The ssu locus plays a key role in organosulfur metabolism in Pseudomonas putida S-313

Current understanding of enzyme structure and function in bacterial two-component flavin-dependent desulfonases: Cleaving C-S bonds of organosulfur compounds

The inherent structural properties of enzymes are critical in defining catalytic function. Often, studies to evaluate the relationship between structure and function are limited to only one defined structural element. The two-component flavin-dependent desulfonase family of enzymes involved in bacterial sulfur acquisition utilize a comprehensive range of structural features to carry out the desulfonation of organosulfur compounds. These metabolically essential two-component FMN-dependent desulfonase systems have been proposed to utilize oligomeric changes, protein-protein interactions for flavin transfer, and common mechanistic steps for carbon-sulfur bond cleavage. This review is focused on our current functional and structural understanding of two-component FMN-dependent desulfonase systems from multiple bacterial sources. Mechanistic and structural comparisons from recent independent studies provide fresh insights into the overall functional properties of these systems and note areas in need of further investigation. The review acknowledges current studies focused on evaluating the structural properties of these enzymes in relationship to their distinct catalytic function. The role of these enzymes in maintaining adequate sulfur levels, coupled with the conserved nature of these enzymes in diverse bacteria, underscore the importance in understanding the functional and structural nuances of these systems.

Impact of Rhizospheric Microbiome on Rice Cultivation

The rhizosphere niche is extremely important for the overall growth and development of plants. Evidently, it is necessary to understand the complete mechanism of plant microbe interactions of the rhizosphere for sustainable and low input productivity. To meet the increasing global food demand, rice (Oryza sativa L.) agriculture seeks optimal conditions. The unique oxic-anoxic interface of rice-growing soil has invited divergent microbes with dynamic biogeochemical cycles. This review provides the systematic analysis of microbes associated with the major biogeochemical cycles with the aim to generate better management strategies of rhizospheric microbiome in the field of rice agriculture. For instance, several methanogenic and methanotrophic bacteria in the rice rhizosphere make an equilibrium for methane concentration in the environment. The carbon sequestration in paddy soil is again done through many rhizospheric microorganisms that can directly assimilate CO2 with their photoautotrophic mode of nutrition. Also the phosphate solubilizing microbes remain to be the most important keys for the PGPR activity of the paddy ecosystem. In addition, rhizospheric microbiome remain crucial in degradation and solubilization of organo-sulfur and insoluble inorganic sulfides which can be taken by the plants. Further, this review elucidates on the advantages of using metagenomic and metaproteomic approaches as an alternative of traditional approaches to understand the overall metabolic pathways operational in paddy-field. These knowledges are expected to open new possibilities for designing the balanced microbiome used as inoculum for intensive farming and will eventually lead to exert positive impacts on rice cultivation.

Antibacterial Activities of the Algal Bromophenol Methylrhodomelol Against Pseudomonas aeruginosa

Methylrhodomelol (1: ) is a bromophenol from the red alga Vertebrata lanosa that has been associated with antimicrobial properties. The aim of the current study was, therefore, to assess the antimicrobial potential of this compound in more detail against the gram-negative pathogen Pseudomonas aeruginosa. 1: exerted weak bacteriostatic activity against different strains when grown in minimal medium, whereas other phenolics were inactive. In addition, 1: (35 and 10 µg/mL) markedly enhanced the susceptibility of multidrug-resistant P. aeruginosa toward the aminoglycoside gentamicin, while it did not affect the viability of Vero kidney cells up to 100 µM. Finally, pyoverdine release was reduced in bacteria treated at sub-inhibitory concentration, but no effect on other virulence factors was observed. Transcriptome analysis of treated versus untreated P. aeruginosa indicated an interference of 1: with bacterial carbon and energy metabolism, which was corroborated by RT-qPCR and decreased ATP-levels in treated bacteria. In summary, the current study characterized the antibacterial properties of methylrhodomelol, revealed its potential as an adjuvant to standard antibiotics, and generated a hypothesis on its mode of action.

Targeting Iron - Respiratory Reciprocity Promotes Bacterial Death

Discovering new bacterial signaling pathways offers unique antibiotic strategies. Here, through an unbiased resistance screen of 3,884 gene knockout strains, we uncovered a previously unknown non-lytic bactericidal mechanism that sequentially couples three transporters and downstream transcription to lethally suppress respiration of the highly virulent P. aeruginosa strain PA14 - one of three species on the WHO's 'Priority 1: Critical' list. By targeting outer membrane YaiW, cationic lacritin peptide 'N-104' translocates into the periplasm where it ligates outer loops 4 and 2 of the inner membrane transporters FeoB and PotH, respectively, to suppress both ferrous iron and polyamine uptake. This broadly shuts down transcription of many biofilm-associated genes, including ferrous iron-dependent TauD and ExbB1. The mechanism is innate to the surface of the eye and is enhanced by synergistic coupling with thrombin peptide GKY20. This is the first example of an inhibitor of multiple bacterial transporters.

Mitigation of biogenic methanethiol using bacteriophages in synthetic wastewater augmented with Pseudomonas putida

Wastewater malodour is the proverbial 'elephant in the room' notwithstanding its severe implications on sanitation, health, and hygiene. The predominant malodorous compounds associated with wastewater treatment plants and toilets are volatile organic compounds, such as hydrogen sulphide, ammonia, methanethiol, and organic acids. Among them, methanethiol warrants more attention owing to its relatively low olfactory threshold and associated cytotoxicity. This requires an efficient odour-abatement method since conventional techniques are either cost-prohibitive or leave recalcitrant byproducts. Bacteriophage-based methodology holds promise, and the described work explores the potential. In this study, a non-lysogenous Pseudomonas putida strain is used as a model organism that produces methanethiol in the presence of methionine. Two double-stranded DNA phages of genome sizes > 10 Kb were isolated from sewage. ɸPh_PP01 and ɸPh_PP02 were stable at suboptimal pH, temperature, and at 10% chloroform. Moreover, they showed adsorption efficiencies of 53% and 89% in 12 min and burst sizes of 507 ± 187 and 105 ± 7 virions per cell, respectively. In augmented synthetic wastewater, ɸPh_PP01 and ɸPh_PP02 reduced methanethiol production by 52% and 47%, respectively, with the concomitant reduction in P. putida by 3 logs in 6 h. On extension of the study in P. putida spiked-sewage sample, maximum reduction in methanethiol production was achieved in 3 h, with 49% and 48% for ɸPh_PP01 and ɸPh_PP02, respectively. But at 6 h, efficiency reduced to 36% with both the phages. The study clearly demonstrates the potential of phages as biocontrol agents in the reduction of malodour in wastewater.

Oligomeric Changes Regulate Flavin Transfer in Two-Component FMN Reductases Involved in Sulfur Metabolism

The FMN reductases (SsuE and MsuE of the alkanesulfonate monooxygenase systems) supply reduced flavin to their partner monooxygenases for the desulfonation of alkanesulfonates. Flavin reductases that comprise two-component systems must be able to regulate both flavin reduction and transfer. One mechanism to control these distinct processes is through changes in the oligomeric state of the enzymes. Despite their similar overall structures, SsuE and MsuE showed clear differences in their oligomeric states in the presence of substrates. The oligomeric state of SsuE was converted from a tetramer to a dimer/tetramer equilibrium in the presence of FMN or NADPH in analytical ultracentrifugation studies. Conversely, MsuE shifted from a dimer to a single tetrameric state with FMN, and the NADPH substrate did not induce a similar oligomeric shift. There was a fast tetramer to dimer equilibrium shift occurring at the dimer/dimer interface in H/D-X investigations with apo SsuE. Formation of the SsuE/FMN complex slowed the tetramer/dimer conversion, leading to a slower exchange along the dimer/dimer interface. The oligomeric shift of the MsuE/FMN complex from a dimer to a distinct tetramer showed a decrease in H/D-X in the region around the π-helices at the dimer/dimer interface. Both SsuE and MsuE showed a comparable and significant increase in the melting temperature with the addition of FMN, indicating the conformers formed by each FMN-bound enzyme had increased stability. A mechanism that supports the different structural shifts is rationalized by the different roles these enzymes play in providing reduced flavin to single or multiple monooxygenase enzymes.

Insights into the Synergistic Antibacterial Activity of Silver Nitrate with Potassium Tellurite against Pseudomonas aeruginosa

The constant, ever-increasing antibiotic resistance crisis leads to the announcement of "urgent, novel antibiotics needed" by the World Health Organization. Our previous works showed a promising synergistic antibacterial activity of silver nitrate with potassium tellurite out of thousands of other metal/metalloid-based antibacterial combinations. The silver-tellurite combined treatment not only is more effective than common antibiotics but also prevents bacterial recovery, decreases the risk of future resistance chance, and decreases the effective concentrations. We demonstrate that the silver-tellurite combination is effective against clinical isolates. Further, this study was conducted to address knowledge gaps in the available data on the antibacterial mechanism of both silver and tellurite, as well as to give insight into how the mixture provides synergism as a combination. Here, we defined the differentially expressed gene profile of Pseudomonas aeruginosa under silver, tellurite, and silver-tellurite combination stress using an RNA sequencing approach to examine the global transcriptional changes in the challenged cultures grown in simulated wound fluid. The study was complemented with metabolomics and biochemistry assays. Both metal ions mainly affected four cellular processes, including sulfur homeostasis, reactive oxygen species response, energy pathways, and the bacterial cell membrane (for silver). Using a Caenorhabditis elegans animal model we showed silver-tellurite has reduced toxicity over individual metal/metalloid salts and provides increased antioxidant properties to the host. This work demonstrates that the addition of tellurite would improve the efficacy of silver in biomedical applications. IMPORTANCE Metals and/or metalloids could represent antimicrobial alternatives for industrial and clinical applications (e.g., surface coatings, livestock, and topical infection control) because of their great properties, such as good stability and long half-life. Silver is the most common antimicrobial metal, but resistance prevalence is high, and it can be toxic to the host above a certain concentration. We found that a silver-tellurite composition has antibacterial synergistic effect and that the combination is beneficial to the host. So, the efficacy and application of silver could increase by adding tellurite in the recommended concentration(s). We used different methods to evaluate the mechanism for how this combination can be so incredibly synergistic, leading to efficacy against antibiotic- and silver-resistant isolates. Our two main findings are that (i) both silver and tellurite mostly target the same pathways and (ii) the coapplication of silver with tellurite tends not to target new pathways but targets the same pathways with an amplified change.

Insights into the physiological and genomic characterization of three bacterial isolates from a highly alkaline, terrestrial serpentinizing system

The terrestrial serpentinite-hosted ecosystem known as "The Cedars" is home to a diverse microbial community persisting under highly alkaline (pH ~ 12) and reducing (Eh < -550 mV) conditions. This extreme environment presents particular difficulties for microbial life, and efforts to isolate microorganisms from The Cedars over the past decade have remained challenging. Herein, we report the initial physiological assessment and/or full genomic characterization of three isolates: Paenibacillus sp. Cedars ('Paeni-Cedars'), Alishewanella sp. BS5-314 ('Ali-BS5-314'), and Anaerobacillus sp. CMMVII ('Anaero-CMMVII'). Paeni-Cedars is a Gram-positive, rod-shaped, mesophilic facultative anaerobe that grows between pH 7-10 (minimum pH tested was 7), temperatures 20-40°C, and 0-3% NaCl concentration. The addition of 10-20 mM CaCl₂ enhanced growth, and iron reduction was observed in the following order, 2-line ferrihydrite > magnetite > serpentinite ~ chromite ~ hematite. Genome analysis identified genes for flavin-mediated iron reduction and synthesis of a bacillibactin-like, catechol-type siderophore. Ali-BS5-314 is a Gram-negative, rod-shaped, mesophilic, facultative anaerobic alkaliphile that grows between pH 10-12 and temperatures 10-40°C, with limited growth observed 1-5% NaCl. Nitrate is used as a terminal electron acceptor under anaerobic conditions, which was corroborated by genome analysis. The Ali-BS5-314 genome also includes genes for benzoate-like compound metabolism. Anaero-CMMVII remained difficult to cultivate for physiological studies; however, growth was observed between pH 9-12, with the addition of 0.01-1% yeast extract. Anaero-CMMVII is a probable oxygen-tolerant anaerobic alkaliphile with hydrogenotrophic respiration coupled with nitrate reduction, as determined by genome analysis. Based on single-copy genes, ANI, AAI and dDDH analyses, Paeni-Cedars and Ali-BS5-314 are related to other species (P. glucanolyticus and A. aestuarii, respectively), and Anaero-CMMVII represents a new species. The characterization of these three isolates demonstrate the range of ecophysiological adaptations and metabolisms present in serpentinite-hosted ecosystems, including mineral reduction, alkaliphily, and siderophore production.

Genetic and functional characterization of multiple thermophilic organosulfur-removal systems reveals desulfurization potentials for waste residue oil cleaning

Heavy oil and petroleum refining residues usually contain high concentrations of recalcitrant hazardous organosulfur compounds, causing long-term serious global environmental pollution during leakage and combustion. Research conducted here identified a unique thermophilic bacterium Parageobacillus thermoglucosidasius W-36 with the notable ability of waste residue oil desulfurization, utilization and tolerance of multiplex hazardous organosulfur pollutants. Genome information mining revealed multiple desulfurization systems in three organosulfur-utilizing gene clusters. Enzymatic characterization, phylogenetic relationships, transcriptional performance and structural prediction indicated four novel key monooxygenases for diverse organosulfur removal. Importantly, all monooxygenases shared obvious commonalities in the predicted tertiary structure backbone and catalytic characteristics of C-S bond cleavage, implying the potential of genetic engineering for broad-spectrum hazardous organosulfur removal. Therefore, this work demonstrated the important application potential of thermophilic bacteria as a promising alternative biodesulfurization way for waste residue oil cleaning.

Whole genome analysis of Flavobacterium aziz-sancarii sp. nov., isolated from Ardley Island (Antarctica), revealed a rich resistome and bioremediation potential

Despite being one of the most isolated regions in the world, Antarctica is at risk of increased contamination with potentially toxic elements and other toxic chemicals through anthropogenic interventions. In this study, a psychrotolerant bacterium was isolated using the lake water collected from Ardley Island (Antarctica), which can grow at temperatures between 4 and 30 °C and pH values between 6.0 and 9.0. The isolate, named AC, had protease, amylase, and lipase activities with no NaCl tolerance and could degrade 1-5% diesel fuel. Multilocus sequence analysis (MLSA) using 16S rRNA, gyrB, tuf, and rpoD genes resulted in 92.91-98.6% sequence similarities between the isolate AC and other Flavobacterium spp. Whole genome analysis indicated that the genome length of Flavobacterium sp. AC is 5.8 Mbp with a GC content of 34.04% and 1274 genes predicted. The strain AC branched independently from other Flavobacterium spp. in the phylogenetic and phylogenomic trees and ranked a new species named Flavobacterium aziz-sancarii. Genome mining identified several cold-inducible genes, including stress-associated genes such as cold-shock proteins, chaperones, carotenoid biosynthetic genes, or oxidative-stress response genes. In addition, virulence, gliding motility, and biofilm-related genes were determined. Its genome contains 35 and 88 open-reading frames related to potentially toxic element and antibiotic resistance, respectively. F. aziz-sancarii showed a remarkable tolerance of Cr and Ni, with minimal inhibitory concentration values of 2.88 and 2.81 mM, respectively. Pb, Cu, and Zn exposure resulted in moderate toxicity (2.14-2.41 mM), while Cd showed the highest inhibitory effect in bacterial growth (0.74 mM). Antibiotic susceptibility testing indicated multidrug-resistant phenotype in correlation to in silico prediction of antibiotic resistance genes. Overall, our results contribute to biodiversity of Antarctica and provide new insights into resistome profile of Antarctic microorganisms. Additionally, the diesel degradation feature of F. aziz-sancarii offers potential use for the bioremediation of hydrocarbon-contaminated polar ecosystems.

Arbuscular Mycorrhiza Support Plant Sulfur Supply through Organosulfur Mobilizing Bacteria in the Hypho- and Rhizosphere

This study aimed to elucidate the role of bacteria colonising mycorrhizal hyphae in organically bound sulfur mobilisation, the dominant soil sulfur source that is not directly plant available. The effect of an intact mycorrhizal symbiosis with access to stable isotope organo-³⁴S enriched soils encased in 35 µm mesh cores was tested in microcosms with Agrostis stolonifera and Plantago lanceolata. Hyphae and associated soil were sampled from static mesh cores with mycorrhizal ingrowth and rotating mesh cores that exclude mycorrhizal ingrowth as well as corresponding rhizosphere soil, while plant shoots were analysed for ³⁴S uptake. Static cores increased uptake of ³⁴S at early stages of plant growth when sulfur demand appeared to be high and harboured significantly larger populations of sulfonate mobilising bacteria. Bacterial and fungal communities were significantly different in the hyphospheres of static cores when compared to rotating cores, not associated with plant hosts. Shifts in bacterial and fungal communities occurred not only in rotated cores but also in the rhizosphere. Arylsulfatase activity was significantly higher in the rhizosphere when cores stayed static, while atsA and asfA gene diversity was distinct in the microcosms with static and rotating cores. This study demonstrated that AM symbioses can promote organo-S mobilization and plant uptake through interactions with hyphospheric bacteria, enabling AM fungal ingrowth into static cores creating a positive feedback-loop, detectable in the microbial rhizosphere communities.

Transcriptomic response of Gordonia sp. strain NB4-1Y when provided with 6:2 fluorotelomer sulfonamidoalkyl betaine or 6:2 fluorotelomer sulfonate as sole sulfur source

Abstract

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are environmental contaminants of concern. We previously described biodegradation of two PFAS that represent components and transformation products of aqueous film-forming foams (AFFF), 6:2 fluorotelomer sulfonamidoalkyl betaine (6:2 FTAB) and 6:2 fluorotelomer sulfonate (6:2 FTSA), by Gordonia sp. strain NB4-1Y. To identify genes involved in the breakdown of these compounds, the transcriptomic response of NB4-1Y was examined when grown on 6:2 FTAB, 6:2 FTSA, a non-fluorinated analog of 6:2 FTSA (1-octanesulfonate), or MgSO₄, as sole sulfur source. Differentially expressed genes were identified as those with ± 1.5 log₂-fold-differences (± 1.5 log₂FD) in transcript abundances in pairwise comparisons. Transcriptomes of cells grown on 6:2 FTAB and 6:2 FTSA were most similar (7.9% of genes expressed ± 1.5 log₂FD); however, several genes that were expressed in greater abundance in 6:2 FTAB treated cells compared to 6:2 FTSA treated cells were noted for their potential role in carbon–nitrogen bond cleavage in 6:2 FTAB. Responses to sulfur limitation were observed in 6:2 FTAB, 6:2 FTSA, and 1-octanesulfonate treatments, as 20 genes relating to global sulfate stress response were more highly expressed under these conditions compared to the MgSO₄ treatment. More highly expressed oxygenase genes in 6:2 FTAB, 6:2 FTSA, and 1-octanesulfonate treatments were found to code for proteins with lower percent sulfur-containing amino acids compared to both the total proteome and to oxygenases showing decreased expression. This work identifies genetic targets for further characterization and will inform studies aimed at evaluating the biodegradation potential of environmental samples through applied genomics.

Graphic Abstract

Electronic supplementary material

The online version of this article (10.1007/s10532-020-09917-8) contains supplementary material, which is available to authorized users.

Comparative transcriptomic analysis revealed the key pathways responsible for organic sulfur removal by thermophilic bacterium Geobacillus thermoglucosidasius W-2

Biodesulfurization is a promising method to desulfurize sulfur-containing compounds in oil with its unique advantages, such as environment-friendly treatments and moderate reaction conditions. In this study, a thermophilic bacterium Geobacillus thermoglucosidasius W-2 was reported to show nearly 40% and 55% desulfurization rates on heavy oil with 2.81% and 0.46% initial total sulfur content, respectively. Subsequently, comparative transcriptome analysis indicated that several possible key desulfurization-related genes of this strain were found to be differentially up-regulated induced by benzothiophene and dibenzothiophene, respectively. These desulfurization-related genes were considered to conduct key step to convert organic sulfur to inorganic sulfur. Moreover, the characterization of thermophilic alkanesulfonate monooxygenase systems SsuD₁/SsuE₁ and SsuD₂/SsuE₂ revealed that the enzymes exhibit considerable thermal and pH stability and wide substrates applicability. These enzymes probably endowed the strain W-2 with the ability to desulfurize oil and eliminate the sulfur-containing surfactants. Thus, this study provides novel alkanesulfonate monooxygenase systems that have the application potential for heavy oil biodesulfurization, oil demulsification and other biocatalytic processes.

Upregulation of peroxide scavenging enzymes and multidrug efflux proteins highlight an active sodium hypochlorite response in Pseudomonas fluorescens biofilms

The oxidative biocide sodium hypochlorite is among the most commonly used antimicrobial agents in the control of surface-attached microbial communities (biofilms). Clarifying the genetic response of microorganisms in biofilms to hypochlorite may contribute to improved biofilm control strategies. Here, RNA-seq was used to investigate the differential gene expression response of industrially relevant Pseudomonas fluorescens biofilms to sub-lethal concentrations of sodium hypochlorite. Pseudomonas biofilms responded to hypochlorite exposure with increased transcription of genes encoding peroxide scavenging enzymes (e.g., alkyl hydroperoxide reductase (Ahp) and hydroperoxide resistance protein (Ohr)), oxidative stress repair enzymes (e.g., the periplasmic sulfoxide reductase YedYZ complex), and multidrug efflux (e.g., MexEF pumps). In addition, genes involved in amino acid synthesis and energy metabolism were down-regulated following hypochlorite exposure. This work improves the current understanding of genetic response mechanisms to biocides and contributes to the optimization of biocides and application strategies.

SfnR2 Regulates Dimethyl Sulfide-Related Utilization in Pseudomonas aeruginosa PAO1

Dimethyl sulfide (DMS) is a volatile sulfur compound produced mainly from the degradation of dimethylsulfoniopropionate (DMSP) in marine environments. DMS undergoes oxidation to form dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO₂), and methanesulfonate (MSA), all of which occur in terrestrial environments and are accessible for consumption by various microorganisms. The purpose of the present study was to determine how the enhancer-binding proteins SfnR1 and SfnR2 contribute to the utilization of DMS and its derivatives in Pseudomonas aeruginosa PAO1. First, results from cell growth experiments showed that deletion of either sfnR2 or sfnG, a gene encoding a DMSO₂-monooxygenase, significantly inhibits the ability of P. aeruginosa PAO1 to use DMSP, DMS, DMSO, and DMSO₂ as sulfur sources. Deletion of the sfnR1 or msuEDC genes, which encode a MSA desulfurization pathway, did not abolish the growth of P. aeruginosa PAO1 on any sulfur compound tested. Second, data collected from β-galactosidase assays revealed that the msuEDC-sfnR1 operon and the sfnG gene are induced in response to sulfur limitation or nonpreferred sulfur sources, such as DMSP, DMS, and DMSO, etc. Importantly, SfnR2 (and not SfnR1) is essential for this induction. Expression of sfnR2 is induced under sulfur limitation but independently of SfnR1 or SfnR2. Finally, the results of this study suggest that the main function of SfnR2 is to direct the initial activation of the msuEDC-sfnR1 operon in response to sulfur limitation or nonpreferred sulfur sources. Once expressed, SfnR1 contributes to the expression of msuEDC-sfnR1, sfnG, and other target genes involved in DMS-related metabolism in P. aeruginosa PAO1.IMPORTANCE Dimethyl sulfide (DMS) is an important environmental source of sulfur, carbon, and/or energy for microorganisms. For various bacteria, including Pseudomonas, Xanthomonas, and Azotobacter, DMS utilization is thought to be controlled by the transcriptional regulator SfnR. Adding more complexity, some bacteria, such as Acinetobacter baumannii, Enterobacter cloacae, and Pseudomonas aeruginosa, possess two, nonidentical SfnR proteins. In this study, we demonstrate that SfnR2 and not SfnR1 is the principal regulator of DMS metabolism in P. aeruginosa PAO1. Results suggest that SfnR1 has a supportive but nonessential role in the positive regulation of genes required for DMS utilization. This study not only enhances our understanding of SfnR regulation but, importantly, also provides a framework for addressing gene regulation through dual SfnR proteins in other bacteria.

Time-resolved transcriptome analysis of Clostridium difficile R20291 response to cysteine

The incidence of Clostridium difficile infection has been steadily rising over the past decade. The increase in the rate of incidence is associated with the specific NAP1/BI/027 strains which are "hypervirulent" and have led to several large outbreaks since their emergence. However, the relation between these outbreaks and virulence regulation mechanisms remains unclear. It has been reported that the major virulence factor TcdA and TcdB in C. difficile could be repressed by cysteine. Here, we investigated the functional and virulence-associated regulation of C. difficile R20291 response to cysteine by using a time-resolved genome-wide transcriptome analysis. Dramatic changes of gene expression in C. difficile revealed functional processes related to transport, metabolism, and regulators in the presence of cysteine during different phases of growth. Flagellar and ribosomal genes were significantly down-regulated in long-term response to cysteine. Many NAP1/BI/027- specific genes were also modulated by cysteine. In addition, cdsB inactivation in C. difficile R20291 could remove the repression of toxin synthesis but could not remove the repression of butyrate production in the presence of cysteine. This suggests that toxin synthesis and butyrate production might have different regulatory controls in response to cysteine. Altogether, our research provides important insights into the regulatory mechanisms of C. difficile response to cysteine.

Multiwall Carbon Nanotubes Induce More Pronounced Transcriptomic Responses in Pseudomonas aeruginosa PG201 than Graphene, Exfoliated Boron Nitride, or Carbon Black

Carbonaceous and boron nitride (BN) nanomaterials have similar applications and hydrophobic properties suggesting common release pathways and exposure to bacteria. While high nanomaterial concentrations can be bactericidal or growth-inhibitory, little is known regarding bacterial transcriptional responses to non-growth-inhibitory nanomaterial concentrations. Here, using one strain of Pseudomonas aeruginosa-a clinically and environmentally important bacterial taxon-we analyzed the comparative transcriptomic response to carbonaceous or BN nanomaterials. We show that, at non-growth-inhibitory, equal mass concentrations (10 mg/L), multiwall carbon nanotubes (MWCNTs) induced differential regulation of 111 genes in P. aeruginosa, while graphene, BN, and carbon black caused differential regulation of 44, 26, and 25 genes, respectively. MWCNTs caused the upregulation of genes encoding general stress response (9 genes), sulfur metabolism (15), and transport of small molecules (7) and downregulation of genes encoding flagellar basal-body rod proteins and other virulence-related factors (6), nitrogen metabolism (7), and membrane proteins (12), including a two-component regulatory system CzcS/R. Because two-component systems are associated with antibiotic resistance, the antibiotic susceptibility of P. aeruginosa was tested following MWCNT exposure. In MWCNT-treated cultures, the minimal inhibitory concentrations (MICs) of meropenem and imipenem decreased from 0.06 to 0.03 μg/mL and from 0.25 to 0.125 μg/mL, respectively. Taken together, whole genome analysis indicated that, in the absence of growth inhibition, nanomaterials can alter bacterial physiology and metabolism. For MWCNTs, such alterations may include downregulation of antibiotic resistance pathways, suggesting that pre-exposure to MWCNTs could potentially render bacteria more susceptible to carbapenems which are often the last resort for the globally concerning, highly antibiotic resistant P. aeruginosa.

Fate and transport of free and conjugated estrogens during soil passage

Endocrine disrupting chemicals, such as the free estrogens 17β-estradiol (E2), estrone (E1) and the conjugated estrogen estrone-sulfate (E1-3S) are found at low concentration levels in the environment. This is somehow contradictory to the strong sorption and high degradation potentials found in laboratory experiments. In particular, the fate and transport behavior of conjugated estrogens is poorly understood, and the importance of enzymes triggering the transformation pathways has received little attention. To address these deficiencies, the present research uses packed laboratory soil columns with pulse injections of free estrogens, either E2 or E1, or E1-3S, to provide sound evidence of the transformation pathways. It is further shown that (i) transport of free estrogens is subject to strong retardation and degradation, (ii) the transport of conjugated estrogens is less retarded and only to a minor degree affected by degradation, and (iii) arylsulfotransferase is the enzyme triggering the transformation reaction.

The role of bacteria and mycorrhiza in plant sulfur supply

Plant growth is highly dependent on bacteria, saprophytic, and mycorrhizal fungi which facilitate the cycling and mobilization of nutrients. Over 95% of the sulfur (S) in soil is present in an organic form. Sulfate-esters and sulfonates, the major forms of organo-S in soils, arise through deposition of biological material and are transformed through subsequent humification. Fungi and bacteria release S from sulfate-esters using sulfatases, however, release of S from sulfonates is catalyzed by a bacterial multi-component mono-oxygenase system. The asfA gene is used as a key marker in this desulfonation process to study sulfonatase activity in soil bacteria identified as Variovorax, Polaromonas, Acidovorax, and Rhodococcus. The rhizosphere is regarded as a hot spot for microbial activity and recent studies indicate that this is also the case for the mycorrhizosphere where bacteria may attach to the fungal hyphae capable of mobilizing organo-S. While current evidence is not showing sulfatase and sulfonatase activity in arbuscular mycorrhiza, their effect on the expression of plant host sulfate transporters is documented. A revision of the role of bacteria, fungi and the interactions between soil bacteria and mycorrhiza in plant S supply was conducted.

Endosulfan induced alteration in bacterial protein profile and RNA yield of Klebsiella sp. M3, Achromobacter sp. M6, and Rhodococcus sp. M2

Three bacterial strains identified as Klebsiella sp. M3, Achromobacter sp. M6 and Rhodococcus sp. M2 were isolated by soil enrichment with endosulfan followed by shake flask enrichment technique. They were efficiently degrading endosulfan in the NSM (non sulfur medium) broth. Degradation of endosulfan was faster with the cell free extract of bacterial cells grown in the sulfur deficient medium (NSM) supplemented with endosulfan than that of nutrient rich medium (Luria Bertani). In the cell free extract of NSM supplemented with endosulfan as sole sulfur source, a unique band was visualized on SDS-PAGE but not with magnesium sulfate as the sole sulfur source in NSM and LB with endosulfan. Expression of a unique polypeptide band was speculated to be induced by endosulfan under sulfur starved condition. These unique polypeptide bands were identified as OmpK35 protein, sulfate binding protein and outer membrane porin protein, respectively, in Klebsiella sp. M3, Achromobacter sp. M6 and Rhodococcus sp. M2. Endosulfan showed dose dependent negative effect on total RNA yield of bacterial strains in nutrient rich medium. Absence of plasmid DNA indicated the presence of endosulfan metabolizing gene on genomic DNA.

Structural and physiological analyses of the alkanesulphonate-binding protein (SsuA) of the citrus pathogen Xanthomonas citri

Background

The uptake of sulphur-containing compounds plays a pivotal role in the physiology of bacteria that live in aerobic soils where organosulfur compounds such as sulphonates and sulphate esters represent more than 95% of the available sulphur. Until now, no information has been available on the uptake of sulphonates by bacterial plant pathogens, particularly those of the Xanthomonas genus, which encompasses several pathogenic species. In the present study, we characterised the alkanesulphonate uptake system (Ssu) of Xanthomonas axonopodis pv. citri 306 strain (X. citri), the etiological agent of citrus canker.

Methodology/Principal Findings

A single operon-like gene cluster (ssuEDACB) that encodes both the sulphur uptake system and enzymes involved in desulphurisation was detected in the genomes of X. citri and of the closely related species. We characterised X. citri SsuA protein, a periplasmic alkanesulphonate-binding protein that, together with SsuC and SsuB, defines the alkanesulphonate uptake system. The crystal structure of SsuA bound to MOPS, MES and HEPES, which is herein described for the first time, provides evidence for the importance of a conserved dipole in sulphate group coordination, identifies specific amino acids interacting with the sulphate group and shows the presence of a rather large binding pocket that explains the rather wide range of molecules recognised by the protein. Isolation of an isogenic ssuA-knockout derivative of the X. citri 306 strain showed that disruption of alkanesulphonate uptake affects both xanthan gum production and generation of canker lesions in sweet orange leaves.

Conclusions/Significance

The present study unravels unique structural and functional features of the X. citri SsuA protein and provides the first experimental evidence that an ABC uptake system affects the virulence of this phytopathogen.

Whole-genome shotgun sequence of Rhodococcus species strain JVH1

Here we present a whole-genome shotgun sequence of Rhodococcus species strain JVH1, an organism capable of degrading a variety of organosulfur compounds. In particular, JVH1 is able to selectively cleave carbon-sulfur bonds within alkyl chains. A large number of oxygenases were identified, consistent with other members of the genus.

Elucidating the structural and conformational factors responsible for the activity and substrate specificity of alkanesulfonate monooxygenase

The mechanism and substrate specificity of alkanesulfonate monooxygenase (SsuD) was investigated by combining molecular dynamics simulations, docking, and a comprehensive quantitative structure activity relationships (QSAR) analysis. The FMNH(2) dependent monooxygenase undergoes a dynamic conformational change of the active site, passing from a closed to an open state. As a consequence, substrates have access to the active site and the cofactor is then regenerated by the associated oxidoreductase FMN reductase SsuE.. Computational analysis of the interaction of SsuD with FMNH(2) based on molecular docking and multiple 20 ns molecular dynamics simulations pointed out that the conformational change is mainly driven by salt bridge formation between Arg297 and Glu20 or Asp111. A set of substrates accepted by SsuD were described by means of ALMOND chemical descriptors and a partial least square (PLS) mathematical model was constructed. The PLS model correlates the structure of substrates and enzyme activity, namely kinetic properties (k (cat)/K (M)). Therefore, information coming from the PLS analysis goes beyond the simple ability of the enzyme to recognize the substrate, but includes the factors that affect the capacity of the enzyme to reduce the activation energy of the rate determining step of the reaction. The two principal components of the model are able to describe both steric and electronic factors and, more importantly, their interactions. Indeed, interactions of factors appear to affect significantly the ability of SsuD of transforming efficiently a substrate.

Global effect of an RNA polymerase β-subunit mutation on gene expression in the radiation-resistant bacterium Deinococcus radiodurans

The β-subunit of RNA polymerase, which is involved in rifampin binding, is highly conserved among prokaryotes, and Rifr mutants detected in many bacteria are the result of amino acid changes. Spontaneous rifampin resistance mutations resulting in amino acid replacement (L420R) and deletion (1258-66 9 bp deletion) have been previously isolated in the rpoB gene of Deinococcus radiodurans. In this study, a β-subunit mutation in D. radiodurans resulted in a unique effect on growth rate. We used DNA microarrays and biochemical assays to investigate how the Rifr mutation in the β-subunit led to changes in growth rate via altered regulation of multiple genes. The expression of genes with predicted functions in metabolism, cellular processes and signaling, and information storage and processing were significantly altered in the 9 bp-deletion rpoB mutant. The consensus promoter sequence of up-regulated genes in the 9 bp-deletion rpoB mutant was identified as an AT-rich sequence. Greater levels of reactive oxygen species accumulated in the L420R and 9 bp-deletion rpoB mutants compared with wild type. These results provide insight into the molecular mechanism of how the β-subunit Rifr mutation alters the regulation of multiple genes.

Expression and functional roles of Bradyrhizobium japonicum genes involved in the utilization of inorganic and organic sulfur compounds in free-living and symbiotic conditions

Strains of Bradyrhizobium spp. form nitrogen-fixing symbioses with many legumes, including soybean. Although inorganic sulfur is preferred by bacteria in laboratory conditions, sulfur in agricultural soil is mainly present as sulfonates and sulfur esters. Here, we show that Bradyrhizobium japonicum and B. elkanii strains were able to utilize sulfate, cysteine, sulfonates, and sulfur-ester compounds as sole sulfur sources for growth. Expression and functional analysis revealed that two sets of gene clusters (bll6449 to bll6455 or bll7007 to bll7011) are important for utilization of sulfonates sulfur source. The bll6451 or bll7010 genes are also expressed in the symbiotic nodules. However, B. japonicum mutants defective in either of the sulfonate utilization operons were not affected for symbiosis with soybean, indicating the functional redundancy or availability of other sulfur sources in planta. In accordance, B. japonicum bacteroids possessed significant sulfatase activity. These results indicate that strains of Bradyrhizobium spp. likely use organosulfur compounds for growth and survival in soils, as well as for legume nodulation and nitrogen fixation.

RNA-seq analysis of sulfur-deprived Chlamydomonas cells reveals aspects of acclimation critical for cell survival

The Chlamydomonas reinhardtii transcriptome was characterized from nutrient-replete and sulfur-depleted wild-type and snrk2.1 mutant cells. This mutant is null for the regulatory Ser-Thr kinase SNRK2.1, which is required for acclimation of the alga to sulfur deprivation. The transcriptome analyses used microarray hybridization and RNA-seq technology. Quantitative RT-PCR evaluation of the results obtained by these techniques showed that RNA-seq reports a larger dynamic range of expression levels than do microarray hybridizations. Transcripts responsive to sulfur deprivation included those encoding proteins involved in sulfur acquisition and assimilation, synthesis of sulfur-containing metabolites, Cys degradation, and sulfur recycling. Furthermore, we noted potential modifications of cellular structures during sulfur deprivation, including the cell wall and complexes associated with the photosynthetic apparatus. Moreover, the data suggest that sulfur-deprived cells accumulate proteins with fewer sulfur-containing amino acids. Most of the sulfur deprivation responses are controlled by the SNRK2.1 protein kinase. The snrk2.1 mutant exhibits a set of unique responses during both sulfur-replete and sulfur-depleted conditions that are not observed in wild-type cells; the inability of this mutant to acclimate to S deprivation probably leads to elevated levels of singlet oxygen and severe oxidative stress, which ultimately causes cell death. The transcriptome results for wild-type and mutant cells strongly suggest the occurrence of massive changes in cellular physiology and metabolism as cells become depleted for sulfur and reveal aspects of acclimation that are likely critical for cell survival.

Microbial conversion of 5-sulfoisophthalic acid into 5-hydroxyisophthalic acid by Ochrobactrum anthropi S9

5-Hydroxyisophthalic acid-producing microorganisms were isolated from enrichment cultures using 5-sulfoisophthalic acid as a sulfur source. One bacterium, Ochrobactrum anthropi S9, had the highest 5-sulfoisophthalic acid-degrading activity, and stoichiometrically formed 5-hydroxyisophthalic acid, a raw material for polymer synthesis. Under optimum culture conditions, 1.3 mM 5-hydroxyisophthalic acid accumulated in the medium by 60 h. The addition of Na(2)SO(4), L: -methionine or L: -cysteine at 2 mM inhibited the conversion of 5-sulfoisophthalic acid. O. anthropi S9 cells converted 5-sulfoisophthalic acid, benzenesulfonic acid, 3-sulfobenzoic acid, 4-aminobenzenesulfonic acid, naphthalene-1-sulfonic acid and naphthalene-2-sulfonic acid into the corresponding hydroxylated compounds.

Stenotrophomonas maltophilia genomes: a start-up comparison

The whole DNA sequences of 2 Stenotrophomonas maltophilia strains isolated from the blood of a cancer patient (K279a) and the poplar Populus trichocarpa (R551-3) have been compared. The 2 chromosomes exhibit extensive synteny, but each is punctuated by about 40 genomic islands (GEIs), which vary in size from 3 to 70kb, and may encode up to about 50 proteins. A large set of smaller DNA sequences, encoding strain-specific 'solo' orfs, contributes to genetic heterogeneity in a significant manner. S. maltophilia GEIs potentially encode several proteins mediating interactions with the environment such as transmembrane proteins, haemagglutinins, components of type I and IV secretion systems, and efflux proteins having a role in metal and/or drug resistance. The presence of specific GEIs in the S. maltophilia population was monitored by PCR and slot-blot analyses. Data suggest that some islands are present at sites different from those identified in K279a and that alternative islands may be integrated at mapped sites.

Comparative genome-wide transcriptional profiling of Azorhizobium caulinodans ORS571 grown under free-living and symbiotic conditions

The whole-genome sequence of the endosymbiotic bacterium Azorhizobium caulinodans ORS571, which forms nitrogen-fixing nodules on the stems and roots of Sesbania rostrata, was recently determined. The sizes of the genome and symbiosis island are 5.4 Mb and 86.7 kb, respectively, and these sizes are the smallest among the sequenced rhizobia. In the present study, a whole-genome microarray of A. caulinodans was constructed, and transcriptomic analyses were performed on free-living cells grown in rich and minimal media and in bacteroids isolated from stem nodules. Transcriptional profiling showed that the genes involved in sulfur uptake and metabolism, acetone metabolism, and the biosynthesis of exopolysaccharide were highly expressed in bacteroids compared to the expression levels in free-living cells. Some mutants having Tn5 transposons within these genes with increased expression were obtained as nodule-deficient mutants in our previous study. A transcriptomic analysis was also performed on free-living cells grown in minimal medium supplemented with a flavonoid, naringenin, which is one of the most efficient inducers of A. caulinodans nod genes. Only 18 genes exhibited increased expression by the addition of naringenin, suggesting that the regulatory mechanism responding to the flavonoid could be simple in A. caulinodans. The combination of our genome-wide transcriptional profiling and our previous genome-wide mutagenesis study has revealed new aspects of nodule formation and maintenance.

Both FMNH2 and FADH2 can be utilized by the dibenzothiophene monooxygenase from a desulfurizing bacterium Mycobacterium goodii X7B

To investigate the flavin utilization by dibenzothiophene monooxygenase (DszC), DszC of a desulfurizing bacterium Mycobacterium goodii X7B was purified from the recombinant Escherichia coli. It was shown to be able to utilize either FMNH(2) or FADH(2) when coupled with a flavin reductase that reduces either FMN or FAD. Sequence analysis indicated that DszC was similar to the C(2) component of p-hydroxyphenylacetate hydroxylase from Acinetobacter baumannii, which can use both FADH(2) and FMNH(2) as substrates. Both flavins at high concentrations could inhibit the activity of DszC due to autocatalytic oxidation of reduced flavins. The results suggest that DszC should be reclassified as an FMNH(2) and FADH(2) both-utilizing monooxygenase component and the flavins should be controlled at properly reduced levels to obtain optimal biodesulfurization results.

Involvement of the oscA gene in the sulphur starvation response and in Cr(VI) resistance in Pseudomonas corrugata 28

Pseudomonas corrugata28 is a Cr(VI)-hyper-resistant bacterium. A Cr(VI)-sensitive mutant was obtained by insertional mutagenesis using EZ-Tn5<R6Kγori/KAN-2>Tnp. The mutant strain was impaired in a gene, here namedoscA(organosulphurcompounds), which encoded a hypothetical small protein of unknown function. The gene was located upstream of a gene cluster that encodes the components of the sulphate ABC transporter, and it formed a transcriptional unit withsbp, which encoded the periplasmic binding protein of the transporter. TheoscA–sbptranscriptional unit was strongly and quickly overexpressed after chromate exposure, suggesting the involvement ofoscAin chromate resistance, which was further confirmed by means of a complementation experiment. Phenotype MicroArray (PM) analysis made it possible to assay 1536 phenotypes and also indicated that theoscAgene was involved in the utilization of organosulphur compounds as a sole source of sulphur. This is believed to be the first evidence thatoscAplays a role in activating a sulphur starvation response, which is required to cope with oxidative stress induced by chromate.

Indole and 7-hydroxyindole diminish Pseudomonas aeruginosa virulence

Indole is an extracellular biofilm signal for Escherichia coli, and many bacterial oxygenases readily convert indole to various oxidized compounds including 7‐hydroxyindole (7HI). Here we investigate the impact of indole and 7HI on Pseudomonas aeruginosa PAO1 virulence and quorum sensing (QS)‐regulated phenotypes; this strain does not synthesize these compounds but degrades them rapidly. Indole and 7HI both altered extensively gene expression in a manner opposite that of acylhomoserine lactones; the most repressed genes encode the mexGHI‐opmD multidrug efflux pump and genes involved in the synthesis of QS‐regulated virulence factors including pyocyanin (phz operon), 2‐heptyl‐3‐hydroxy‐4(1H)‐quinolone (PQS) signal (pqs operon), pyochelin (pch operon) and pyoverdine (pvd operon). Corroborating these microarray results, indole and 7HI decreased production of pyocyanin, rhamnolipid, PQS and pyoverdine and enhanced antibiotic resistance. In addition, indole affected the utilization of carbon, nitrogen and phosphorus, and 7HI abolished swarming motility. Furthermore, 7HI reduced pulmonary colonization of P. aeruginosa in guinea pigs and increased clearance in lungs. Hence, indole‐related compounds have potential as a novel antivirulence approach for the recalcitrant pathogen P. aeruginosa.

Crystal structures of NADH:FMN oxidoreductase (EmoB) at different stages of catalysis

EDTA has become a major organic pollutant in the environment because of its extreme usage and resistance to biodegradation. Recently, two critical enzymes, EDTA monooxygenase (EmoA) and NADH:FMN oxidoreductase (EmoB), belonging to the newly established two-component flavin-diffusible monooxygenase family, were identified in the EDTA degradation pathway in Mesorhizobium sp. BNC1. EmoA is an FMNH2-dependent enzyme that requires EmoB to provide FMNH2 for the conversion of EDTA to ethylenediaminediacetate. To understand the molecular basis of this FMN-mediated reaction, the crystal structures of the apo-form, FMN.FMN complex, and FMN.NADH complex of EmoB were determined at 2.5 angstroms resolution. The structure of EmoB is a homotetramer consisting of four alpha/beta-single-domain monomers of five parallel beta-strands flanked by five alpha-helices, which is quite different from those of other known two-component flavin-diffusible monooxygenase family members, such as PheA2 and HpaC, in terms of both tertiary and quaternary structures. For the first time, the crystal structures of both the FMN.FMN and FMN.NADH complexes of an NADH:FMN oxidoreductase were determined. Two stacked isoalloxazine rings and nicotinamide/isoalloxazine rings were at a proper distance for hydride transfer. The structures indicated a ping-pong reaction mechanism, which was confirmed by activity assays. Thus, the structural data offer detailed mechanistic information for hydride transfer between NADH to an enzyme-bound FMN and between the bound FMNH2 and a diffusible FMN.

Sulfoacetate released during the assimilation of taurine-nitrogen by Neptuniibacter caesariensis: purification of sulfoacetaldehyde dehydrogenase

Taurine (2-aminoethanesulfonate) is a widespread natural product whose nitrogen moiety was recently shown to be assimilated by bacteria, usually with excretion of an organosulfonate via undefined novel pathways; other data involve transcriptional regulator TauR in taurine metabolism. A screen of genome sequences for TauR with the BLAST algorithm allowed the hypothesis that the marine gammaproteobacterium Neptuniibacter caesariensis MED92 would inducibly assimilate taurine-nitrogen and excrete sulfoacetate. The pathway involved an ABC transporter (TauABC), taurine:pyruvate aminotransferase (Tpa), a novel sulfoacetaldehyde dehydrogenase (SafD) and exporter(s) of sulfoacetate (SafE) (DUF81). Ten candidate genes in two clusters involved three sets of paralogues (for TauR, Tpa and SafE). Inducible Tpa and SafD were detected in cell extracts. SafD was purified 600-fold to homogeneity in two steps. The monomer had a molecular mass of 50 kDa (SDS-PAGE); data from gel filtration chromatography indicated a tetrameric native protein. SafD was specific for sulfoacetaldehyde with a K (m)-value of 0.12 mM. The N-terminal amino acid sequence of SafD confirmed the identity of the safD gene. The eight pathway genes were transcribed inducibly, which indicated expression of the whole hypothetical pathway. We presume that this pathway is one source of sulfoacetate in nature, where this compound is dissimilated by many bacteria.

Transcriptional regulation of the sulfate-starvation-induced gene sfnA by a sigma54-dependent activator of Pseudomonas putida

The sigma(54)-dependent transcriptional regulator SfnR is essential for the use of dimethyl sulfone (DMSO(2)) as a sulfur source by Pseudomonas putida DS1. SfnR binds three SfnR-binding sites (sites 1, 2 and 3) within an intergenic region of the divergently transcribed sfnAB and sfnFG gene clusters. The site 1 region, proximal to the sfnF gene, is indispensable for the expression of the sfnFG operon, which encodes components of DMSO(2) monooxygenase. We investigated the transcriptional regulation of the sfnAB operon and possible functions of the sfnA gene. RT-PCR analysis revealed that the sfnAB gene cluster, which is similar to homologues of the acyl-CoA dehydrogenase family, was transcribed as an operon, and its expression was regulated by SfnR under conditions of sulfate starvation. Deletion analyses using lacZ as a reporter demonstrated that the region up to at least -138 bp from the transcription start point of sfnA (containing sites 2 and 3) was necessary for the expression of the sfnAB operon. A growth test of the sfnA-disrupted mutant revealed the possibility that sfnA may be involved in the use of methanethiol as a sulfur source.

Desulfurization of aromatic sulfonates by rhizosphere bacteria: high diversity of the asfA gene

The plant growth-promoting effect of Pseudomonas putida S-313 is associated with its ability to desulfurize arylsulfonates. To understand this further, other plant-associated bacteria able to desulfurize a range of arylsulfonates were isolated from the rhizospheres of winter and spring barley. The isolates belonged to the beta-proteobacteria, including bacteria from the Variovorax paradoxus group and from the Acidovorax genus. They desulfurized toluenesulfonate to p-cresol, and were found to contain orthologues of the P. putida S-313 asfA gene (> 70% sequence identity to AsfA), which is required for aryldesulfonation in this species. Further putative asfA orthologues were identified in several bacteria and cyanobacteria whose genomes have been sequenced, but of these only Cupriavidus (Ralstonia) metallidurans was able to utilize arylsulfonates as sulfur source. Cultivation of V. paradoxus, C. metallidurans or P. putida S-313 with toluenesulfonate as sulfur source led to a 100-fold increase in expression of the asfA homologues, which was largely repressed when sulfate was added. Polymerase chain reaction with degenerate primers was used to generate asfAB clone libraries from spring- and winter-barley rhizosphere DNA. Cluster analysis of 76 sequenced AsfA fragments revealed a broad diversity, with the majority of the sequences clustered together with AsfA from bacteria that are able to utilize toluenesulfonate as sulfur source. The diversity of asfA in barley rhizosphere underlines the importance of the desulfonation process for bacteria that inhabit the plant rhizosphere.

A global response to sulfur starvation in Pseudomonas putida and its relationship to the expression of low-sulfur-content proteins

Sulfur is essential for life on Earth, but its availability is limited in many environments. Here the sulfur-starvation response of the model soil bacterium Pseudomonas putida KT2440 is shown to be associated with an approximately fivefold reduction in the total soluble thiol content of the cell. A bioinformatic survey of the P. putida KT2440 genome identified 646 genes encoding proteins with a significantly lower than average sulfur content (low sulfur-content proteins, LSPs), the expression of which may have a role in the global reduction of cellular thiol content during sulfur starvation. Analysis of the genetic organization of the LSP-encoding genes showed that 31% were potentially transcriptionally associated with at least one other gene encoding a protein defined as an LSP. In particular, 55 LSP genes were located in three large clusters, termed low-sulfur islands (LSIs) here. The predicted identities of the proteins encoded by the LSIs strongly suggest that the LSIs have a role in acquiring sulfur from organic sulfur sources during sulfur starvation. This hypothesis was supported by transcription fusion studies on a limited number of LSP promoters under low-sulfur conditions. In a wider survey of bacterial species, LSIs were found to be more prevalent in free-living, Gram-negative bacteria than in Gram-positive or obligately intracellular bacteria.

Uncoupling of choline-O-sulphate utilization from osmoprotection in Pseudomonas putida

The genomic context of the recognized bet genes for choline-O-sulphate (COS) utilization in Pseudomonas putida KT2440 is such that betC (choline sulphatase) lies adjacent to an ATP-binding cassette transporter and a LysR type regulator, but well away from betBA, encoding enzymes for transformation of choline into glycine betaine. The consequences of such genetic layout of the functions for COS metabolism have been examined with a suite of genetic and biochemical approaches. An early clue of the utilities of the betencoded products was exposed by the phenotypes of a betC deletion. This mutant still accumulated intact COS but failed to use this compound as carbon or nitrogen source. Furthermore, betC expression was downregulated at high salt concentrations, showing that the principal role of this gene lied in COS metabolism, not in osmoprotection. In contrast, the betBA genes were required for choline transformation into the highly effective compatible solute glycine betaine (and the concomitant endurance to high salt) and also for its utilization as carbon or nitrogen source. Thus, unlike in the cases of Bacillus subtilis and Sinorhizobium meliloti, betC is unrelated to osmoprotection in Pseudomonas putida while the betBA genes are required for both betaine synthesis and tolerance to high osmotic pressure.

Regulation of sulfur assimilation pathways in Burkholderia cenocepacia: identification of transcription factors CysB and SsuR and their role in control of target genes

Two genes encoding transcriptional regulators involved in sulfur assimilation pathways in Burkholderia cenocepacia strain 715j have been identified and characterized functionally. Knockout mutations in each of the B. cenocepacia genes were constructed and introduced into the genome of 715j by allelic replacement. Studies on the utilization of various sulfur sources by 715j and the obtained mutants demonstrated that one of the B. cenocepacia regulators, designated CysB, is preferentially involved in the control of sulfate transport and reduction, while the other, designated SsuR, is required for aliphatic sulfonate utilization. Using transcriptional promoter-lacZ fusions and DNA-binding experiments, we identified several target promoters for positive control by CysB and/or SsuR--sbpp (preceding the sbp cysT cysW cysA ssuR cluster), cysIp (preceding the cysI cysD1 cysN cysH cysG cluster), cysD2p (preceding a separate cluster, cysD2 cysNC), and ssuDp (located upstream of the ssuDCB operon)--and we demonstrated overlapping functions of CysB and SsuR at particular promoters. We also demonstrated that the cysB gene is negatively controlled by both CysB and SsuR but the ssuR gene itself is not significantly regulated as a separate transcription unit. The function of B. cenocepacia CysB (in vivo and in vitro) appeared to be independent of the presence of acetylserine, the indispensable coinducer of the CysB regulators of Escherichia coli and Salmonella. The phylogenetic relationships among members of the "CysB family" in the gamma and beta subphyla are presented.

Two transsulfurylation pathways in Klebsiella pneumoniae

In most bacteria, inorganic sulfur is assimilated into cysteine, which provides sulfur for methionine biosynthesis via transsulfurylation. Here, cysteine is transferred to the terminal carbon of homoserine via its sulfhydryl group to form cystathionine, which is cleaved to yield homocysteine. In the enteric bacteria Escherichia coli and Salmonella enterica, these reactions are catalyzed by irreversible cystathionine-gamma-synthase and cystathionine-beta-lyase enzymes. Alternatively, yeast and some bacteria assimilate sulfur into homocysteine, which serves as a sulfhydryl group donor in the synthesis of cysteine by reverse transsulfurylation with a cystathionine-beta-synthase and cystathionine-gamma-lyase. Herein we report that the related enteric bacterium Klebsiella pneumoniae encodes genes for both transsulfurylation pathways; genetic and biochemical analyses show that they are coordinately regulated to prevent futile cycling. Klebsiella uses reverse transsulfurylation to recycle methionine to cysteine during periods of sulfate starvation. This methionine-to-cysteine (mtc) transsulfurylation pathway is activated by cysteine starvation via the CysB protein, by adenosyl-phosphosulfate starvation via the Cbl protein, and by methionine excess via the MetJ protein. While mtc mutants cannot use methionine as a sulfur source on solid medium, they will utilize methionine in liquid medium via a sulfide intermediate, suggesting that an additional nontranssulfurylation methionine-to-cysteine recycling pathway(s) operates under these conditions.

Importance of organosulfur utilization for survival of Pseudomonas putida in soil and rhizosphere

The sulfur present in both agricultural and uncultivated soils is largely in the form of sulfonates and sulfate esters and not as free, bioavailable inorganic sulfate. Desulfurization of the former compounds in vitro has previously been studied in Pseudomonas putida, a common rhizosphere inhabitant. Survival of P. putida strains was now investigated in three sulfur-deficient Danish soils which were found to contain 60 to 70% of their sulfur in sulfonate or sulfate ester form, as determined by X-ray near-edge spectroscopy. The soil fitness of P. putida S-313 was compared with that of isogenic strains with mutations in the sftR and asfA genes (required for in vitro desulfurization of sulfate esters and arylsulfonates, respectively) and in the ssu locus (required in vitro for the desulfurization of both sulfonates and sulfate esters). asfA or sftR mutants showed significantly reduced survival compared to the parent strain in bulk soil that had been enriched with carbon and nitrogen to mimic rhizosphere conditions, but this reduced survival was not observed in the absence of these additives. In a tomato rhizosphere grown in compost, survival of sftR and ssu mutants was reduced relative to the parent strain. The results demonstrate that the ability to desulfurize sulfonates and sulfate esters is critical for survival of bacteria in the rhizosphere but less so in bulk soils outside the influence of plant roots, where carbon is the limiting nutrient for growth.

Role of the ssu and seu genes of Corynebacterium glutamicum ATCC 13032 in utilization of sulfonates and sulfonate esters as sulfur sources

Corynebacterium glutamicum ATCC 13032 was found to be able to utilize a broad range of sulfonates and sulfonate esters as sulfur sources. The two gene clusters potentially involved in sulfonate utilization, ssuD1CBA and ssuI-seuABC-ssuD2, were identified in the genome of C. glutamicum ATCC 13032 by similarity searches. While the ssu genes encode proteins resembling Ssu proteins from Escherichia coli or Bacillus subtilis, the seu gene products exhibited similarity to the dibenzothiophene-degrading Dsz monooxygenases of Rhodococcus strain IGTS8. Growth tests with the C. glutamicum wild-type and appropriate mutant strains showed that the clustered genes ssuC, ssuB, and ssuA, putatively encoding the components of an ABC-type transporter system, are required for the utilization of aliphatic sulfonates. In C. glutamicum sulfonates are apparently degraded by sulfonatases encoded by ssuD1 and ssuD2. It was also found that the seu genes seuA, seuB, and seuC can effectively replace ssuD1 and ssuD2 for the degradation of sulfonate esters. The utilization of all sulfonates and sulfonate esters tested is dependent on a novel putative reductase encoded by ssuI. Obviously, all monooxygenases encoded by the ssu and seu genes, including SsuD1, SsuD2, SeuA, SeuB, and SeuC, which are reduced flavin mononucleotide dependent according to sequence similarity, have SsuI as an essential component. Using real-time reverse transcription-PCR, the ssu and seu gene cluster was found to be expressed considerably more strongly during growth on sulfonates and sulfonate esters than during growth on sulfate.

The sigma54-dependent transcriptional activator SfnR regulates the expression of the Pseudomonas putida sfnFG operon responsible for dimethyl sulphone utilization

Pseudomonas putida DS1 is able to utilize dimethyl sulphide through dimethyl sulphoxide, dimethyl sulphone (DMSO2), methanesulphonate (MSA) and sulphite as a sulphur source. We previously demonstrated that sfnR encoding a sigma54-dependent transcriptional regulator is essential for DMSO2 utilization by P. putida DS1. To identify the target genes of SfnR, we carried out transposon mutagenesis on an sfnR disruptant (DMSO2-utilization-defective phenotype) using mini-Tn5, which contains two outward-facing constitutively active promoters; as a result, we obtained a mutant that restored the ability to utilize DMSO2. The DMSO2-positive mutant carried a mini-Tn5 insertion in the intergenic region between two opposite-facing operons, sfnAB and sfnFG. Both sfnA and sfnB products were similar to acyl-CoA dehydrogenase family proteins, whereas sfnF and sfnG encoded a putative NADH-dependent FMN reductase (SfnF) and an FMNH2-dependent monooxygenase (SfnG). Disruption and complementation of the sfn genes indicated that the sfnG product is essential for DMSO2 utilization by P. putida DS1. Furthermore, an enzyme assay demonstrated that SfnG is an FMNH2-dependent DMSO2 monooxygenase that converts DMSO2 to MSA. It was revealed that the expression of the sfnFG operon is directly activated by the binding of SfnR at its upstream region. Site-directed mutagenesis of the SfnR binding sequences allowed us to define a potential recognition sequence for SfnR. These results provided insight into regulation of sulphate starvation-induced genes in bacteria.