with an emphasis on bacterial chaperones. We describe the structure and conformational dynamics of these chaperones and their interactions with each other and with client proteins. The physiological roles of Hsp90 in Escherichia coli and other bacteria are also discussed. We anticipate that the information gained from exploring the mechanism of the bacterial chaperone system will provide the groundwork for understanding the more complex eukaryotic Hsp90 system and its modulation by Hsp90 co-chaperones.

Stimulation of the ATPase activity of Hsp90 by zerumbone modification of its cysteine residues destabilizes its clients and causes cytotoxicity

Hsp90 is an ATP-dependent molecular chaperone that assists folding and conformational maturation/maintenance of many proteins. It is a potential cancer drug target because it chaperones oncoproteins. A prokaryotic homolog of Hsp90 (HtpG) is essential for thermo-tolerance in some bacteria and virulence of zoonotic pathogens. To identify a new class of small molecules which target prokaryotic and eukaryotic Hsp90s, we studied the effects of a naturally occurring cyclic sesquiterpene, zerumbone, which inhibits proliferation of a wide variety of tumor cells, on the activity of Hsp90. Zerumbone enhanced the ATPase activity of cyanobacterial Hsp90 (Hsp90SE), yeast Hsp90, and human Hsp90α. It also enhanced the catalytic efficiency of Hsp90SE by greatly increasing kcat. Mass analysis showed that zerumbone binds to cysteine side chains of Hsp90SE covalently. Mutational studies identified 3 cysteine residues (one per each domain of Hsp90SE) that are involved in the enhancement, suggesting the presence of allosteric sites in the middle and C-terminal domains of Hsp90SE. Treatment of cyanobacterial cells with zerumbone caused them to become very temperature-sensitive, a phenotype reminiscent of cyanobacterial Hsp90 mutants, and also decreased the cellular level of linker polypeptides that are clients for Hsp90SE. Zerumbone showed cellular toxicity on cancer-derived mammalian cells by inducing apoptosis. In addition, zerumbone inhibited the binding of Hsp90/Cdc37 to client kinases. Altogether, we conclude that modification of cysteine residues of Hsp90 by zerumbone enhances its ATPase activity and inhibits physiological Hsp90 function. The activation of Hsp90 may provide new strategies to inhibit its chaperone function in cells.

ParA-like protein influences the distribution of multi-copy chromosomes in cyanobacterium Synechococcus elongatus PCC 7942

While many bacteria, such as Escherichia coli and Bacillus subtilis, harbour a single-copy chromosome, freshwater cyanobacteria have multiple copies of each chromosome per cell. Although it has been reported that multi-copy chromosomes are evenly distributed along the major axis of the cell in cyanobacterium Synechococcus elongatus PCC 7942, the distribution mechanism of these chromosomes remains unclear. In S. elongatus, the carboxysome, a metabolic microcompartment for carbon fixation that is distributed in a similar manner to the multi-copy chromosomes, is regulated by ParA-like protein (hereafter ParA). To elucidate the role of ParA in the distribution of multi-copy chromosomes, we constructed and analysed ParA disruptant and overexpressing strains of S. elongatus. Our fluorescence in situ hybridization assay revealed that the parA disruptants displayed an aberrant distribution of their multi-copy chromosomes. In the parA disruptant the multiple origin and terminus foci, corresponding to the intracellular position of each chromosomal region, were aggregated, which was compensated by the expression of exogenous ParA from other genomic loci. The parA disruptant is sensitive to UV-C compared to the WT strain. Additionally, giant cells appeared under ParA overexpression at the late stage of growth indicating that excess ParA indirectly inhibits cell division. Screening of the ParA-interacting proteins by yeast two-hybrid analysis revealed four candidates that are involved in DNA repair and cell membrane biogenesis. These results suggest that ParA is involved in the pleiotropic cellular functions with these proteins, while parA is dispensable for cell viability in S. elongatus.

