Structure and Function of Formate-Dependent Cytochrome c Nitrite Reductase, NrfA

Recent mechanistic developments for cytochrome c nitrite reductase, the key enzyme in the dissimilatory nitrate reduction to ammonium pathway

Cytochrome c nitrite reductase, NrfA, is a soluble, periplasmic pentaheme cytochrome responsible for the reduction of nitrite to ammonium in the Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway, a vital reaction in the global nitrogen cycle. NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we summarize the latest progress in elucidating the reaction mechanism of ammonia production, including new findings about the active site architecture of NrfA, as well as recent results that elucidate electron transfer and storage in the pentaheme scaffold of this enzyme.

Interactive effects of Pseudomonas entomophila strain 23S and Clavibacter michiganensis subsp. michiganensis on proteome and anti-Cmm compound production

Pseudomonas entomophila strain 23S is an effective biocontrol bacterium for tomato bacterial canker caused by Clavibacter michiganensis subsp. michiganensis (Cmm); it produces an inhibitory compound affecting the growth of Cmm. In this study, the interactions between pure cultures of P. entomophila 23S and Cmm were investigated. First, the population dynamics of each bacterium during the interaction was determined using the selective media. Second, the amount of anti-Cmm compound produced by P. entomophila 23S in the presence of Cmm was quantified using HPLC. Lastly, a label-free shotgun proteomics study of P. entomophila 23S, Cmm, and a co-culture was conducted to understand the effects of the interaction of each bacterium at the proteomic level. Compared with the pure culture grown, the total number of proteins decreased in the interaction for both bacteria. P. entomophila 23S secreted stress-related proteins, such as chaperonins, peptidases, ABC-transporters and elongation factors. The bacterium also produced more proteins related with purine, pyrimidine, carbon and nitrogen metabolisms in the presence of Cmm. The population enumeration study revealed that the Cmm population declined dramatically during the interaction, while the population of P. entomophila 23S maintained. The quantification of anti-Cmm compound indicated that P. entomophila 23S produced significantly higher amount of anti-Cmm compound when it was cultured with Cmm. Overall, the study suggested that P. entomophila 23S, although is cidal to Cmm, was also negatively affected by the presence of Cmm, while trying to adapt to the stress condition, and that such an environment favored increased production of the anti-Cmm compound by P. entomophila 23S. SIGNIFICANCE: Pseudomonas entomophila strain 23S is an effective biocontrol bacterium for tomato bacterial canker caused by Clavibacter michiganensis subsp. michiganensis (Cmm); it produces an inhibitory compound affecting the growth of Cmm. In this study, secreted proteome of pure cultures of P. entomophila 23S and Cmm, and also of a co-culture was first time identified. Furthermore, the study found that P. entomophila strain 23S produced significantly higher amount of anti-Cmm compound when the bacterium was grown together with Cmm. Co-culture enhancing anti-Cmm compound production by P. entomophila 23S is useful information, particularly from a commercial point of view of biocontrol application, and for scale-up of anti-Cmm compound production.

Architecture of the Heme-translocating CcmABCD/E complex required for Cytochrome c maturation

Mono- and multiheme cytochromes c are post-translationally matured by the covalent attachment of heme. For this, Escherichia coli employs the most complex type of maturation machineries, the Ccm-system (for cytochrome c maturation). It consists of two membrane protein complexes, one of which shuttles heme across the membrane to a mobile chaperone that then delivers the cofactor to the second complex, an apoprotein:heme lyase, for covalent attachment. Here we report cryo-electron microscopic structures of the heme translocation complex CcmABCD from E. coli, alone and bound to the heme chaperone CcmE. CcmABCD forms a heterooctameric complex centered around the ABC transporter CcmAB that does not by itself transport heme. Our data suggest that the complex flops a heme group from the inner to the outer leaflet at its CcmBC interfaces, driven by ATP hydrolysis at CcmA. A conserved heme-handling motif (WxWD) at the periplasmic side of CcmC rotates the heme by 90° for covalent attachment to the heme chaperone CcmE that we find interacting exclusively with the CcmB subunit.

An unusual active site architecture in cytochrome c nitrite reductase NrfA-1 from Geobacter metallireducens

Dissimilatory nitrate reduction to ammonia (DNRA) is a central pathway in the biogeochemical nitrogen cycle, allowing for the utilization of nitrate or nitrite as terminal electron acceptors. In contrast to the competing denitrification to N2, a major part of the essential nutrient nitrogen in DNRA is retained within the ecosystem and made available as ammonium to serve as a nitrogen source for other organisms. The second step of DNRA is mediated by the pentahaem cytochrome c nitrite reductase NrfA that catalyzes the six-electron reduction of nitrite to ammonium and is widely distributed among bacteria. A recent crystal structure of an NrfA ortholog from Geobacter lovleyi was the first characterized representative of a novel subclass of NrfA enzymes that lacked the canonical Ca2+ ion close to the active site haem 1. Here, we report the structural and functional characterization of NrfA from the closely related G. metallireducens. We established the recombinant production of catalytically active NrfA with its unique, lysine-coordinated active site haem heterologously in Escherichia coli and determined its three-dimensional structure by X-ray crystallography to 1.9 Å resolution. The structure confirmed GmNrfA as a further calcium-independent NrfA protein, and it also shows an altered active site that contained an unprecedented aspartate residue, D80, close to the substrate-binding site. This residue formed part of a loop that also caused a changed arrangement of the conserved substrate/product channel relative to other NrfA proteins and rendered the protein insensitive to the inhibitor sulphate. To elucidate the relevance of D80, we produced and studied the variants D80A and D80N that showed significantly reduced catalytic activity.

