Mutation of a broadly conserved operon (RL3499-RL3502) from Rhizobium leguminosarum biovar viciae causes defects in cell morphology and envelope integrity

Potential roles of gut microbiota in metal mixture and bone mineral density and osteoporosis risk association: an epidemiologic study in Wuhan

Few studies have focused on the effects of multiple metal mixtures on bone health and the underlying mechanisms related to alterations in the gut microbiota. This study aimed to examine the potential roles of gut microbiota alterations in metal mixtures and their association with osteoporosis traits. Adults aged ≥ 55 years were recruited from two community healthcare centers in Wuhan City during 2016-2019. The plasma concentrations of six metals (zinc, iron, selenium, lead, cadmium, and arsenic) were measured using an inductively coupled plasma mass spectrometer. The k-means clustering method was employed to explore the exposure profiles of metal mixtures for all participants. 16S rRNA gene sequencing was used to profile the gut microbiota of participants. Combining these results with those of our previous study, we identified overlapping taxa and evaluated their potential roles. A total of 806 participants (516 females), with an average age of 67.36 years were included. The participants were grouped into three clusters using k-means clustering: Cluster 1 (n = 458), Cluster 2 (n = 199), and Cluster 3 (n = 149). The high-exposure group for iron, zinc, lead, and cadmium (Cluster 3) showed a negative association with lumbar spine 1-4 bone mineral density (BMD). A total of 201 individuals (121 females) underwent sequencing of the gut microbiota. Both alpha and beta diversities were statistically different among the three groups. Bacteroidaceae, Lachnospiraceae, Bifidobacteriaceae, Bacteroides, and Lachnospiraceae_incertae_sedis were identified as overlapping taxa associated with the metal mixtures and BMD. Interaction analysis revealed that Cluster 3 interacted with Bacteroidaceae/Bacteroides, resulting in a positive effect on LS1-4 BMD (β = 0.358 g/cm2, 95% CI: 0.047 to 0.669, P = 0.025). Our findings indicate associations between multiple metal mixtures and BMD as well as gut microbiota alterations. Exploring the interaction between metal mixtures and the gut microbiota provides new perspectives for the precise prevention and treatment of osteoporosis.

Leptospira interrogans Bat proteins impair host hemostasis by fibrinogen cleavage and platelet aggregation inhibition

Leptospirosis is a worldwide spread zoonosis, caused by pathogenic Leptospira. Evidences suggest that compromised hemostasis might be involved in the leptospirosis pathophysiology. In the genome of L. interrogans serovar Copenhageni, we found two genes coding for proteins which comprise von Willebrand factor (VWF) A domains (BatA and BatB). As VWF A domains exhibit multiple binding sites which contributes to human VWF hemostatic functions, we hypothesized that the L. interrogans BatA and BatB proteins could be involved in the hemostatic impairment during leptospirosis. We have cloned, expressed in Escherichia coli, and purified recombinant BatA and BatB. The influence of recombinant BatA and BatB on different in vitro hemostatic assays evaluating the enzymatic activity, platelet aggregation and fibrinogen integrity was investigated. We describe BatB as a new serine protease which is able to cleave thrombin chromogenic substrate, fibrin, fibrinogen, gelatin and casein; while BatA is active only towards fibrinogen. BatA and BatB interfere with the platelet aggregation induced by VWF/ristocetin and thrombin. Our results suggest an important role of the L. interrogans serovar Copenhageni Bat proteins in the hemostasis dysfunction observed during leptospirosis and contribute to the understanding of the leptospirosis pathophysiological mechanisms.

Shedding light on the presymbiontic phase of C. arietinum

A complex network of symbiotic events between plants and bacteria allows the biosphere to exploit the atmospheric reservoir of molecular nitrogen. In seeds, a series of presymbiotic steps are already identified during imbibition, while interactions between the host and its symbiont begin in the early stages of germination. In the present study, a detailed analysis of the substances' complex delivered by Cicer arietinum seeds during imbibition showed a relevant presence of proteins and amino acids, which, except for cysteine, occurred with the whole proteinogenic pool. The imbibing solution was found to provide essential probiotic properties able to sustain the growth of the specific chickpea symbiont Mesorhizobium ciceri. Moreover, the imbibing solution, behaving as a complete medium, was found to be critically important for the symbiont's attraction, a fact this that is strictly related to the presence of the amino acids glycine, serine, and threonine. Here, the presence of these amino acids is constantly supported by the presence of the enzymes serine hydroxymethyltransferase and formyltetrahydrofolate deformylase, which are both involved in their biosynthesis. The reported findings are discussed in the light of the pivotal role played by the imbibing solution in attracting and sustaining symbiosis between the host and its symbiont.

Brucella Periplasmic Protein EipB Is a Molecular Determinant of Cell Envelope Integrity and Virulence

The Gram-negative cell envelope is a remarkable structure with core components that include an inner membrane, an outer membrane, and a peptidoglycan layer in the periplasmic space between. Multiple molecular systems function to maintain integrity of this essential barrier between the interior of the cell and its surrounding environment. We show that a conserved DUF1849 family protein, EipB, is secreted to the periplasmic space of Brucella species, a monophyletic group of intracellular pathogens. In the periplasm, EipB folds into an unusual 14-stranded β-spiral structure that resembles the LolA and LolB lipoprotein delivery system, though the overall fold of EipB is distinct from LolA/LolB. Deletion of eipB results in defects in Brucella cell envelope integrity in vitro and in maintenance of spleen colonization in a mouse model of Brucella abortus infection. Transposon disruption of ttpA, which encodes a periplasmic protein containing tetratricopeptide repeats, is synthetically lethal with eipB deletion. ttpA is a reported virulence determinant in Brucella, and our studies of ttpA deletion and overexpression strains provide evidence that this gene also contributes to cell envelope function. We conclude that eipB and ttpA function in the Brucella periplasmic space to maintain cell envelope integrity, which facilitates survival in a mammalian host.IMPORTANCE Brucella species cause brucellosis, a global zoonosis. A gene encoding a conserved DUF1849-family protein, which we have named EipB, is present in all sequenced Brucella and several other genera in the class Alphaproteobacteria The manuscript provides the first functional and structural characterization of a DUF1849 protein. We show that EipB is secreted to the periplasm where it forms a spiral-shaped antiparallel β protein that is a determinant of cell envelope integrity in vitro and virulence in an animal model of disease. eipB genetically interacts with ttpA, which also encodes a periplasmic protein. We propose that EipB and TtpA function as part of a system required for cell envelope homeostasis in select Alphaproteobacteria.

Novel Genes and Regulators That Influence Production of Cell Surface Exopolysaccharides in Sinorhizobium meliloti

Sinorhizobium meliloti is a soil-dwelling alphaproteobacterium that engages in a nitrogen-fixing root nodule symbiosis with leguminous plants. Cell surface polysaccharides are important both for adapting to stresses in the soil and for the development of an effective symbiotic interaction. Among the polysaccharides characterized to date, the acidic exopolysaccharides I (EPS-I; succinoglycan) and II (EPS-II; galactoglucan) are particularly important for protection from abiotic stresses, biofilm formation, root colonization, and infection of plant roots. Previous genetic screens discovered mutants with impaired EPS production, allowing the delineation of EPS biosynthetic pathways. Here we report on a genetic screen to isolate mutants with mucoid colonial morphologies that suggest EPS overproduction. Screening with Tn5-110, which allows the recovery of both null and upregulation mutants, yielded 47 mucoid mutants, most of which overproduce EPS-I; among the 30 unique genes and intergenic regions identified, 14 have not been associated with EPS production previously. We identified a new protein-coding gene, emmD, which may be involved in the regulation of EPS-I production as part of the EmmABC three-component regulatory circuit. We also identified a mutant defective in EPS-I production, motility, and symbiosis, where Tn5-110 was not responsible for the mutant phenotypes; these phenotypes result from a missense mutation in rpoA corresponding to the domain of the RNA polymerase alpha subunit known to interact with transcription regulators.IMPORTANCE The alphaproteobacterium Sinorhizobium meliloti converts dinitrogen to ammonium while inhabiting specialized plant organs termed root nodules. The transformation of S. meliloti from a free-living soil bacterium to a nitrogen-fixing plant symbiont is a complex developmental process requiring close interaction between the two partners. As the interface between the bacterium and its environment, the S. meliloti cell surface plays a critical role in adaptation to varied soil environments and in interaction with plant hosts. We isolated and characterized S. meliloti mutants with increased production of exopolysaccharides, key cell surface components. Our diverse set of mutants suggests roles for exopolysaccharide production in growth, metabolism, cell division, envelope homeostasis, biofilm formation, stress response, motility, and symbiosis.

Characterization of the inner membrane protein BB0173 from Borrelia burgdorferi

BACKGROUND

The bacterial spirochete Borrelia burgdorferi is the causative agent of the most commonly reported arthropod-borne illness in the United States, Lyme disease. A family of proteins containing von Willebrand Factor A (VWFA) domains adjacent to a MoxR AAA+ ATPase have been found to be highly conserved in the genus Borrelia. Previously, a VWFA domain containing protein of B. burgdorferi, BB0172, was determined to be an outer membrane protein capable of binding integrin α3β1. In this study, the characterization of a new VWFA domain containing membrane protein, BB0173, is evaluated in order to define the location and topology of this multi-spanning membrane protein. In addition, functional predictions are made.

RESULTS

Our results show that BB0173, in contrast to BB0172, is an inner membrane protein, in which the VWFA domain is exposed to the periplasmic space. Further, BB0173 was predicted to have an aerotolerance regulator domain, and expression of BB0173 and the surrounding genes was evaluated under aerobic and microaerophilic conditions, revealing that these genes are downregulated under aerobic conditions. Since the VWFA domain containing proteins of B. burgdorferi are highly conserved, they are likely required for survival of the pathogen through sensing diverse environmental oxygen conditions.

CONCLUSIONS

Presently, the complex mechanisms that B. burgdorferi uses to detect and respond to environmental changes are not completely understood. However, studying the mechanisms that allow B. burgdorferi to survive in the highly disparate environments of the tick vector and mammalian host could allow for the development of novel methods of preventing acquisition, survival, or transmission of the spirochete. In this regard, a putative membrane protein, BB0173, was characterized. BB0173 was found to be highly conserved across pathogenic Borrelia, and additionally contains several truly transmembrane domains, and a Bacteroides aerotolerance-like domain. The presence of these functional domains and the highly conserved nature of this protein, strongly suggests a required function of BB0173 in the survival of B. burgdorferi.

An uncharacterized gene coding a conserved lytic transglycosylase domain (RL4716) is required for proper cell envelope function in Rhizobium leguminosarum

Rhizobium leguminosarum is a plant-associated bacterium that can form a symbiotic relationship with leguminous plants. Rhizobia must respond to significantly different environments during their biphasic lifestyle. The cell envelope is an important cellular feature that must be able to adapt to changing environments. Mutations in rhizobial genes required for proper cell envelope development have been identified based on growth deficiencies on peptide-rich media. Using transposon mutagenesis and screening of mutants for loss of growth on peptide-rich media, this study identified RL4716 as being required for proper cell envelope function in R. leguminosarum. Mutation of RL4716 results in an altered cell morphology, and an increase in permeability to the non-polar probe 1-N-phenylnaphthylamine, indicating a role of RL4716 in maintaining cell envelope integrity. The mutation also affected phenotypes that are known to be dependent on genes associated with a functional cell envelope including decreased desiccation tolerance and a decreased ability to form biofilms.

A Biosensor-Based Leaf Punch Assay for Glutamine Correlates to Symbiotic Nitrogen Fixation Measurements in Legumes to Permit Rapid Screening of Rhizobia Inoculants under Controlled Conditions

Legumes are protein sources for billions of humans and livestock. These traits are enabled by symbiotic nitrogen fixation (SNF), whereby root nodule-inhabiting rhizobia bacteria convert atmospheric nitrogen (N) into usable N. Unfortunately, SNF rates in legume crops suffer from undiagnosed incompatible/suboptimal interactions between crop varieties and rhizobia strains. There are opportunities to test much large numbers of rhizobia strains if cost/labor-effective diagnostic tests become available which may especially benefit researchers in developing countries. Inside root nodules, fixed N from rhizobia is assimilated into amino acids including glutamine (Gln) for export to shoots as the major fraction (amide-exporting legumes) or as the minor fraction (ureide-exporting legumes). Here, we have developed a new leaf punch based technique to screen rhizobia inoculants for SNF activity following inoculation of both amide exporting and ureide exporting legumes. The assay is based on measuring Gln output using the GlnLux biosensor, which consists of Escherichia coli cells auxotrophic for Gln and expressing a constitutive lux operon. Subsistence farmer varieties of an amide exporter (lentil) and two ureide exporters (cowpea and soybean) were inoculated with different strains of rhizobia under controlled conditions, then extracts of single leaf punches were incubated with GlnLux cells, and light-output was measured using a 96-well luminometer. In the absence of external N and under controlled conditions, the results from the leaf punch assay correlated with ¹⁵N-based measurements, shoot N percentage, and shoot total fixed N in all three crops. The technology is rapid, inexpensive, high-throughput, requires minimum technical expertise and very little tissue, and hence is relatively non-destructive. We compared and contrasted the benefits and limitations of this novel diagnostic assay to methods.

The Use of Transposon Insertion Sequencing to Interrogate the Core Functional Genome of the Legume Symbiont Rhizobium leguminosarum

The free-living legume symbiont Rhizobium leguminosarum is of significant economic value because of its ability to provide fixed nitrogen to globally important leguminous food crops, such as peas and lentils. Discovery based research into the genetics and physiology of R. leguminosarum provides the foundational knowledge necessary for understanding the bacterium's complex lifestyle, necessary for augmenting its use in an agricultural setting. Transposon insertion sequencing (INSeq) facilitates high-throughput forward genetic screening at a genomic scale to identify individual genes required for growth in a specific environment. In this study we applied INSeq to screen the genome of R. leguminosarum bv. viciae strain 3841 (RLV3841) for genes required for growth on minimal mannitol containing medium. Results from this study were contrasted with a prior INSeq experiment screened on peptide rich media to identify a common set of functional genes necessary for basic physiology. Contrasting the two growth conditions indicated that approximately 10% of the chromosome was required for growth, under both growth conditions. Specific genes that were essential to singular growth conditions were also identified. Data from INSeq screening on mannitol as a sole carbon source were used to reconstruct a metabolic map summarizing growth impaired phenotypes observed in the Embden-Meyerhof-Parnas pathway, Entner-Doudoroff pathway, pentose phosphate pathway, and tricarboxylic acid cycle. This revealed the presence of mannitol dependent and independent metabolic pathways required for growth, along with identifying metabolic steps with isozymes or possible carbon flux by-passes. Additionally, genes were identified on plasmids pRL11 and pRL12 that are likely to encode functional activities important to the central physiology of RLV3841.

Construction of a mariner-based transposon vector for use in insertion sequence mutagenesis in selected members of the Rhizobiaceae

Background

The Rhizobiaceae family of Gram-negative bacteria often engage in symbiosis with plants of economic importance. Historically, genetic studies to identify the function of individual genes, and characterize the biology of these bacteria have relied on the use of classical transposon mutagenesis. To increase the rate of scientific discovery in the Rhizobiaceae there is a need to adapt high-throughput genetic screens like insertion sequencing for use in this family of bacteria. Here we describe a Rhizobiaceae compatible MmeI-adapted mariner transposon that can be used with insertion sequencing for high-throughput genetic screening.

Results

The newly constructed mariner transposon pSAM_Rl mutagenized R. leguminosarum, S. meliloti, and A. tumefaciens at a high frequency. In R. leguminosarum, mutant pools were generated that saturated 88% of potential mariner insertions sites in the genome. Analysis of the R. leguminosarum transposon insertion sequencing data with a previously described hidden Markov model-based method resulted in assignment of the contribution of all annotated genes in the R. leguminosarum 3841 genome for growth on a complex medium. Good concordance was observed between genes observed to be required for growth on the complex medium, and previous studies.

Conclusions

The newly described Rhizobiaceaee compatible mariner transposon insertion sequencing vector pSAM_Rl has been shown to mutagenize at a high frequency and to be an effective tool for use in high-throughput genetic screening. The construction and validation of this transposon insertion sequencing tool for use in the Rhizobiziaceae will provide an opportunity for researchers in the Rhizobiaceae community to use high-throughput genetic screening, allowing for significant increase in the rate of genetic discovery, particularly given the recent release of genome sequences from many Rhizobiaceae strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0298-z) contains supplementary material, which is available to authorized users.

A previously uncharacterized tetratricopeptide-repeat-containing protein is involved in cell envelope function in Rhizobium leguminosarum

Rhizobium leguminosarum is a soil bacterium that is an intracellular symbiont of leguminous plants through the formation of nitrogen-fixing root nodules. Due to the changing environments that rhizobia encounter, the cell is often faced with a variety of cell altering stressors that can compromise the cell envelope integrity. A previously uncharacterized operon (RL3499-RL3502) has been linked to proper cell envelope function, and mutants display pleiotropic phenotypes including an inability to grow on peptide-rich media. In order to identify functional partners to the operon, suppressor mutants capable of growth on complex, peptide-rich media were isolated. A suppressor mutant of a non-polar mutation to RL3500 was chosen for further characterization. Transposon mutagenesis, screening for loss of the suppressor phenotype, led to the identification of a Tn5 insertion in an uncharacterized tetratricopeptide-repeat-containing protein RL0936. Furthermore, RL0936 had a 3.5-fold increase in gene expression in the suppressor strain when compared with the WT and a 1.5-fold increase in the original RL3500 mutant. Mutation of RL0936 decreased desiccation tolerance and lowered the ability to form biofilms when compared with the WT strain. This work has identified a potential interaction between RL0936 and the RL3499-RL3502 operon that is involved in cell envelope development in R. leguminosarum, and has described phenotypic activities to a previously uncharacterized conserved hypothetical gene.

Proteomic analysis of free-living Bradyrhizobium diazoefficiens: highlighting potential determinants of a successful symbiosis

BACKGROUND

Strain CPAC 7 (=SEMIA 5080) was recently reclassified into the new species Bradyrhizobium diazoefficiens; due to its outstanding efficiency in fixing nitrogen, it has been used in commercial inoculants for application to crops of soybean [Glycine max (L.) Merr.] in Brazil and other South American countries. Although the efficiency of B. diazoefficiens inoculant strains is well recognized, few data on their protein expression are available.

RESULTS

We provided a two-dimensional proteomic reference map of CPAC 7 obtained under free-living conditions, with the successful identification of 115 spots, representing 95 different proteins. The results highlighted the expression of molecular determinants potentially related to symbiosis establishment (e.g. inositol monophosphatase, IMPase), fixation of atmospheric nitrogen (N2) (e.g. NifH) and defenses against stresses (e.g. chaperones). By using bioinformatic tools, it was possible to attribute probable functions to ten hypothetical proteins. For another ten proteins classified as "NO related COG" group, we analyzed by RT-qPCR the relative expression of their coding-genes in response to the nodulation-gene inducer genistein. Six of these genes were up-regulated, including blr0227, which may be related to polyhydroxybutyrate (PHB) biosynthesis and competitiveness for nodulation.

CONCLUSIONS

The proteomic map contributed to the identification of several proteins of B. diazoefficiens under free-living conditions and our approach-combining bioinformatics and gene-expression assays-resulted in new information about unknown genes that might play important roles in the establishment of the symbiosis with soybean.

Architecture and characterization of a thermostable MoxR family AAA(+) ATPase from Thermococcus kodakarensis KOD1

AAA(+) ATPases are ubiquitous enzymes that can function as molecular chaperones, employing the energy obtained from ATP hydrolysis to remodel macromolecules. In this report, the MoxR enzyme from Thermococcus kodakarensis KOD1 (TkMoxR) was shown to have two native forms: a two-stack hexameric ring and a hexameric structure, under physiological conditions and cold stress, respectively. TkMoxR was altered to a microtubule-like form in the presence of ATP and tightly interacted with dsDNA molecules of various lengths. In addition, the two-stack hexameric protein catalyzed dsDNA decomposition to form and then release ssDNA, whereas the hexamer TkMoxR structure interacted with but did not release dsDNA. These results suggest that TkMoxR has DNA helicase activity involved in gene expression control.

The MoxR ATPase RavA and its cofactor ViaA interact with the NADH:ubiquinone oxidoreductase I in Escherichia coli

MoxR ATPases are widespread throughout bacteria and archaea. The experimental evidence to date suggests that these proteins have chaperone-like roles in facilitating the maturation of dedicated protein complexes that are functionally diverse. In Escherichia coli, the MoxR ATPase RavA and its putative cofactor ViaA are found to exist in early stationary-phase cells at 37 °C at low levels of about 350 and 90 molecules per cell, respectively. Both proteins are predominantly localized to the cytoplasm, but ViaA was also unexpectedly found to localize to the cell membrane. Whole genome microarrays and synthetic lethality studies both indicated that RavA-ViaA are genetically linked to Fe-S cluster assembly and specific respiratory pathways. Systematic analysis of mutant strains of ravA and viaA indicated that RavA-ViaA sensitizes cells to sublethal concentrations of aminoglycosides. Furthermore, this effect was dependent on RavA's ATPase activity, and on the presence of specific subunits of NADH:ubiquinone oxidoreductase I (Nuo Complex, or Complex I). Importantly, both RavA and ViaA were found to physically interact with specific Nuo subunits. We propose that RavA-ViaA facilitate the maturation of the Nuo complex.

Homoserine catabolism by Rhizobium leguminosarum bv. viciae 3841 requires a plasmid-borne gene cluster that also affects competitiveness for nodulation

Homoserine represents a substantial component of pea root exudate that may be important for plant-microbe interactions in the rhizosphere. We identified a gene cluster on plasmid pRL8JI that is required for homoserine utilization by Rhizobium leguminosarum bv. viciae. The genes are arranged as two divergently expressed predicted operons that were induced by L-homoserine, pea root exudate, and were expressed on pea roots. A mutation in gene pRL80083 that prevented utilization of homoserine as a sole carbon and energy source affected the mutant's ability to nodulate peas and lentils competitively. The homoserine gene cluster was present in approximately 47% of natural R. leguminosarum isolates (n = 59) and was strongly correlated with homoserine utilization. Conjugation of pRL8JI to R. leguminosarum 4292 or Agrobacterium tumefaciens UBAPF2 was sufficient for homoserine utilization. The presence of L-homoserine increased conjugation efficiency of pRL8JI from R. leguminosarum to a pRL8JI-cured derivative of R. leguminosarum 1062 and to A. tumefaciens UBAPF2, and induced expression of the plasmid transfer gene trbB; however, there was no difference in conjugation efficiency or trbB expression with A. tumefaciens UBAPF2pRL8-Gm as the donor suggesting that other genes in R. leguminosarum may contribute to regulating conjugation of pRL8 in the presence of homoserine.

Genetic analysis reveals links between lipid A structure and expression of the outer membrane protein gene, ropB, in Rhizobium leguminosarum

The fabXL genes encode enzymes that synthesize the very-long-chain fatty acid - a unique acyl modification located at the 2' position of the lipid A of Gram-negative bacteria in the order Rhizobiales. Mutation of the fabXL genes causes sensitivity to outer membrane stressors and other envelope-related stresses; however, the underlying mechanisms for increased sensitivity are poorly understood. We found that expression of the outer membrane protein gene ropB is down-regulated in an acpXL mutant. Furthermore, constitutive expression of ropB in an acpXL or fabF2XL, fabF1XL mutant restores tolerance to detergents, hyperosmotic stress, and acidic pH. The fabF2XL, fabF1XL mutant also has a delayed nodulation phenotype, whereas a ropB mutant has no observable defects in nodulation, demonstrating that mutation of the fabXL genes results in pleiotropic phenotypes that can be classified as either ropB dependent or ropB independent. Ex-nodule isolates of the mutant strains display restored tolerance to detergents and hyperosmotic and acidic stress conditions; however, the rescued phenotypes are not owing to increased ropB expression. Finally, we found that the fabXL genes are induced by the sensor kinase ChvG in response to peptide-rich growth conditions, which is similar to the results reported for induction of ropB.

Novel structural and functional insights into the MoxR family of AAA+ ATPases

The MoxR family of AAA+ ATPases is widespread among bacteria and archaea, although their cellular functions are not well characterized. Based on recent studies, MoxR ATPases are proposed to have chaperone-like function for the maturation of specific protein complexes or for the insertion of cofactors into proteins. MoxR proteins have been found to be important modulators of multiple stress response pathways in different organisms. For example, the respective MoxR proteins have been found to play important roles in the cell envelope stress response in Rhizobium leguminosarum, in the oxidative stress, acid stress, and heat stress responses in Francisella tularensis, in the acid stress and stringent responses in Escherichia coli, in viral tail formation in the crenarchaeal Acidianus two-tailed virus, and in the utilization of carbon monoxide as the sole carbon source by the Gram-negative chemolithoautotrophe Oligotropha carboxidovorans. Recent structural studies on the MoxR proteins from E. coli and Cytophaga hutchinsonii show the unique spatial arrangement of the αβα and all-α subdomains of the AAA+ domain in these proteins compared to the typical arrangement found in canonical AAA+ proteins such as HslU. The spatial organization of the subdomains in the AAA+ domain of MoxR proteins is similar to that found in the ATPase component of the magnesium chelatase complexes, possibly suggesting a similar mechanism of function. In this review, we provide an overview of the newly identified functions and the newly obtained structures of MoxR AAA+ ATPases.

Mutation of the sensor kinase chvG in Rhizobium leguminosarum negatively impacts cellular metabolism, outer membrane stability, and symbiosis

Two-component signal transduction systems (TCS) are a main strategy used by bacteria to sense and adapt to changes in their environment. In the legume symbiont Rhizobium leguminosarum biovar viciae VF39, mutation of chvG, a histidine kinase, caused a number of pleiotropic phenotypes. ChvG mutants are unable to grow on proline, glutamate, histidine, or arginine as the sole carbon source. The chvG mutant secreted smaller amounts of acidic and neutral surface polysaccharides and accumulated abnormally large amounts of poly-ß-hydroxybutyrate. Mutation of chvG caused symbiotic defects on peas, lentils, and vetch; nodules formed by the chvG mutant were small and white and contained only a few cells that had failed to differentiate into bacteroids. Mutation of chvG also destabilized the outer membrane of R. leguminosarum, resulting in increased sensitivity to membrane stressors. Constitutive expression of ropB, the outer membrane protein-encoding gene, restored membrane stability and rescued the sensitivity phenotypes described above. Similar phenotypes have been described for mutations in other ChvG-regulated genes encoding a conserved operon of unknown function and in the fabXL genes required for synthesis of the lipid A very-long-chain fatty acid, suggesting that ChvG is a key component of the envelope stress response in Rhizobium leguminosarum. Collectively, the results of this study demonstrate the important and unique role the ChvG/ChvI TCS plays in the physiology, metabolism, and symbiotic competency of R. leguminosarum.