ATP-dependent molecular chaperones in plastids--More complex than expected

Plastids are a class of essential plant cell organelles comprising photosynthetic chloroplasts of green tissues, starch-storing amyloplasts of roots and tubers or the colorful pigment-storing chromoplasts of petals and fruits. They express a few genes encoded on their organellar genome, called plastome, but import most of their proteins from the cytosol. The import into plastids, the folding of freshly-translated or imported proteins, the degradation or renaturation of denatured and entangled proteins, and the quality-control of newly folded proteins all require the action of molecular chaperones. Members of all four major families of ATP-dependent molecular chaperones (chaperonin/Cpn60, Hsp70, Hsp90 and Hsp100 families) have been identified in plastids from unicellular algae to higher plants. This review aims not only at giving an overview of the most current insights into the general and conserved functions of these plastid chaperones, but also into their specific plastid functions. Given that chloroplasts harbor an extreme environment that cycles between reduced and oxidized states, that has to deal with reactive oxygen species and is highly reactive to environmental and developmental signals, it can be presumed that plastid chaperones have evolved a plethora of specific functions some of which are just about to be discovered. Here, the most urgent questions that remain unsolved are discussed, and guidance for future research on plastid chaperones is given. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Binding of the RNA chaperone Hfq to the type IV pilus base is crucial for its function in Synechocystis sp. PCC 6803

The bacterial RNA-binding protein Hfq functions in post-transcriptional regulation of gene expression. There is evidence in a range of bacteria for specific subcellular localization of Hfq; however, the mechanism and role of Hfq localization remain unclear. Cyanobacteria harbour a subfamily of Hfq that is structurally conserved but exhibits divergent RNA binding sites. Mutational analysis in the cyanobacterium Synechocystis sp. PCC 6803 revealed that several conserved amino acids on the proximal side of the Hfq hexamer are crucial not only for Hfq-dependent RNA accumulation but also for phototaxis, the latter of which depends on type IV pili. Co-immunoprecipitation and yeast two-hybrid analysis show that the secretion ATPase PilB1 (a component of the type IV pilus base) is an interaction partner of Hfq. Fluorescence microscopy revealed that Hfq is localized to the cytoplasmic membrane in a PilB1-dependent manner. Concomitantly, Hfq-dependent RNA accumulation is abrogated in a ΔpilB1 mutant, indicating that localization to the pilus base via interaction with PilB1 is essential for Hfq function in cyanobacteria.

Translational control of small heat shock genes in mesophilic and thermophilic cyanobacteria by RNA thermometers

Cyanobacteria constitute a heterogeneous phylum of oxygen-producing, photosynthetic prokaryotes. They are susceptible to various stress conditions like heat, salt, or light stress, all inducing the cyanobacterial heat shock response (HSR). Cyanobacterial small heat shock proteins (sHsps) are known to preserve thylakoid membrane integrity under stress conditions, thereby protecting the photosynthesis machinery. In Synechocystis sp PCC 6803, synthesis of the sHsp Hsp17 is regulated by an RNA thermometer (RNAT) in the 5′-untranslated region (5′-UTR) of the hsp17 mRNA. RNATs are direct temperature sensors that control expression of many bacterial heat shock and virulence genes. They hinder translation at low temperatures by base pairing, thus blocking ribosome access to the mRNA.

 

To explore the temperature range in which RNATs act, we studied various RNAT candidates upstream of sHsp genes from mesophilic and thermophilic cyanobacteria. The mesophilic cyanobacteria Anabaena variabilis and Nostoc sp chromosomally encode two sHsps each. Reporter gene studies suggested RNAT-mediated post-transcriptional regulation of shsp expression in both organisms. Detailed structural analysis of the two A. variabilis candidates revealed two novel RNAT types. The first, avashort, regulates translation primarily by masking of the AUG translational start codon. The second, featuring an extended initial hairpin, thus named avalong, presumably makes use of complex tertiary interaction. The 5′-UTR of the small heat shock gene hspA in the thermophile Thermosynechococcus elongatus is predicted to adopt an extended secondary structure. Structure probing revealed that the ribosome binding site was blocked at temperatures below 55 °C. The results of this study demonstrate that cyanobacteria commonly use RNATs to control expression of their small heat shock genes.

Cyanobacterial heat-shock response: role and regulation of molecular chaperones

Cyanobacteria constitute a morphologically diverse group of oxygenic photoautotrophic microbes which range from unicellular to multicellular, and non-nitrogen-fixing to nitrogen-fixing types. Sustained long-term exposure to changing environmental conditions, during their three billion years of evolution, has presumably led to their adaptation to diverse ecological niches. The ability to maintain protein conformational homeostasis (folding–misfolding–refolding or aggregation–degradation) by molecular chaperones holds the key to the stress adaptability of cyanobacteria. Although cyanobacteria possess several genes encoding DnaK and DnaJ family proteins, these are not the most abundant heat-shock proteins (Hsps), as is the case in other bacteria. Instead, the Hsp60 family of proteins, comprising two phylogenetically conserved proteins, and small Hsps are more abundant during heat stress. The contribution of the Hsp100 (ClpB) family of proteins and of small Hsps in the unicellular cyanobacteria (Synechocystis and Synechococcus) as well as that of Hsp60 proteins in the filamentous cyanobacteria (Anabaena) to thermotolerance has been elucidated. The regulation of chaperone genes by several cis-elements and trans-acting factors has also been well documented. Recent studies have demonstrated novel transcriptional and translational (mRNA secondary structure) regulatory mechanisms in unicellular cyanobacteria. This article provides an insight into the heat-shock response: its organization, and ecophysiological regulation and role of molecular chaperones, in unicellular and filamentous nitrogen-fixing cyanobacterial strains.

Identification of highly-disrupted tRNA genes in nuclear genome of the red alga, Cyanidioschyzon merolae 10D

The limited locations of tRNA introns are crucial for eukaryal tRNA-splicing endonuclease recognition. However, our analysis of the nuclear genome of an early-diverged red alga, Cyanidioschyzon merolae, demonstrated the first evidence of nuclear-encoded tRNA genes that contain ectopic and/or multiple introns. Some genes exhibited both intronic and permuted structures in which the 3′-half of the tRNA coding sequence lies upstream of the 5′-half, and an intron is inserted into either half. These highly disrupted tRNA genes, which account for 63% of all nuclear tRNA genes, are expressed via the orderly and sequential processing of bulge-helix-bulge (BHB) motifs at intron-exon junctions and termini of permuted tRNA precursors, probably by a C. merolae tRNA-splicing endonuclease with an unidentified subunit architecture. The results revealed a considerable diversity in eukaryal tRNA intron properties and endonuclease architectures, which will help to elucidate the acquisition mechanism of the BHB-mediated disrupted tRNA genes.

Physical interaction between bacterial heat shock protein (Hsp) 90 and Hsp70 chaperones mediates their cooperative action to refold denatured proteins

In eukaryotes, heat shock protein 90 (Hsp90) is an essential ATP-dependent molecular chaperone that associates with numerous client proteins. HtpG, a prokaryotic homolog of Hsp90, is essential for thermotolerance in cyanobacteria, and in vitro it suppresses the aggregation of denatured proteins efficiently. Understanding how the non-native client proteins bound to HtpG refold is of central importance to comprehend the essential role of HtpG under stress. Here, we demonstrate by yeast two-hybrid method, immunoprecipitation assays, and surface plasmon resonance techniques that HtpG physically interacts with DnaJ2 and DnaK2. DnaJ2, which belongs to the type II J-protein family, bound DnaK2 or HtpG with submicromolar affinity, and HtpG bound DnaK2 with micromolar affinity. Not only DnaJ2 but also HtpG enhanced the ATP hydrolysis by DnaK2. Although assisted by the DnaK2 chaperone system, HtpG enhanced native refolding of urea-denatured lactate dehydrogenase and heat-denatured glucose-6-phosphate dehydrogenase. HtpG did not substitute for DnaJ2 or GrpE in the DnaK2-assisted refolding of the denatured substrates. The heat-denatured malate dehydrogenase that did not refold by the assistance of the DnaK2 chaperone system alone was trapped by HtpG first and then transferred to DnaK2 where it refolded. Dissociation of substrates from HtpG was either ATP-dependent or -independent depending on the substrate, indicating the presence of two mechanisms of cooperative action between the HtpG and the DnaK2 chaperone system.

Heat shock transcriptional responses in an MC-Producing Cyanobacterium (Planktothrix agardhii) and its MC-deficient mutant under high light conditions

Microcystins (MCs) are the most commonly-reported hepatotoxins produced by various cyanobacterial taxa in fresh waters to constitute a potential threat to human and animal health. The biological role of MCs in the producer organisms is not known, and it would be very useful to understand the driving force behind the toxin production. Recent studies have suggested that MCs may have a protective function in cells facing environmental stress. Following this starting premise, we speculate that under adverse conditions the expression of stress-related genes coding for Heat Shock Proteins (Hsp) might be different in an MC-producing strain and its MC-deficient mutant. We therefore used RT-qPCR to compare the expression of 13 hsp genes of an MC-producing strain of Planktothrix agardhii (CYA126/8) and its MC-deficient ΔmcyD mutant over different periods of exposure to high light stress (HL). Three reference genes (RGs) were selected from six candidates to normalize the RT-qPCR data. Of these three RGs (rsh, rpoD, and gltA), gltA is used here for the first time as an RG in prokaryotes. Under HL stress, five genes were found to be strongly up-regulated in both strains (htpG, dnaK, hspA, groES, and groEL). Unexpectedly, we found that the MC-producing wild type strain accumulated higher levels of htpG and dnaK transcripts in response to HL stress than the MC-deficient mutant. In addition, a significant increase in the mcyE transcript was detected in the mutant, suggesting that MCs are required under HL conditions. We discuss several possible roles of MCs in the response to HL stress through their possible involvement in the protective mechanisms of the cells.

Characterization of the Bat proteins in the oxidative stress response of Leptospira biflexa

Background

Leptospires lack many of the homologs for oxidative defense present in other bacteria, but do encode homologs of the Bacteriodes aerotolerance (Bat) proteins, which have been proposed to fulfill this function. Bat homologs have been identified in all families of the phylum Spirochaetes, yet a specific function for these proteins has not been experimentally demonstrated.

Results

We investigated the contribution of the Bat proteins in the model organism Leptospira biflexa for their potential contributions to growth rate, morphology and protection against oxidative challenges. A genetically engineered mutant strain in which all bat ORFs were deleted did not exhibit altered growth rate or morphology, relative to the wild-type strain. Nor could we demonstrate a protective role for the Bat proteins in coping with various oxidative stresses. Further, pre-exposing L. biflexa to sublethal levels of reactive oxygen species did not appear to induce a general oxidative stress response, in contrast to what has been shown in other bacterial species. Differential proteomic analysis of the wild-type and mutant strains detected changes in the abundance of a single protein only – HtpG, which is encoded by the gene immediately downstream of the bat loci.

Conclusion

The data presented here do not support a protective role for the Leptospira Bat proteins in directly coping with oxidative stress as previously proposed. L. biflexa is relatively sensitive to reactive oxygen species such as superoxide and H₂O₂, suggesting that this spirochete lacks a strong, protective defense against oxidative damage despite being a strict aerobe.

Interactions between histidine kinase NblS and the response regulators RpaB and SrrA are involved in the bleaching process of the cyanobacterium Synechococcus elongatus PCC 7942

Cyanobacteria have developed a light-harvesting antenna complex known as the phycobilisome. When cells are starved for nutrients or exposed to high light, the phycobilisome is rapidly degraded (bleaching). It has been suggested that in the cyanobacterium Synechococcus elongatus PCC 7942, the bleaching process is regulated by a two-component histidine kinase, NblS. To clarify the signaling pathway involving NblS, we identified the NblS-interacting response regulators RpaB and SrrA. In vitro assays using recombinant proteins showed that both RpaB and SrrA can receive phosphoryl groups from autophosphorylated NblS; the NblS-interacting protein SipA clearly enhances the phosphotransfer activity from NblS to RpaB and SrrA. In addition, NblS prefers SrrA over RpaB as the phosphotransfer target with or without SipA. Gel mobility shift assay revealed that both RpaB and SrrA can bind to the upstream region of nblA, a major regulatory factor in the bleaching process. nblA transcript accumulates in nblS or rpaB mutants even under normal growth conditions, while in the srrA disruptant the nblA transcripts are slightly up-regulated under stress conditions. These observations suggest that the bleaching signal transduction pathway via NblS is regulated by RpaB and that SrrA is partially involved.

HtpG is involved in the pathogenesis of Edwardsiella tarda

Hsp90 is a molecular chaperone that is involved in diverse cellular processes including protein folding/repairing and signal transduction. Edwardsiella tarda is a serious fish pathogen that affects fish aquaculture worldwide. The aim of this study was to investigate the potential importance of HtpG, the prokaryotic homologue of Hsp90, in the pathogenesis of E. tarda. E. tarda HtpG is 627-residue in length and contains domain structures that are conserved among Hsp90 family members. Quantitative real time RT-PCR analysis indicated that expression of htpG is induced by heat shock and oxidative stress. Recombinant HtpG (rHtpG) purified from Escherichia coli exhibits apparent ATPase activity, which is optimal at 40°C. Mutation of htpG (i) affects bacterial growth at elevated temperature and renders the cells more sensitive to stress induced by reactive oxygen species, (ii) causes dramatic reduction in blood dissemination and general bacterial virulence, (iii) weakens the ability of E. tarda to block head kidney macrophage activation and to resist against the bactericidal effect of macrophages, and (iv) upregulates the expression of pro-inflammatory cytokines in macrophages. Taken together, these results indicate that HtpG is a biologically active protein that is required for E. tarda to cope with various stress conditions especially that encountered in vivo the host system during infection.

Ribosomal protein L2 associates with E. coli HtpG and activates its ATPase activity

Although eukaryotic Hsp90 has been studied extensively, the function of its bacterial homologue HtpG remains elusive. Here we report that 50S ribosomal protein L2 was found as an associated protein with His-tagged HtpG from Escherichia coli cultured in minimum medium at 45 °C. L2 specifically activated ATPase activity of HtpG, but other denatured proteins did not. The analysis using domain derivatives of HtpG and L2 showed that C-terminal domain of L2 and the middle to C-terminal domain of HtpG are important for interaction. At physiological salt concentration, L2 was denatured state and was recognized by HtpG as well as other chaperones, DnaK/DnaJ/GrpE and GroEL/GroES. The ATPase of HtpG at increasing concentration of L2 indicated that an L2 molecule bound to a dimer HtpG with apparent K(D) of 0.3 μM at 100mM KCl and 3.3 μM at 200 mM KCl.

HtpG, the prokaryotic homologue of Hsp90, stabilizes a phycobilisome protein in the cyanobacterium Synechococcus elongatus PCC 7942

HtpG, a homologue of HSP90, is essential for thermotolerance in cyanobacteria. It is not known how it plays this important role. We obtained evidence that HtpG interacts with linker polypeptides of phycobilisome in the cyanobacterium Synechococcus elongatus PCC 7942. In an htpG mutant, the 30 kDa rod linker polypeptide was reduced. In vitro studies with purified HtpG and phycobilisome showed that HtpG interacts with the linker polypeptide as well as other linker polypeptides to suppress their thermal aggregation with a stoichiometry of one linker polypeptide/HtpG dimer. We constructed various domain-truncated derivatives of HtpG to identify putative chaperone sites at which HtpG binds linker polypeptides. The middle domain and the N-terminal domain, although less efficiently, prevented the aggregation of denatured polypeptides, while the C-terminal domain did not. Truncation of the C-terminal domain that is involved in the dimerization of HtpG led to decrease in the anti-aggregation activity, while fusion of the N-terminal domain to the middle domain lowered the activity. In vitro studies with HtpG and the isolated 30 kDa rod linker polypeptide provided basically similar results to those with HtpG and phycobilisome. ADP inhibited the anti-aggregation activity, indicating that a compact ADP conformational state provides weaker aggregation protection compared with the others.

Immunoglobulin G (IgG) class, but Not IgA or IgM, antibodies to peptides of the Porphyromonas gingivalis chaperone HtpG predict health in subjects with periodontitis by a fluorescence enzyme-linked immunosorbent assay

Chaperones are molecules found in all cells and are critical in stabilization of synthesized proteins, in repair/removal of defective proteins, and as immunodominant antigens in innate and adaptive immunity. Subjects with gingivitis colonized by the oral pathogen Porphyromonas gingivalis previously demonstrated levels of anti-human chaperone Hsp90 that were highest in individuals with the best oral health. We hypothesized that similar antibodies to pathogen chaperones might be protective in periodontitis. This study examined the relationship between antibodies to P. gingivalis HtpG and clinical statuses of healthy and periodontitis-susceptible subjects. We measured the humoral responses (immunoglobulin G [IgG], IgA, and IgM) to peptides of a unique insert (P18) found in Bacteroidaceae HtpG by using a high-throughput, quantitative fluorescence enzyme-linked immunosorbent assay. Indeed, higher levels of IgG class anti-P. gingivalis HtpG P18 peptide (P < 0.05) and P18alpha, consisting of the N-terminal 16 amino acids of P18 (P < 0.05), were associated with better oral health; these results were opposite of those found with anti-P. gingivalis whole-cell antibodies and levels of the bacterium in the subgingival biofilm. When we examined the same sera for IgA and IgM class antibodies, we found no significant relationship to subject clinical status. The relationship between anti-P18 levels and clinical populations and individual subjects was found to be improved when we normalized the anti-P18alpha values to those for anti-P18gamma (the central 16 amino acids of P18). That same ratio correlated with the improvement in tissue attachment gain after treatment (P < 0.05). We suggest that anti-P. gingivalis HtpG P18alpha antibodies are protective in periodontal disease and may have prognostic value for guidance of individual patient treatment.

A 'foldosome' in the chloroplast?

The proper functioning of many cytosolic proteins involved in signal transduction depends on protein folding steps carried out cooperatively by a multichaperone complex containing the Hsp90 and Hsp70 machineries. We have recently found that also in the chloroplast the Hsp90 and Hsp70 machineries form a multichaperone complex, although chloroplast Hsp90 and Hsp70 are from eukaryotic and prokaryotic origin, respectively. In earlier work by others it was shown that plants expressing a mutated form of a chloroplast-targeted Hsp90 were impaired in the light induction of several nuclear genes. These data suggest that, like in the cytosol, the folding of chloroplast proteins involved in chloroplast-to-nucleus signalling might depend on the cooperative action of the chloroplast Hsp70–Hsp90 machineries.

Interaction of the molecular chaperone HtpG with uroporphyrinogen decarboxylase in the cyanobacterium Synechococcus elongatus PCC 7942

Uroporphyrinogen decarboxylase (HemE) is important due to its location at the first branch-point in tetrapyrrole biosynthesis. We detected a complex formation between full-length polypeptides of HtpG and HemE by biochemical studies in vivo and in vitro. The interaction suppressed the enzyme activity, suggesting a regulatory role of HtpG in tetrapyrrole biosynthesis.

Membrane regulation of the stress response from prokaryotic models to mammalian cells

"Membrane regulation" of stress responses in various systems is widely studied. In poikilotherms, membrane rigidification could be the first reaction to cold perception: reducing membrane fluidity of membranes at physiological temperatures is coupled with enhanced cold inducibility of a number of genes, including desaturases (see J.L. Harwood's article in this Proceedings volume). A similar role of changes in membrane physical state in heat (oxidative stress, etc.) sensing- and signaling gained support recently from prokaryotes to mammalian cells. Stress-induced remodeling of membrane lipids could influence generation, transduction, and deactivation of stress signals, either through global effects on the fluidity of the membrane matrix, or by specific interactions of boundary (or raft) lipids with receptor proteins, lipases, ion channels, etc. Our data point to membranes not only as targets of stress, but also as sensors in activating a stress response.

Protection of psbAII transcript from ribonuclease degradation in vitro by DnaK2 and DnaJ2 chaperones of the cyanobacterium Synechococcus elongatus PCC 7942

Three dnaK and four dnaJ genes have been identified in the genome of cyanobacterium Synechococcus elongatus PCC 7942. Our comprehensive analysis of yeast two-hybrid screening revealed a specific interaction among DnaK2, DnaJ2, and RNase E, an essential endoribonuclease. We examined the effects of DnaK2 and DnaJ2 on RNase E activity by monitoring the digestion of psbAII transcript in vitro. The addition of DnaK2 and DnaJ2 obviously inhibited RNase E activity in an ATP-dependent manner. These results suggest that DnaK2 and DnaJ2 are involved in RNA degradation through interaction with RNase E.