A Dual Enrichment Strategy Provides Soil- and Digestate-Competent Nitrous Oxide-Respiring Bacteria for Mitigating Climate Forcing in Agriculture

Manipulating soil metabolism through heavy inoculation with microbes is feasible if organic wastes can be utilized as the substrate for growth and vector as a fertilizer. This, however, requires organisms active in both digestate and soil (generalists). Here, we present a dual enrichment strategy to enrich and isolate such generalists among N₂O-respiring bacteria (NRB) in soil and digestates, to be used as an inoculum for strengthening the N₂O-reduction capacity of soils. The enrichment strategy utilizes sequential batch enrichment cultures alternating between sterilized digestate and soil as substrates, with each batch initiated with limited O₂ and unlimited N₂O. The cultures were monitored for gas kinetics and community composition. As predicted by a Lotka-Volterra competition model, cluster analysis identified generalist operational taxonomic units (OTUs) which became dominant, digestate/soil-specialists which did not, and a majority that were gradually diluted out. We isolated several NRBs circumscribed by generalist OTUs. Their denitrification genes and phenotypes predicted a variable capacity to act as N₂O-sinks, while all genomes predicted broad catabolic capacity. The latter contrasts with previous attempts to enrich NRB by anaerobic incubation of unsterilized digestate only, which selected for organisms with a catabolic capacity limited to fermentation products. The two isolates with the most promising characteristics as N₂O sinks were aPseudomonas sp. with a full-fledged denitrification-pathway and a Cloacibacterium sp. carrying only N₂O reductase (clade II), and soil experiments confirmed their capacity to reduce N₂O-emissions from soil. The successful enrichment of NRB with broad catabolic spectra suggests that the concept of dual enrichment should also be applicable for enrichment of generalists with traits other than N₂O reduction.

Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen

Since the characterization of cytochrome c₅₅₂ as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH₄⁺ produced from NO₂^- is released as NH₃ leading to nitrogen loss, similar to denitrification which generates NO, N₂O, and N₂. NH₄⁺ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO₂^- to NH₄⁺, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.

Two homologous Salmonella serogroup C1-specific genes are required for flagellar motility and cell invasion

Background

Salmonella is a major bacterial pathogen associated with a large number of outbreaks of foodborne diseases. Many highly virulent serovars that cause human illness belong to Salmonella serogroup C1, and Salmonella ser. Choleraesuis is a prominent cause of invasive infections in Asia. Comparative genomic analysis in our previous study showed that two homologous genes, SC0368 and SC0595 in Salmonella ser. Choleraesuis were unique to serogroup C1. In this study, two single-deletion mutants (Δ0368 and Δ0595) and one double-deletion mutant (Δ0368Δ0595) were constructed based on the genome. All these mutants and the wild-type strain were subjected to RNA-Seq analysis to reveal functional relationships of the two serogroup C1-specific genes.

Results

Data from RNA-Seq indicated that deletion of SC0368 resulted in defects in motility through repression of σ²⁸ in flagellar regulation Class 3. Consistent with RNA-Seq data, results from transmission electron microcopy (TEM) showed that flagella were not present in △0368 and △0368△0595 mutants resulting in both swimming and swarming defects. Interestingly, the growth rates of two non-motile mutants △0368 and △0368△0595 were significantly greater than the wild-type, which may be associated with up-regulation of genes encoding cytochromes, enhancing bacterial proliferation. Moreover, the △0595 mutant was significantly more invasive in Caco-2 cells as shown by bacterial enumeration assays, and the expression of lipopolysaccharide (LPS) core synthesis-related genes (rfaB, rfaI, rfaQ, rfaY, rfaK, rfaZ) was down-regulated only in the △0368△0595 mutant. In addition, this study also speculated that these two genes might be contributing to serotype conversion for Salmonella C1 serogroup based on their apparent roles in biosynthesis of LPS and the flagella.

Conclusion

A combination of biological and transcriptomic (RNA-Seq) analyses has shown that the SC0368 and SC0595 genes are involved in biosynthesis of flagella and complete LPS, as well as in bacterial growth and virulence. Such information will aid to revealing the role of these specific genes in bacterial physiology and evolution within the serogroup C1.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07759-z.

Review on Stress Tolerance in Campylobacter jejuni

Campylobacter spp. are the leading global cause of bacterial colon infections in humans. Enteropathogens are subjected to several stress conditions in the host colon, food complexes, and the environment. Species of the genus Campylobacter, in collective interactions with certain enteropathogens, can manage and survive such stress conditions. The stress-adaptation mechanisms of Campylobacter spp. diverge from other enteropathogenic bacteria, such as Escherichia coli, Salmonella enterica serovar Typhi, S. enterica ser. Paratyphi, S. enterica ser. Typhimurium, and species of the genera Klebsiella and Shigella. This review summarizes the different mechanisms of various stress-adaptive factors on the basis of species diversity in Campylobacter, including their response to various stress conditions that enhance their ability to survive on different types of food and in adverse environmental conditions. Understanding how these stress adaptation mechanisms in Campylobacter, and other enteric bacteria, are used to overcome various challenging environments facilitates the fight against resistance mechanisms in Campylobacter spp., and aids the development of novel therapeutics to control Campylobacter in both veterinary and human populations.

How Thermophilic Gram-Positive Organisms Perform Extracellular Electron Transfer: Characterization of the Cell Surface Terminal Reductase OcwA

Thermophilic Gram-positive organisms were recently shown to be a promising class of organisms to be used in bioelectrochemical systems for the production of electrical energy. These organisms present a thick peptidoglycan layer that was thought to preclude them to perform extracellular electron transfer (i.e., exchange catabolic electrons with solid electron acceptors outside the cell). In this paper, we describe the structure and functional mechanisms of the multiheme cytochrome OcwA, the terminal reductase of the Gram-positive bacterium Thermincola potens JR found at the cell surface of this organism. The results presented here show that this protein can take the role of a respiratory “Swiss Army knife,” allowing this organism to grow in environments with soluble and insoluble substrates. Moreover, it is shown that it is unrelated to terminal reductases found at the cell surface of other electroactive organisms. Instead, OcwA is similar to terminal reductases of soluble electron acceptors. Our data reveal that terminal oxidoreductases of soluble and insoluble substrates are evolutionarily related, providing novel insights into the evolutionary pathway of multiheme cytochromes.

Extracellular electron transfer is the key process underpinning the development of bioelectrochemical systems for the production of energy or added-value compounds. Thermincola potens JR is a promising Gram-positive bacterium to be used in these systems because it is thermophilic. In this paper, we describe the structural and functional properties of the nonaheme cytochrome OcwA, which is the terminal reductase of this organism. The structure of OcwA, determined at 2.2-Å resolution, shows that the overall fold and organization of the hemes are not related to other metal reductases and instead are similar to those of multiheme cytochromes involved in the biogeochemical cycles of nitrogen and sulfur. We show that, in addition to solid electron acceptors, OcwA can also reduce soluble electron shuttles and oxyanions. These data reveal that OcwA can work as a multipurpose respiratory enzyme allowing this organism to grow in environments with rapidly changing availability of terminal electron acceptors without the need for transcriptional regulation and protein synthesis.

Identification, Expression and Activity of Candidate Nitrite Reductases From Orange Beggiatoaceae, Guaymas Basin

Orange filamentous Beggiatoaceae form massive microbial mats on hydrothermal sediments in Guaymas Basin; these bacteria are considered to oxidize sulfide with nitrate and nitrite as electron acceptors. From a previously analyzed genome of an orange Beggiatoaceae filament, three candidate genes for enzymes with nitrite-reducing function - an orange octaheme cytochrome, a nirS nitrite reductase, and a nitrite/tetrathionate-reducing octaheme cytochrome - were cloned and expressed in Escherichia coli. The expressed and purified orange cytochrome showed reduced nitrite-reducing activity compared to the multifunctional native protein obtained from microbial mats. The nirS gene product showed in vitro but no in-gel nitrite-reducing activity; and the nitrite/tetrathionate-reducing octaheme cytochrome was capable of reducing both nitrite and tetrathionate in vitro. Phylogenetic analysis shows that the orange Beggiatoaceae nirS, in contrast to the other candidate nitrite reductases, does not form monophyletic lineages with its counterparts in other large sulfur-oxidizing bacteria, and most likely represents a recent acquisition by lateral gene transfer. The nitrite/tetrathionate-reducing enzyme of the orange Beggiatoaceae is related to nitrite- and tetrathionate reductases harbored predominantly by Gammaproteobacteria, including obligate endosymbionts of hydrothermal vent tubeworms. Thus, the orange Guaymas Basin Beggiatoaceae have a repertoire of at least three different functional enzymes for nitrite reduction. By demonstrating the unusual diversity of enzymes with a potential role in nitrite reduction, we show that bacteria in highly dynamic, sulfide-rich hydrothermal vent habitats adapt to these conditions that usually prohibit nitrate and nitrite reduction. In the case of the orange Guaymas Beggiatoaceae, classical denitrification appears to be replaced by different multifunctional enzymes for nitrite and tetrathionate reduction; the resulting ecophysiological flexibility provides a new key to the dominance of these Beggiatoaceae in hydrothermal hot spots.

Optimization of PCR primers to detect phylogenetically diverse nrfA genes associated with nitrite ammonification

Dissimilatory nitrate reduction to ammonium (DNRA) is now known to be a more prevalent process in terrestrial ecosystems than previously thought. The key enzyme, a pentaheme cytochrome c nitrite reductase NrfA associated with respiratory nitrite ammonification, is encoded by the nrfA gene in a broad phylogeny of bacteria. The lack of reliable and comprehensive molecular tools to detect diverse nrfA from environmental samples has hampered efforts to meaningfully characterize the genetic potential for DNRA in environmental systems. In this study, modifications were made to optimize the amplification efficiency of previously-designed PCR primers, targeting the diagnostic region of NrfA between the conserved third- and fourth heme binding domains, and to increase coverage to include detection of environmentally relevant Geobacteraceae-like nrfA. Using an alignment of the primers to >270 bacterial nrfA genes affiliated with 18 distinct clades, modifications to the primer sequences improved coverage, minimized amplification artifacts, and yielded the predicted product sizes from reference-, soil-, and groundwater DNA. Illumina sequencing of amplicons showed the successful recovery of nrfA gene fragments from environmental DNA based on alignments of the translated sequences. The new primers developed in this study are more efficient in PCR reactions, although gene targets with high GC content affect efficiency. Furthermore, the primers have a broader spectrum of detection and were validated rigorously for use in detecting nrfA from natural environments. These are suitable for conventional PCR, qPCR, and use in PCR access array technologies that allow multiplex gene amplification for downstream high throughput sequencing platforms.

Cytochromes c Constitute a Layer of Protection against Nitric Oxide but Not Nitrite

Nitric oxide (NO) is a radical gas that reacts with various biological molecules in complex ways to inhibit growth as a bacteriostatic agent. NO is nearly ubiquitous because it can be generated both biotically and abiotically. To protect the cell from NO damage, bacteria have evolved many strategies, with the production of detoxifying enzymatic systems being the most efficient. Here, we report that c-type cytochromes (cytochromes c) constitute a primary NO protection system in Shewanella oneidensis, a Gram-negative environmental bacterium renowned for respiratory versatility due to its high cytochrome c content. By using mutants producing cytochromes c at varying levels, we found that the content of these proteins is inversely correlated with the growth inhibition imposed by NO, whereas the effect of each individual cytochrome c is negligible. This NO-protecting system has no effect on nitrite inhibition. In the absence of cytochromes c, other NO targets and protective proteins, such as NnrS, emerge to show physiological influences during the NO stress. We further demonstrate that cytochromes c also play a similar role in Escherichia coli, albeit only modestly. Our data thus identify the in vivo function of an important group of proteins in alleviating NO stress.IMPORTANCE It is widely accepted that the antibacterial effects of nitrite are attributable to nitric oxide (NO) formation, suggesting a correlation of bacterial susceptibilities to these two chemicals. However, compared to E. coli, S. oneidensis is highly sensitive to nitrite but resistant to NO, implying the presence of robust NO-protective systems. Here, we show that c-type cytochromes (cytochromes c) play a main role in protecting S. oneidensis against damages from NO but not from nitrite. In their absence, impacts of proteins that promote NO tolerance and that are targets of NO inhibition become evident. Our data thus reveal the specific activity of cytochromes c in alleviating the stress caused by NO but not nitrite.

Distinct Nitrite and Nitric Oxide Physiologies in Escherichia coli and Shewanella oneidensis

Nitrite has been used as a bacteriostatic agent for centuries in food preservation. It is widely accepted that this biologically inert molecule functions indirectly, serving as a stable reservoir of bioactive nitric oxide (NO) and other reactive nitrogen species to impact physiology. As a result, to date, we know surprisingly little about in vivo targets of nitrite. Here, we carry out comparative analyses of nitrite and NO physiology in Escherichia coli and in Shewanella oneidensis, a Gram-negative environmental bacterium renowned for respiratory versatility. These two bacteria differ from each other in many aspects of nitrite and NO physiology, including NO generation, NO degradation, and unexpectedly, their contrary susceptibility to nitrite and NO. In cell extracts of both bacteria, most of the NO targets are also susceptible to nitrite, and vice versa. However, with respect to growth inhibition caused by NO, the targets are impacted distinctly; NO targets are responsible for the inhibition of growth of E. coli but not of S. oneidensis More surprisingly, all proteins identified to be implicated in NO tolerance in other bacteria appear to play a dispensable role in protecting S. oneidensis against NO. These data suggest that S. oneidensis is equipped with a robust but yet unknown NO protecting system. In the case of nitrite, it is clear that the target of physiological significance in both bacteria is cytochrome heme-copper oxidase.IMPORTANCE Nitrite is toxic to living organisms at high levels, but such antibacterial effects of nitrite are attributable to the formation of nitric oxide (NO), a highly reactive radical gas molecule. Here, we report that Shewanella oneidensis is highly resistant to NO but sensitive to nitrite compared to Escherichia coli by approximately 4-fold. In both bacteria, nitrite inhibits bacterial growth by targeting cytochrome heme-copper oxidase. In contrast, the targets of NO are diverse. Although these targets are similar in E. coli and S. oneidensis, they are responsible for growth inhibition caused by NO in the former but not in the latter. Overall, the presented data, along with the previous data, solidify a proposal that the in vivo targets of NO and nitrite in bacteria are largely different.

Genomic features of "Candidatus Venteria ishoeyi", a new sulfur-oxidizing macrobacterium from the Humboldt Sulfuretum off Chile

The Humboldt Sulfuretum (HS), in the productive Humboldt Eastern Boundary Current Upwelling Ecosystem, extends under the hypoxic waters of the Peru-Chile Undercurrent (ca. 6°S and ca. 36°S). Studies show that primeval sulfuretums held diverse prokaryotic life, and, while rare today, still sustain species-rich giant sulfur-oxidizing bacterial communities. We here present the genomic features of a new bacteria of the HS, "Candidatus Venteria ishoeyi" ("Ca. V. ishoeyi") in the family Thiotrichaceae.Three identical filaments were micro-manipulated from reduced sediments collected off central Chile; their DNA was extracted, amplified, and sequenced by a Roche 454 GS FLX platform. Using three sequenced libraries and through de novo genome assembly, a draft genome of 5.7 Mbp, 495 scaffolds, and a N50 of 70 kbp, was obtained. The 16S rRNA gene phylogenetic analysis showed that "Ca. V. ishoeyi" is related to non-vacuolate forms presently known as Beggiatoa or Beggiatoa-like forms. The complete set of genes involved in respiratory nitrate-reduction to dinitrogen was identified in "Ca. V. ishoeyi"; including genes likely leading to ammonification. As expected, the sulfur-oxidation pathway reported for other sulfur-oxidizing bacteria were deduced and also, key inorganic and organic carbon acquisition related genes were identified. Unexpectedly, the genome of "Ca. V. ishoeyi" contained numerous CRISPR repeats and an I-F CRISPR-Cas type system gene coding array. Findings further show that, as a member of an eons-old marine ecosystem, "Ca. V. ishoeyi" contains the needed metabolic plasticity for life in an increasingly oxygenated and variable ocean.

Responses of the picoprasinophyte Micromonas commoda to light and ultraviolet stress

Micromonas is a unicellular marine green alga that thrives from tropical to polar ecosystems. We investigated the growth and cellular characteristics of acclimated mid-exponential phase Micromonas commoda RCC299 over multiple light levels and over the diel cycle (14:10 hour light:dark). We also exposed the light:dark acclimated M. commoda to experimental shifts from moderate to high light (HL), and to HL plus ultraviolet radiation (HL+UV), 4.5 hours into the light period. Cellular responses of this prasinophyte were quantified by flow cytometry and changes in gene expression by qPCR and RNA-seq. While proxies for chlorophyll a content and cell size exhibited similar diel variations in HL and controls, with progressive increases during day and decreases at night, both parameters sharply decreased after the HL+UV shift. Two distinct transcriptional responses were observed among chloroplast genes in the light shift experiments: i) expression of transcription and translation-related genes decreased over the time course, and this transition occurred earlier in treatments than controls; ii) expression of several photosystem I and II genes increased in HL relative to controls, as did the growth rate within the same diel period. However, expression of these genes decreased in HL+UV, likely as a photoprotective mechanism. RNA-seq also revealed two genes in the chloroplast genome, ycf2-like and ycf1-like, that had not previously been reported. The latter encodes the second largest chloroplast protein in Micromonas and has weak homology to plant Ycf1, an essential component of the plant protein translocon. Analysis of several nuclear genes showed that the expression of LHCSR2, which is involved in non-photochemical quenching, and five light-harvesting-like genes, increased 30 to >50-fold in HL+UV, but was largely unchanged in HL and controls. Under HL alone, a gene encoding a novel nitrite reductase fusion protein (NIRFU) increased, possibly reflecting enhanced N-assimilation under the 625 μmol photons m^-2 s^-1 supplied in the HL treatment. NIRFU’s domain structure suggests it may have more efficient electron transfer than plant NIR proteins. Our analyses indicate that Micromonas can readily respond to abrupt environmental changes, such that strong photoinhibition was provoked by combined exposure to HL and UV, but a ca. 6-fold increase in light was stimulatory.

Thermodynamic integration network study of electron transfer: from proteins to aggregates

We describe electron transfer through the NrfHA nitrite reductase using a thermodynamic integration scheme. Driving forces are hardly affected by dimerization, but the transport mechanism only emerges simulating the dimer.

Exploring membrane respiratory chains

Acquisition of energy is central to life. In addition to the synthesis of ATP, organisms need energy for the establishment and maintenance of a transmembrane difference in electrochemical potential, in order to import and export metabolites or to their motility. The membrane potential is established by a variety of membrane bound respiratory complexes. In this work we explored the diversity of membrane respiratory chains and the presence of the different enzyme complexes in the several phyla of life. We performed taxonomic profiles of the several membrane bound respiratory proteins and complexes evaluating the presence of their respective coding genes in all species deposited in KEGG database. We evaluated 26 quinone reductases, 5 quinol:electron carriers oxidoreductases and 18 terminal electron acceptor reductases. We further included in the analyses enzymes performing redox or decarboxylation driven ion translocation, ATP synthase and transhydrogenase and we also investigated the electron carriers that perform functional connection between the membrane complexes, quinones or soluble proteins. Our results bring a novel, broad and integrated perspective of membrane bound respiratory complexes and thus of the several energetic metabolisms of living systems. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Nitrous Oxide Metabolism in Nitrate-Reducing Bacteria: Physiology and Regulatory Mechanisms

Nitrous oxide (N2O) is an important greenhouse gas (GHG) with substantial global warming potential and also contributes to ozone depletion through photochemical nitric oxide (NO) production in the stratosphere. The negative effects of N2O on climate and stratospheric ozone make N2O mitigation an international challenge. More than 60% of global N2O emissions are emitted from agricultural soils mainly due to the application of synthetic nitrogen-containing fertilizers. Thus, mitigation strategies must be developed which increase (or at least do not negatively impact) on agricultural efficiency whilst decrease the levels of N2O released. This aim is particularly important in the context of the ever expanding population and subsequent increased burden on the food chain. More than two-thirds of N2O emissions from soils can be attributed to bacterial and fungal denitrification and nitrification processes. In ammonia-oxidizing bacteria, N2O is formed through the oxidation of hydroxylamine to nitrite. In denitrifiers, nitrate is reduced to N2 via nitrite, NO and N2O production. In addition to denitrification, respiratory nitrate ammonification (also termed dissimilatory nitrate reduction to ammonium) is another important nitrate-reducing mechanism in soil, responsible for the loss of nitrate and production of N2O from reduction of NO that is formed as a by-product of the reduction process. This review will synthesize our current understanding of the environmental, regulatory and biochemical control of N2O emissions by nitrate-reducing bacteria and point to new solutions for agricultural GHG mitigation.

Comparative Genomics of the Genus Porphyromonas Identifies Adaptations for Heme Synthesis within the Prevalent Canine Oral Species Porphyromonas cangingivalis

Porphyromonads play an important role in human periodontal disease and recently have been shown to be highly prevalent in canine mouths. Porphyromonas cangingivalis is the most prevalent canine oral bacterial species in both plaque from healthy gingiva and plaque from dogs with early periodontitis. The ability of P. cangingivalis to flourish in the different environmental conditions characterized by these two states suggests a degree of metabolic flexibility. To characterize the genes responsible for this, the genomes of 32 isolates (including 18 newly sequenced and assembled) from 18 Porphyromonad species from dogs, humans, and other mammals were compared. Phylogenetic trees inferred using core genes largely matched previous findings; however, comparative genomic analysis identified several genes and pathways relating to heme synthesis that were present in P. cangingivalis but not in other Porphyromonads. Porphyromonas cangingivalis has a complete protoporphyrin IX synthesis pathway potentially allowing it to synthesize its own heme unlike pathogenic Porphyromonads such as Porphyromonas gingivalis that acquire heme predominantly from blood. Other pathway differences such as the ability to synthesize siroheme and vitamin B12 point to enhanced metabolic flexibility for P. cangingivalis, which may underlie its prevalence in the canine oral cavity.

Three transcription regulators of the Nss family mediate the adaptive response induced by nitrate, nitric oxide or nitrous oxide in Wolinella succinogenes

Sensing potential nitrogen-containing respiratory substrates such as nitrate, nitrite, hydroxylamine, nitric oxide (NO) or nitrous oxide (N2 O) in the environment and subsequent upregulation of corresponding catabolic enzymes is essential for many microbial cells. The molecular mechanisms of such adaptive responses are, however, highly diverse in different species. Here, induction of periplasmic nitrate reductase (Nap), cytochrome c nitrite reductase (Nrf) and cytochrome c N2 O reductase (cNos) was investigated in cells of the Epsilonproteobacterium Wolinella succinogenes grown either by fumarate, nitrate or N2 O respiration. Furthermore, fumarate respiration in the presence of various nitrogen compounds or NO-releasing chemicals was examined. Upregulation of each of the Nap, Nrf and cNos enzyme systems was found in response to the presence of nitrate, NO-releasers or N2 O, and the cells were shown to employ three transcription regulators of the Crp-Fnr superfamily (homologues of Campylobacter jejuni NssR), designated NssA, NssB and NssC, to mediate the upregulation of Nap, Nrf and cNos. Analysis of single nss mutants revealed that NssA controls production of the Nap and Nrf systems in fumarate-grown cells, while NssB was required to induce the Nap, Nrf and cNos systems specifically in response to NO-generators. NssC was indispensable for cNos production under any tested condition. The data indicate dedicated signal transduction routes responsive to nitrate, NO and N2 O and imply the presence of an N2 O-sensing mechanism.

Complete genome sequence of the chromate-reducing bacterium Thermoanaerobacter thermohydrosulfuricus strain BSB-33

Thermoanaerobacter thermohydrosulfuricus BSB-33 is a thermophilic gram positive obligate anaerobe isolated from a hot spring in West Bengal, India. Unlike other T. thermohydrosulfuricus strains, BSB-33 is able to anaerobically reduce Fe(III) and Cr(VI) optimally at 60 °C. BSB-33 is the first Cr(VI) reducing T. thermohydrosulfuricus genome sequenced and of particular interest for bioremediation of environmental chromium contaminations. Here we discuss features of T. thermohydrosulfuricus BSB-33 and the unique genetic elements that may account for the peculiar metal reducing properties of this organism. The T. thermohydrosulfuricus BSB-33 genome comprises 2597606 bp encoding 2581 protein genes, 12 rRNA, 193 pseudogenes and has a G + C content of 34.20 %. Putative chromate reductases were identified by comparative analyses with other Thermoanaerobacter and chromate-reducing bacteria.

Regulation of Nitrite Stress Response in Desulfovibrio vulgaris Hildenborough, a Model Sulfate-Reducing Bacterium

Sulfate-reducing bacteria (SRB) are sensitive to low concentrations of nitrite, and nitrite has been used to control SRB-related biofouling in oil fields. Desulfovibrio vulgaris Hildenborough, a model SRB, carries a cytochrome c-type nitrite reductase (nrfHA) that confers resistance to low concentrations of nitrite. The regulation of this nitrite reductase has not been directly examined to date. In this study, we show that DVU0621 (NrfR), a sigma54-dependent two-component system response regulator, is the positive regulator for this operon. NrfR activates the expression of the nrfHA operon in response to nitrite stress. We also show that nrfR is needed for fitness at low cell densities in the presence of nitrite because inactivation of nrfR affects the rate of nitrite reduction. We also predict and validate the binding sites for NrfR upstream of the nrfHA operon using purified NrfR in gel shift assays. We discuss possible roles for NrfR in regulating nitrate reductase genes in nitrate-utilizing Desulfovibrio spp.

IMPORTANCE

The NrfA nitrite reductase is prevalent across several bacterial phyla and required for dissimilatory nitrite reduction. However, regulation of the nrfA gene has been studied in only a few nitrate-utilizing bacteria. Here, we show that in D. vulgaris, a bacterium that does not respire nitrate, the expression of nrfHA is induced by NrfR upon nitrite stress. This is the first report of regulation of nrfA by a sigma54-dependent two-component system. Our study increases our knowledge of nitrite stress responses and possibly of the regulation of nitrate reduction in SRB.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

Structural and Functional Studies of NirC from Salmonella typhimurium

NirC is a pentameric transport system for monovalent anions that is expressed in the context of assimilatory nitrite reductase NirBD in a wide variety of enterobacterial species. A NirC pentamer contains individual pores in each protomer that mediate the passage of at least the nitrite [Formula: see text] and nitrate [Formula: see text] anions. As a member of the formate/nitrite transporter family of membrane transport proteins, NirC shares a range of structural and functional features with the formate channel FocA and the hydrosulfide channel AsrD (HSC). NirC from the enteropathogen Salmonella typhimurium has been studied by X-ray crystallography, proton uptake assays, and different electrophysiological techniques, and the picture that has emerged shows a fast and versatile transport system for nitrite that doubles as a defense system during the enteric life of the bacterium. Structural and functional assays are described, which shed light on the transport mechanism of this important molecular machine.

The octahaem MccA is a haem c-copper sulfite reductase

The six-electron reduction of sulfite to sulfide is the pivot point of the biogeochemical cycle of the element sulfur. The octahaem cytochrome c MccA (also known as SirA) catalyses this reaction for dissimilatory sulfite utilization by various bacteria. It is distinct from known sulfite reductases because it has a substantially higher catalytic activity and a relatively low reactivity towards nitrite. The mechanistic reasons for the increased efficiency of MccA remain to be elucidated. Here we show that anoxically purified MccA exhibited a 2- to 5.5-fold higher specific sulfite reductase activity than the enzyme isolated under oxic conditions. We determined the three-dimensional structure of MccA to 2.2 Å resolution by single-wavelength anomalous dispersion. We find a homotrimer with an unprecedented fold and haem arrangement, as well as a haem bound to a CX15CH motif. The heterobimetallic active-site haem 2 has a Cu(I) ion juxtaposed to a haem c at a Fe-Cu distance of 4.4 Å. While the combination of metals is reminiscent of respiratory haem-copper oxidases, the oxidation-labile Cu(I) centre of MccA did not seem to undergo a redox transition during catalysis. Intact MccA tightly bound SO2 at haem 2, a dehydration product of the substrate sulfite that was partially turned over due to photoreduction by X-ray irradiation, yielding the reaction intermediate SO. Our data show the biometal copper in a new context and function and provide a chemical rationale for the comparatively high catalytic activity of MccA.

Genomic resolution of linkages in carbon, nitrogen, and sulfur cycling among widespread estuary sediment bacteria

BACKGROUND

Estuaries are among the most productive habitats on the planet. Bacteria in estuary sediments control the turnover of organic carbon and the cycling of nitrogen and sulfur. These communities are complex and primarily made up of uncultured lineages, thus little is known about how ecological and metabolic processes are partitioned in sediments.

RESULTS

De novo assembly and binning resulted in the reconstruction of 82 bacterial genomes from different redox regimes of estuary sediments. These genomes belong to 23 bacterial groups, including uncultured candidate phyla (for example, KSB1, TA06, and KD3-62) and three newly described phyla (White Oak River (WOR)-1, WOR-2, and WOR-3). The uncultured phyla are generally most abundant in the sulfate-methane transition (SMTZ) and methane-rich zones, and genomic data predict that they mediate essential biogeochemical processes of the estuarine environment, including organic carbon degradation and fermentation. Among the most abundant organisms in the sulfate-rich layer are novel Gammaproteobacteria that have genes for the oxidation of sulfur and the reduction of nitrate and nitrite. Interestingly, the terminal steps of denitrification (NO3 to N2O and then N2O to N2) are present in distinct bacterial populations.

CONCLUSIONS

This dataset extends our knowledge of the metabolic potential of several uncultured phyla. Within the sediments, there is redundancy in the genomic potential in different lineages, often distinct phyla, for essential biogeochemical processes. We were able to chart the flow of carbon and nutrients through the multiple geochemical layers of bacterial processing and reveal potential ecological interactions within the communities.

Storage, transport, release: heme versatility in nitrite reductase electron transfer studied by molecular dynamics simulations

Using molecular dynamics simulations of the thermodynamic integration type, we study the energetics and kinetics of electron transfer through the nitrite reductase enzyme of Sulfurospirillum deleyianum, Wolinella succinogenes and Campylobacter jejuni.

Production and consumption of nitrous oxide in nitrate-ammonifying Wolinella succinogenes cells

Global warming is moving more and more into the public consciousness. Besides the commonly mentioned carbon dioxide and methane, nitrous oxide (N2O) is a powerful greenhouse gas in addition to its contribution to depletion of stratospheric ozone. The increasing concern about N2O emission has focused interest on underlying microbial energy-converting processes and organisms harbouring N2O reductase (NosZ), such as denitrifiers and ammonifiers of nitrate and nitrite. Here, the epsilonproteobacterial model organism Wolinella succinogenes is investigated with regard to its capacity to produce and consume N2O during growth by anaerobic nitrate ammonification. This organism synthesizes an unconventional cytochrome c nitrous oxide reductase (cNosZ), which is encoded by the first gene of an atypical nos gene cluster. However, W. succinogenes lacks a nitric oxide (NO)-producing nitrite reductase of the NirS- or NirK-type as well as an NO reductase of the Nor-type. Using a robotized incubation system, the wild-type strain and suitable mutants of W. succinogenes that either produced or lacked cNosZ were analysed as to their production of NO, N2O and N2 in both nitrate-sufficient and nitrate-limited growth medium using formate as electron donor. It was found that cells growing in nitrate-sufficient medium produced small amounts of N2O, which derived from nitrite and, most likely, from the presence of NO. Furthermore, cells employing cNosZ were able to reduce N2O to N2. This reaction, which was fully inhibited by acetylene, was also observed after adding N2O to the culture headspace. The results indicate that W. succinogenes cells are competent in N2O and N2 production despite being correctly grouped as respiratory nitrate ammonifiers. N2O production is assumed to result from NO detoxification and nitrosative stress defence, while N2O serves as a terminal electron acceptor in anaerobic respiration. The ecological implications of these findings are discussed.

Refined NrfA phylogeny improves PCR-based nrfA gene detection

Dissimilatory nitrate reduction to ammonium (DNRA) and denitrification are contrasting microbial processes in the terrestrial nitrogen (N) cycle, in that the former promotes N retention and the latter leads to N loss (i.e., the formation of gaseous products). The nitrite reductase NrfA catalyzes nitrite reduction to ammonium, the enzyme associated with respiratory nitrite ammonification and the key step in DNRA. Although well studied biochemically, the diversity and phylogeny of this enzyme had not been rigorously analyzed. A phylogenetic analysis of 272 full-length NrfA protein sequences distinguished 18 NrfA clades with robust statistical support (>90% Bayesian posterior probabilities). Three clades possessed a CXXCH motif in the first heme-binding domain, whereas all other clades had a CXXCK motif in this location. The analysis further identified a KXRH or KXQH motif between the third and fourth heme-binding motifs as a conserved and diagnostic feature of all pentaheme NrfA proteins. PCR primers targeting a portion of the heme-binding motifs that flank this diagnostic region yielded the expected 250-bp-long amplicons with template DNA from eight pure cultures and 16 new nrfA-containing isolates. nrfA amplicons obtained with template DNA from two geomorphically distinct agricultural soils could be assigned to one of the 18 NrfA clades, providing support for this expanded classification. The extended NrfA phylogeny revealed novel diagnostic features of DNRA populations and will be useful to assess nitrate/nitrite fate in natural and engineered ecosystems.

No laughing matter: the unmaking of the greenhouse gas dinitrogen monoxide by nitrous oxide reductase

The gas nitrous oxide (N₂O) is generated in a variety of abiotic, biotic, and anthropogenic processes and it has recently been under scrutiny for its role as a greenhouse gas. A single enzyme, nitrous oxide reductase, is known to reduce N₂O to uncritical N₂, in a two-electron reduction process that is catalyzed at two unusual metal centers containing copper. Nitrous oxide reductase is a bacterial metalloprotein from the metabolic pathway of denitrification, and it forms a 130 kDa homodimer in which the two metal sites CuA and CuZ from opposing monomers are brought into close contact to form the active site of the enzyme. CuA is a binuclear, valence-delocalized cluster that accepts and transfers a single electron. The CuA site of nitrous oxide reductase is highly similar to that of respiratory heme-copper oxidases, but in the denitrification enzyme the site additionally undergoes a conformational change on a ligand that is suggested to function as a gate for electron transfer from an external donor protein. CuZ, the tetranuclear active center of nitrous oxide reductase, is isolated under mild and anoxic conditions as a unique [4Cu:2S] cluster. It is easily desulfurylated to yield a [4Cu:S] state termed CuZ (*) that is functionally distinct. The CuZ form of the cluster is catalytically active, while CuZ (*) is inactive as isolated in the [3Cu(1+):1Cu(2+)] state. However, only CuZ (*) can be reduced to an all-cuprous state by sodium dithionite, yielding a form that shows higher activities than CuZ. As the possibility of a similar reductive activation in the periplasm is unconfirmed, the mechanism and the actual functional state of the enzyme remain under debate. Using enzyme from anoxic preparations with CuZ in the [4Cu:2S] state, N2O was shown to bind between the CuA and CuZ sites, suggesting direct electron transfer from CuA to the substrate after its activation by CuZ.

The production of ammonia by multiheme cytochromes C

The global biogeochemical nitrogen cycle is essential for life on Earth. Many of the underlying biotic reactions are catalyzed by a multitude of prokaryotic and eukaryotic life forms whereas others are exclusively carried out by microorganisms. The last century has seen the rise of a dramatic imbalance in the global nitrogen cycle due to human behavior that was mainly caused by the invention of the Haber-Bosch process. Its main product, ammonia, is a chemically reactive and biotically favorable form of bound nitrogen. The anthropogenic supply of reduced nitrogen to the biosphere in the form of ammonia, for example during environmental fertilization, livestock farming, and industrial processes, is mandatory in feeding an increasing world population. In this chapter, environmental ammonia pollution is linked to the activity of microbial metalloenzymes involved in respiratory energy metabolism and bioenergetics. Ammonia-producing multiheme cytochromes c are discussed as paradigm enzymes.

An introduction to nitric oxide sensing and response in bacteria

Nitric oxide (NO) is a radical gas that has been intensively studied for its role as a bacteriostatic agent. NO reacts in complex ways with biological molecules, especially metal centers and other radicals, to generate other bioactive compounds that inhibit enzymes, oxidize macromolecules, and arrest bacterial growth. Bacteria encounter not only NO derived from the host during infection but also NO derived from other bacteria and inorganic sources. The transcriptional responses used by bacteria to respond to NO are diverse but usually involve an iron-containing transcription factor that binds NO and alters its affinity for either DNA or factors involved in transcription, leading to the production of enzymatic tolerance systems. Some of these systems, such as flavohemoglobin and flavorubredoxin, directly remove NO. Some do not but are still important for NO tolerance through other mechanisms. The targets of NO that are protected by these systems include many metabolic pathways such as the tricarboxylic acid cycle and branched chain amino acid synthesis. This chapter discusses these topics and others and serves as a general introduction to microbial NO biology.

Unique organizational and functional features of the cytochrome c maturation system in Shewanella oneidensis

Shewanella are renowned for their ability to respire on a wide range of electron acceptors, which has been partially accredited to the presence of a large number of the c-type cytochromes. In the model species S. oneidensis MR-1, at least 41 genes encode c-type cytochromes that are predicted to be intact, thereby likely functional. Previously, in-frame deletion mutants for 36 of these genes were obtained and characterized. In this study, first we completed the construction of an entire set of c-type cytochrome mutants utilizing a newly developed att-based mutagenesis approach, which is more effective and efficient than the approach used previously by circumventing the conventional cloning. Second, we investigated the cytochrome c maturation (Ccm) system in S. oneidensis. There are two loci predicted to encode components of the Ccm system, SO0259-SO0269 and SO0476-SO0478. The former is proven essential for cytochrome c maturation whereas the latter is dispensable. Unlike the single operon organization observed in other γ-proteobacteria, genes at the SO0259-SO0269 locus are uniquely organized into four operons, ccmABCDE, scyA, SO0265, and ccmFGH-SO0269. Functional analysis revealed that the SO0265 gene rather than the scyA and SO0269 genes are relevant to cytochrome c maturation.

Sulfide oxidation, nitrate respiration, carbon acquisition, and electron transport pathways suggested by the draft genome of a single orange Guaymas Basin Beggiatoa (Cand. Maribeggiatoa) sp. filament

A near-complete draft genome has been obtained for a single vacuolated orange Beggiatoa (Cand. Maribeggiatoa) filament from a Guaymas Basin seafloor microbial mat, the third relatively complete sequence for the Beggiatoaceae. Possible pathways for sulfide oxidation; nitrate respiration; inorganic carbon fixation by both Type II RuBisCO and the reductive tricarboxylic acid cycle; acetate and possibly formate uptake; and energy-generating electron transport via both oxidative phosphorylation and the Rnf complex are discussed here. A role in nitrite reduction is suggested for an abundant orange cytochrome produced by the Guaymas strain; this has a possible homolog in Beggiatoa (Cand. Isobeggiatoa) sp. PS, isolated from marine harbor sediment, but not Beggiatoa alba B18LD, isolated from a freshwater rice field ditch. Inferred phylogenies for the Calvin-Benson-Bassham (CBB) cycle and the reductive (rTCA) and oxidative (TCA) tricarboxylic acid cycles suggest that genes encoding succinate dehydrogenase and enzymes for carboxylation and/or decarboxylation steps (including RuBisCO) may have been introduced to (or exported from) one or more of the three genomes by horizontal transfer, sometimes by different routes. Sequences from the two marine strains are generally more similar to each other than to sequences from the freshwater strain, except in the case of RuBisCO: only the Guaymas strain encodes a Type II enzyme, which (where studied) discriminates less against oxygen than do Type I RuBisCOs. Genes subject to horizontal transfer may represent key steps for adaptation to factors such as oxygen and carbon dioxide concentration, organic carbon availability, and environmental variability.

Microbial sulfite respiration

Despite its reactivity and hence toxicity to living cells, sulfite is readily converted by various microorganisms using distinct assimilatory and dissimilatory metabolic routes. In respiratory pathways, sulfite either serves as a primary electron donor or terminal electron acceptor (yielding sulfate or sulfide, respectively), and its conversion drives electron transport chains that are coupled to chemiosmotic ATP synthesis. Notably, such processes are also seen to play a general role in sulfite detoxification, which is assumed to have an evolutionary ancient origin. The diversity of sulfite conversion is reflected by the fact that the range of microbial sulfite-converting enzymes displays different cofactors such as siroheme, heme c, or molybdopterin. This chapter aims to summarize the current knowledge of microbial sulfite metabolism and focuses on sulfite catabolism. The structure and function of sulfite-converting enzymes and the emerging picture of the modular architecture of the corresponding respiratory/detoxifying electron transport chains is emphasized.

Nitrite reductase NirBD is induced and plays an important role during in vitro dormancy of Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the strongest reducers of nitrate among all mycobacteria. