HU histone-like DNA-binding protein from Thermus thermophilus: structural and evolutionary analyses

The 14-3-3 protein CaTFT7 interacts with transcription factor CaHDZ27 to positively regulate pepper immunity against Ralstonia solanacearum

Bacterial wilt, caused by Ralstonia solanacearum, is a devastating disease affecting plants in the Solanaceae family. In our previous study, CaHDZ27 was shown to act crucially in the pepper defense response to R. solanacearum. However, the molecular basis underlying CaHDZ27 function remains unexplored. In this study, we demonstrate that CaHDZ27 is post-translationally regulated by the 14-3-3 protein CaTFT7, which functions as a positive regulator in pepper immunity against R. solanacearum. RT-qPCR analysis revealed that CaTFT7 is transcriptionally induced by R. solanacearum infection. The data from virus-induced gene silencing revealed that CaTFT7 positively affects pepper immunity, which was further confirmed by the data of CaTFT7-overexpressing Nicotiana benthamiana. CaTFT7 interacted with CaHDZ27, thereby promoting the stability of CaHDZ27 and enhancing CaHDZ27 binding to the promoter of cysteine-rich receptor-like protein kinase 5 (CaCRK5), a gene that positively affects pepper defense against R. solanacearum. The above data indicated that CaTFT7 enhanced CaHDZ27 stability and promoted its ability to activate pepper immunity, shedding light on the mechanisms underlying pepper resistance to bacterial wilt.

CaIAA2-CaARF9 module mediates the trade-off between pepper growth and immunity

To challenge the invasion of various pathogens, plants re-direct their resources from plant growth to an innate immune defence system. However, the underlying mechanism that coordinates the induction of the host immune response and the suppression of plant growth remains unclear. Here we demonstrate that an auxin response factor, CaARF9, has dual roles in enhancing the immune resistance to Ralstonia solanacearum infection and in retarding plant growth by repressing the expression of its target genes as exemplified by Casmc4, CaLBD37, CaAPK1b and CaRROP1. The expression of these target genes not only stimulates plant growth but also negatively impacts pepper resistance to R. solanacearum. Under normal conditions, the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 is active when promoter-bound CaARF9 is complexed with CaIAA2. Under R. solanacearum infection, however, degradation of CaIAA2 is triggered by SA and JA-mediated signalling defence by the ubiquitin-proteasome system, which enables CaARF9 in the absence of CaIAA2 to repress the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 and, in turn, impeding plant growth while facilitating plant defence to R. solanacearum infection. Our findings uncover an exquisite mechanism underlying the trade-off between plant growth and immunity mediated by the transcriptional repressor CaARF9 and its deactivation when complexed with CaIAA2.

CaSTH2 disables CaWRKY40 from activating pepper thermotolerance and immunity against Ralstonia solanacearum via physical interaction

CaWRKY40 coordinately activates pepper immunity against Ralstonia solanacearum infection (RSI) and high temperature stress (HTS), forms positive feedback loops with other positive regulators and is promoted by CaWRKY27b/CaWRKY28 through physical interactions; however, whether and how it is regulated by negative regulators to function appropriately remain unclear. Herein, we provide evidence that CaWRKY40 is repressed by a SALT TOLERANCE HOMOLOG2 in pepper (CaSTH2). Our data from gene silencing and transient overexpression in pepper and epoptic overexpression in Nicotiana benthamiana plants showed that CaSTH2 acted as negative regulator in immunity against RSI and thermotolerance. Our data from BiFC, CoIP, pull down, and MST indicate that CaSTH2 interacted with CaWRKY40, by which CaWRKY40 was prevented from activating immunity or thermotolerance-related genes. It was also found that CaSTH2 repressed CaWRKY40 at least partially through blocking interaction of CaWRKY40 with CaWRKY27b/CaWRKY28, but not through directly repressing binding of CaWRKY40 to its target genes. The results of study provide new insight into the mechanisms underlying the coordination of pepper immunity and thermotolerance.

Differential CaKAN3-CaHSF8 associations underlie distinct immune and heat responses under high temperature and high humidity conditions

High temperature and high humidity (HTHH) conditions increase plant susceptibility to a variety of diseases, including bacterial wilt in solanaceous plants. Some solanaceous plant cultivars have evolved mechanisms to activate HTHH-specific immunity to cope with bacterial wilt disease. However, the underlying mechanisms remain poorly understood. Here we find that CaKAN3 and CaHSF8 upregulate and physically interact with each other in nuclei under HTHH conditions without inoculation or early after inoculation with R. solanacearum in pepper. Consequently, CaKAN3 and CaHSF8 synergistically confer immunity against R. solanacearum via activating a subset of NLRs which initiates immune signaling upon perception of unidentified pathogen effectors. Intriguingly, when HTHH conditions are prolonged without pathogen attack or the temperature goes higher, CaHSF8 no longer interacts with CaKAN3. Instead, it directly upregulates a subset of HSP genes thus activating thermotolerance. Our findings highlight mechanisms controlling context-specific activation of high-temperature-specific pepper immunity and thermotolerance mediated by differential CaKAN3-CaHSF8 associations.

CaSWC4 regulates the immunity-thermotolerance tradeoff by recruiting CabZIP63/CaWRKY40 to target genes and activating chromatin in pepper

Pepper (Capsicum annuum) responds differently to high temperature stress (HTS) and Ralstonia solanacearum infection (RSI) but employs some shared transcription factors (TFs), such as CabZIP63 and CaWRKY40, in both cases. How the plant activates and balances these distinct responses, however, was unclear. Here, we show that the protein CaSWC4 interacts with CaRUVBL2 and CaTAF14b and they all act positively in pepper response to RSI and thermotolerance. CaSWC4 activates chromatin of immunity or thermotolerance related target genes of CaWRKY40 or CabZIP63 by promoting deposition of H2A.Z, H3K9ac and H4K5ac, simultaneously recruits CabZIP63 and CaWRKY40 through physical interaction and brings them to their targets (immunity- or thermotolerance-related genes) via binding AT-rich DNA element. The above process relies on the recruitment of CaRUVBL2 and TAF14 by CaSWC4 via physical interaction, which occurs at loci of immunity related target genes only when the plants are challenged with RSI, and at loci of thermotolerance related target genes only upon HTS. Collectively, our data suggest that CaSWC4 regulates rapid, accurate responses to both RSI and HTS by modulating chromatin of specific target genes opening and recruiting the TFs, CaRUVBL2 and CaTAF14b to the specific target genes, thereby helping achieve the balance between immunity and thermotolerance.

A CaCDPK29-CaWRKY27b module promotes CaWRKY40-mediated thermotolerance and immunity to Ralstonia solanacearum in pepper

CaWRKY40 in pepper (Capsicum annuum) promotes immune responses to Ralstonia solanacearum infection (RSI) and to high-temperature, high-humidity (HTHH) stress, but how it interacts with upstream signalling components remains poorly understood. Here, using approaches of reverse genetics, biochemical and molecular biology we functionally characterised the relationships among the WRKYGMK-containing WRKY protein CaWRKY27b, the calcium-dependent protein kinase CaCDPK29, and CaWRKY40 during pepper response to RSI or HTHH. Our data indicate that CaWRKY27b is upregulated and translocated from the cytoplasm to the nucleus upon phosphorylation of Ser137 in the nuclear localisation signal by CaCDPK29. Using electrophoretic mobility shift assays and microscale thermophoresis, we observed that, due to the replacement of Q by M in the conserved WRKYGQK, CaWRKY27b in the nucleus failed to bind to W-boxes in the promoters of immunity- and thermotolerance-related marker genes. Instead, CaWRKY27b interacted with CaWRKY40 and promoted its binding and positive regulation of the tested marker genes including CaNPR1, CaDEF1 and CaHSP24. Notably, mutation of the WRKYGMK motif in CaWRKY27b to WRKYGQK restored the W-box binding ability. Our data therefore suggest that CaWRKY27b is phosphorylated by CaCDPK29 and acts as a transcriptional activator of CaWRKY40 during the pepper response to RSI and HTHH.

Pepper NAC-type transcription factor NAC2c balances the trade-off between growth and defense responses

Plant responses to pathogen attacks and high-temperature stress (HTS) are distinct in nature but generally share several signaling components. How plants produce specific responses through these common signaling intermediates remains elusive. With the help of reverse-genetics approaches, we describe here the mechanism underlying trade-offs in pepper (Capsicum annuum) between growth, immunity, and thermotolerance. The NAC-type transcription factor CaNAC2c was induced by HTS and Ralstonia solanacearum infection (RSI). CaNAC2c-inhibited pepper growth, promoted immunity against RSI by activating jasmonate-mediated immunity and H2O2 accumulation, and promoted HTS responses by activating Heat shock factor A5 (CaHSFA5) transcription and blocking H2O2 accumulation. We show that CaNAC2c physically interacts with CaHSP70 and CaNAC029 in a context-specific manner. Upon HTS, CaNAC2c-CaHSP70 interaction in the nucleus protected CaNAC2c from degradation and resulted in the activation of thermotolerance by increasing CaNAC2c binding and transcriptional activation of its target promoters. CaNAC2c did not induce immunity-related genes under HTS, likely due to the degradation of CaNAC029 by the 26S proteasome. Upon RSI, CaNAC2c interacted with CaNAC029 in the nucleus and activated jasmonate-mediated immunity but was prevented from activating thermotolerance-related genes. In non-stressed plants, CaNAC2c was tethered outside the nucleus by interaction with CaHSP70, and thus was unable to activate either immunity or thermotolerance. Our results indicate that pepper growth, immunity, and thermotolerance are coordinately and tightly regulated by CaNAC2c via its inducible expression and differential interaction with CaHSP70 and CaNAC029.

CaWRKY28 Cys249 is Required for Interaction with CaWRKY40 in the Regulation of Pepper Immunity to Ralstonia solanacearum

WRKY transcription factors have been implicated in plant response to pathogens but how WRKY-mediated networks are organized and operate to produce appropriate transcriptional outputs remains largely unclear. Here, we identify a member of the WRKY family from pepper (Capsicum annuum), CaWRKY28, that physically interacts with CaWRKY40, a positive regulator of pepper immunity and thermotolerance. We confirmed CaWRKY28-CaWRKY40 interaction by coimmunoprecipitation, bimolecular fluorescence complementation, and microscale thermophoresis. Our findings supported the idea that CaWRKY28 is a nuclear protein that acts as positive regulator in pepper responses to infection by the pathogenic bacterium Ralstonia solanacearum. It performs its function not by directly modulating the W-box containing immunity-related genes but by promoting CaWRKY40 via physical interaction to bind and activate its immunity-related target genes, including CaPR1, CaNPR1, CaDEF1, and CaABR1, but not its thermotolerance-related target gene, CaHSP24. All of these data indicate that CaWRKY28 interacts with and potentiates CaWRKY40 in regulating immunity against R. solanacearum infection but not thermotolerance. Importantly, we discovered that CaWRKY28 Cys249, shared by CaWRKY28 and its orthologs probably only in the family Solanaceae, is crucial for the CaWRKY28-CaWRKY40 interaction. These results highlight how CaWRKY28 associates with CaWRKY40 during the establishment of WRKY networks, and how CaWRKY40 achieves its functional specificity during pepper responses to R. solanacearum infection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Ca14-3-3 Interacts With CaWRKY58 to Positively Modulate Pepper Response to Low-Phosphorus Starvation

Low-phosphorus stress (LPS) and pathogen attack are two important stresses frequently experienced by plants in their natural habitats, but how plant respond to them coordinately remains under-investigated. Here, we demonstrate that CaWRKY58, a known negative regulator of the pepper (Capsicum annuum) response to attack by Ralstonia solanacearum, is upregulated by LPS. Virus-induced gene silencing (VIGS) and overexpression of CaWRKY58 in Nicotiana benthamiana plants in combination with chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays (EMSA) demonstrated that CaWRKY58 positively regulates the response of pepper to LPS by directly targeting and regulating genes related to phosphorus-deficiency tolerance, including PHOSPHATE STARVATION RESPONSE1 (PHR1). Yeast two-hybrid assays revealed that CaWRKY58 interacts with a 14-3-3 protein (Ca14-3-3); this interaction was confirmed by pull-down, bimolecular fluorescence complementation (BiFC), and microscale thermophoresis (MST) assays. The interaction between Ca14-3-3 and CaWRKY58 enhanced the activation of PHR1 expression by CaWRKY58, but did not affect the expression of the immunity-related genes CaNPR1 and CaDEF1, which are negatively regulated by CaWRKY58 in pepper upon Ralstonia solanacearum inoculation. Collectively, our data indicate that CaWRKY58 negatively regulates immunity against Ralstonia solanacearum, but positively regulates tolerance to LPS and that Ca14-3-3 transcriptionally activates CaWRKY58 in response to LPS.

A novel protein-engineered dsDNA-binding protein (HU-Simulacrum) inspired by HU, a nucleoid-associated DNABII protein

HU, a DNA-binding protein, has a helical N-terminal region (NTR) of ∼44 residues and a beta strand- and IDR-rich C-terminal region (CTR) of ∼46 residues. CTR binds to DNA through (i) a clasp (two arginine/lysine-rich, IDR-rich beta hairpins that bind to phosphate groups in the minor groove), (ii) a flat surface (comprising four antiparallel beta strands that abut the major groove), and (iii) a charge cluster (two lysine residues upon a short C-terminal helix). HU forms a dimer displaying extensive inter-subunit CTR-CTR contacts. A single-chain simulacrum of these contacts (HU-Simul) incorporating all DNA-binding elements was created by fusing together the CTRs of Escherichia coli HU-A and Thermus thermophilus HU. HU-Simul is monomeric, binds to dsDNA and cruciform DNA, but not to ssDNA.

Nanoscale surface structures of DNA bound to Deinococcus radiodurans HU unveiled by atomic force microscopy

The Deinococcus radiodurans protein HU (DrHU) was shown to be critical for nucleoid activities, yet its functional and structural properties remain largely unexplored. We have applied atomic force microscopy (AFM) imaging to study DrHU binding to pUC19-DNA in vitro and analyzed the topographic structures formed at the nanoscale. At the single-molecule level, AFM imaging allows visualization of super-helical turns on naked DNA surfaces and characterization of free DrHU molecules observed as homodimers. When enhancing the molecular surface structures of AFM images by the Laplacian weight filter, the distribution of bound DrHUs was visibly varied as a function of the DrHU/DNA molar ratio. At a low molar ratio, DrHU binding was found to reduce the volume of condensed DNA configuration by about 50%. We also show that DrHU is capable of bridging distinct DNA segments. Moreover, at a low molar ratio, the binding orientation of individual DrHU dimers could be perceived on partially "open" DNA configuration. At a high molar ratio, DrHU stiffened the DNA molecule and enlarged the spread of the open DNA configuration. Furthermore, a lattice-like pattern could be seen on the surface of DrHU-DNA complex, indicating that DrHU multimerization had occurred leading to the formation of a higher order architecture. Together, our results show that the functional plasticity of DrHU in mediating DNA organization is subject to both the conformational dynamics of DNA molecules and protein abundance.

CaASR1 promotes salicylic acid- but represses jasmonic acid-dependent signaling to enhance the resistance of Capsicum annuum to bacterial wilt by modulating CabZIP63

CabZIP63 acts positively in the resistance of pepper (Capsicum annuum) to bacterial wilt caused by Ralstonia solanacearum or tolerance to high-temperature/high-humidity stress, but it is unclear how CabZIP63 achieves its functional specificity against R. solanacearum. Here, CaASR1, an abscisic acid-, stress-, and ripening-inducible protein of C. annuum, was functionally characterized in modulating the functional specificity of CabZIP63 during the defense response of pepper to R. solanacearum. In pepper plants inoculated with R. solanacearum, CaASR1 was up-regulated before 24 h post-inoculation but down-regulated thereafter, and was down-regulated by high-temperature/high-humidity stress. Data from gene silencing and transient overexpression experiments indicated that CaASR1 acts as a positive regulator in the immunity of pepper against R. solanacearum and a negative regulator of thermotolerance. Pull-down combined with mass spectrometry revealed that CaASR1 interacted with CabZIP63 upon R. solanacearum infection; the interaction was confirmed by microscale thermophoresis and bimolecular fluorescence complementation assays.CaASR1 silencing upon R. solanacearum inoculation repressed CabZIP63-mediated transcription from the promoters of the salicylic acid (SA)-dependent CaPR1 and CaNPR1, but derepressed transcription of CaHSP24 and the jasmonic acid (JA)-dependent CaDEF1. Our findings suggest that CaASR1 acts as a positive regulator of the defense response of pepper to R. solanacearum by interacting with CabZIP63, enabling it to promote SA-dependent but repress JA-dependent immunity and thermotolerance during the early stages of infection.

Pepper CaMLO6 Negatively Regulates Ralstonia solanacearum Resistance and Positively Regulates High Temperature and High Humidity Responses

Plant mildew-resistance locus O (MLO) proteins influence susceptibility to powdery mildew. However, their roles in plant responses to other pathogens and heat stress remain unclear. Here, we showed that CaMLO6, a pepper (Capsicum annuum) member of MLO clade V, is a protein targeted to plasma membrane and probably endoplasmic reticulum. The transcript expression level of CaMLO6 was upregulated in the roots and leaves of pepper plants challenged with high temperature and high humidity (HTHH) and was upregulated in leaves but downregulated in roots of plants infected with the bacterial pathogen Ralstonia solanacearum. CaMLO6 was also directly upregulated by CaWRKY40 upon HTHH but downregulated by CaWRKY40 upon R. solanacearum infection. Virus-induced gene silencing of CaMLO6 significantly decreased pepper HTHH tolerance and R. solanacearum susceptibility. Moreover, CaMLO6 overexpression enhanced the susceptibility of Nicotiana benthamiana and pepper plants to R. solanacearum and their tolerance to HTHH, effects that were associated with the expression of immunity- and thermotolerance-associated marker genes, respectively. These results suggest that CaMLO6 acts as a positive regulator in response to HTHH but a negative regulator in response to R. solanacearum. Moreover, CaMLO6 is transcriptionally affected by R. solanacearum and HTHH; these transcriptional responses are at least partially regulated by CaWRKY40.

FoldX accurate structural protein-DNA binding prediction using PADA1 (Protein Assisted DNA Assembly 1)

The speed at which new genomes are being sequenced highlights the need for genome-wide methods capable of predicting protein–DNA interactions. Here, we present PADA1, a generic algorithm that accurately models structural complexes and predicts the DNA-binding regions of resolved protein structures. PADA1 relies on a library of protein and double-stranded DNA fragment pairs obtained from a training set of 2103 DNA–protein complexes. It includes a fast statistical force field computed from atom-atom distances, to evaluate and filter the 3D docking models. Using published benchmark validation sets and 212 DNA–protein structures published after 2016 we predicted the DNA-binding regions with an RMSD of <1.8 Å per residue in >95% of the cases. We show that the quality of the docked templates is compatible with FoldX protein design tool suite to identify the crystallized DNA molecule sequence as the most energetically favorable in 80% of the cases. We highlighted the biological potential of PADA1 by reconstituting DNA and protein conformational changes upon protein mutagenesis of a meganuclease and its variants, and by predicting DNA-binding regions and nucleotide sequences in proteins crystallized without DNA. These results opens up new perspectives for the engineering of DNA–protein interfaces.

Identification and functional characterization of Histone-like DNA-binding protein in Nocardia seriolae (NsHLP) involved in cell apoptosis

Nocardia seriolae, a facultative intracellular bacterium, is the main pathogen of fish nocardiosis. Bioinformatic analysis showed that the histone-like DNA-binding protein (HLP) gene of N. seriolae (nshlp) encoded a secreted protein and might target the mitochondria in the host cell. To further study the preliminary function of HLP in N. seriolae (NsHLP), the gene cloning, extracellular products identification, subcellular localization, overexpression and apoptosis detection assay were carried out in this study. Mass spectrometry analysis of the extracellular products from N. seriolae showed that NsHLP was a secreted protein. Subcellular localization of HLP-GFP fusion proteins mainly assembled in the nucleus, which indicated that the NsHLP was co-located with the nucleus rather than mitochondria in fathead minnow (FHM) cells. Notably, the expression of NsHLP had changed the distribution of mitochondria into lumps in the FHM cell. In addition, apoptotic features were found in the transfected FHM cells by overexpression of NsHLP. Quantitative assays of mitochondrial membrane potential value, caspase-3 activity and pro-apoptotic genes mRNA (Bad, Bid and Bax) expression level demonstrated that the cell apoptosis was induced in the transfected FHM cells. All the results presented in this study provided insight on the function of NsHLP, which suggested that it may participate in the cell apoptosis regulation and play an important role in the pathogenesis of N. seriolae.

DNA silencing by prokaryotic Argonaute proteins adds a new layer of defense against invading nucleic acids

Argonaute (Ago) proteins are encoded in all three domains of life and are responsible for the regulation of intracellular nucleic acid levels. Whereas some Ago variants are able to cleave target nucleic acids by their endonucleolytic activity, others only bind to their target nucleic acids while target cleavage is mediated by other effector proteins. Although all Ago proteins show a high degree of overall structural homology, the nature of the nucleic acid binding partners differs significantly. Recent structural and functional data have provided intriguing new insights into the mechanisms of archaeal and bacterial Ago variants demonstrating the mechanistic diversity within the prokaryotic Ago family with astonishing differences in nucleic acid selection and nuclease specificity. In this review, we provide an overview of the structural organisation of archaeal Ago variants and discuss the current understanding of their biological functions that differ significantly from their eukaryotic counterparts.

Capsicum annuum HsfB2a Positively Regulates the Response to Ralstonia solanacearum Infection or High Temperature and High Humidity Forming Transcriptional Cascade with CaWRKY6 and CaWRKY40

The responses of pepper (Capsicum annuum) plants to inoculation with the pathogenic bacterium Ralstonia solanacearum and to high-temperature-high-humidity (HTHH) conditions were previously found to be coordinated by the transcription factors CaWRKY6 and CaWRKY40; however, the underlying molecular mechanism was unclear. Herein, we identified and functionally characterized CaHsfB2a, a nuclear-localized heat shock factor involved in pepper immunity to R. solanacearum inoculation (RSI) and tolerance to HTHH. CaHsfB2a is transcriptionally induced in pepper plants by RSI or HTHH and by exogenous application of salicylic acid (SA), methyl jasmonate (MeJA), ethylene (ETH), or abscisic acid (ABA). Virus-induced gene silencing (VIGS) of CaHsfB2a significantly impaired pepper immunity to RSI, hampered HTHH tolerance, and curtailed expression of immunity- and thermotolerance-associated marker genes such as CaHIR1, CaNPR1, CaABR1, and CaHSP24. Likewise, transient overexpression of CaHsfB2a in pepper leaves induced hypersensitive response (HR)-like cell death and H2O2 accumulation and upregulated the above-mentioned marker genes as well as CaWRKY6 and CaWRKY40. Chromatin immunoprecipitation (ChIP) and microscale thermophoresis (MST) analysis revealed that CaHsfB2a bound the promoters of both CaWRKY6 and CaWRKY40. In a parallel experiment, we determined by ChIP-PCR and MST that CaHsfB2a was regulated directly by CaWRKY40 but indirectly by CaWRKY6. Cumulatively, our results suggest that CaHsfB2a positively regulates plant immunity against RSI and tolerance to HTHH, via transcriptional cascades and positive feedback loops involving CaWRKY6 and CaWRKY40.

CaC3H14 encoding a tandem CCCH zinc finger protein is directly targeted by CaWRKY40 and positively regulates the response of pepper to inoculation by Ralstonia solanacearum

Tandem CCCH zinc finger (TZnF) proteins have been implicated in plant defence, but their role in pepper (Capsicum annuum) is unclear. In the present study, the role of CaC3H14, a pepper TZnF protein, in the immune response of pepper plants to Ralstonia solanacearum infection was characterized. When fused to the green fluorescent protein, CaC3H14 was localized exclusively to the nuclei in leaf cells of Nicotiana benthamiana plants transiently overexpressing CaC3H14. Transcript abundance of CaC3H14 was up-regulated by inoculation with R. solanacearum. Virus-induced silencing of CaC3H14 increased the susceptibility of the plants to R. solanacearum and down-regulated the genes associated with the hypersensitive response (HR), specifically HIR1 and salicylic acid (SA)-dependent PR1a. By contrast, silencing resulted in the up-regulation of jasmonic acid (JA)-dependent DEF1 and ethylene (ET) biosynthesis-associated ACO1. Transient overexpression of CaC3H14 in pepper triggered an intensive HR, indicated by cell death and hydrogen peroxide (H₂ O₂ ) accumulation, up-regulated PR1a and down-regulated DEF1 and ACO1. Ectopic overexpression of CaC3H14 in tobacco plants significantly decreased the susceptibility of tobacco plants to R. solanacearum. It also up-regulated HR-associated HSR515, immunity-associated GST1 and the SA-dependent marker genes NPR1 and PR2, but down-regulated JA-dependent PR1b and ET-dependent EFE26. The CaC3H14 promoter and was bound and its transcription was up-regulated by CaWRKY40. Collectively, these results indicate that CaC3H14 is transcriptionally targeted by CaWRKY40, is a modulator of the antagonistic interaction between SA and JA/ET signalling, and enhances the defence response of pepper plants to infection by R. solanacearum.

NMR elucidation of monomer-dimer transition and conformational heterogeneity in histone-like DNA binding protein of Helicobacter pylori

Helicobacter pylori (H. pylori) colonizes under harsh acidic/oxidative stress conditions of human gastrointestinal tract and can survive there for infinitely longer durations of host life. The bacterium expresses several harbinger proteins to facilitate its persistent colonization under such conditions. One such protein in H. pylori is histone-like DNA binding protein (Hup), which in its homo-dimeric form binds to DNA to perform various DNA dependent cellular activities. Further, it also plays an important role in protecting the genomic DNA from oxidative stress and acidic denaturation. Legitimately, if the binding of Hup to DNA is suppressed, it will directly impact on the survival of the bacterium, thus making Hup a potential therapeutic target for developing new anti-H. pylori agents. However, to inhibit the binding of Hup to DNA, it is necessary to gain detailed insights into the molecular and structural basis of Hup-dimerization and its binding mechanism to DNA. As a first step in this direction, we report here the nuclear magnetic resonance (NMR) assignments and structural features of Hup at pH 6.0. The study revealed the occurrence of dynamic equilibrium between its monomer and dimer conformations. The dynamic equilibrium was found to shifting towards dimer both at low temperature and low pH; whereas DNA binding studies evidenced that the protein binds to DNA in its dimeric form. These preliminary investigations correlate very well with the diverse functionality of protein and will form the basis for future studies aiming to develop novel anti-H. pylori agents employing structure-based-rational drug discovery approach.

Structural plasticity and thermal stability of the histone-like protein from Spiroplasma melliferum are due to phenylalanine insertions into the conservative scaffold

The histone-like (HU) protein is one of the major nucleoid-associated proteins of the bacterial nucleoid, which shares high sequence and structural similarity with IHF but differs from the latter in DNA-specificity. Here, we perform an analysis of structural-dynamic properties of HU protein from Spiroplasma melliferum and compare its behavior in solution to that of another mycoplasmal HU from Mycoplasma gallisepticum. The high-resolution heteronuclear NMR spectroscopy was coupled with molecular-dynamics study and comparative analysis of thermal denaturation of both mycoplasmal HU proteins. We suggest that stacking interactions in two aromatic clusters in the HUSpm dimeric interface determine not only high thermal stability of the protein, but also its structural plasticity experimentally observed as slow conformational exchange. One of these two centers of stacking interactions is highly conserved among the known HU and IHF proteins. Second aromatic core described recently in IHFs and IHF-like proteins is considered as a discriminating feature of IHFs. We performed an electromobility shift assay to confirm high affinities of HUSpm to both normal and distorted dsDNA, which are the characteristics of HU protein. MD simulations of HUSpm with alanine mutations of the residues forming the non-conserved aromatic cluster demonstrate its role in dimer stabilization, as both partial and complete distortion of the cluster enhances local flexibility of HUSpm.

Comparison of histone-like HU protein DNA-binding properties and HU/IHF protein sequence alignment

Background

The structure and function of bacterial nucleoid are controlled by histone-like proteins of HU/IHF family, omnipresent in bacteria and also founding archaea and some eukaryotes.HU protein binds dsDNA without sequence specificity and avidly binds DNA structures with propensity to be inclined such as forks, three/four-way junctions, nicks, overhangs and DNA bulges. Sequence comparison of thousands of known histone-like proteins from diverse bacteria phyla reveals relation between HU/IHF sequence, DNA–binding properties and other protein features.

Methodology and principal findings

Performed alignment and clusterization of the protein sequences show that HU/IHF family proteins can be unambiguously divided into three groups, HU proteins, IHF_A and IHF_B proteins. HU proteins, IHF_A and IHF_B proteins are further partitioned into several clades for IHF and HU; such a subdivision is in good agreement with bacterial taxonomy. We also analyzed a hundred of 3D fold comparative models built for HU sequences from all revealed HU clades. It appears that HU fold remains similar in spite of the HU sequence variations. We studied DNA–binding properties of HU from N. gonorrhoeae, which sequence is similar to one of E.coli HU, and HU from M. gallisepticum and S. melliferum which sequences are distant from E.coli protein. We found that in respect to dsDNA binding, only S. melliferum HU essentially differs from E.coli HU. In respect to binding of distorted DNA structures, S. melliferum HU and E.coli HU have similar properties but essentially different from M. gallisepticum HU and N. gonorrhea HU. We found that in respect to dsDNA binding, only S. melliferum HU binds DNA in non-cooperative manner and both mycoplasma HU bend dsDNA stronger than E.coli and N. gonorrhoeae. In respect to binding to distorted DNA structures, each HU protein has its individual profile of affinities to various DNA-structures with the increased specificity to DNA junction.

Conclusions and significance

HU/IHF family proteins sequence alignment and classification are updated. Comparative modeling demonstrates that HU protein 3D folding’s even more conservative than HU sequence. For the first time, DNA binding characteristics of HU from N. gonorrhoeae, M. gallisepticum and S. melliferum are studied. Here we provide detailed analysis of the similarity and variability of DNA-recognizing and bending of four HU proteins from closely and distantly related HU clades.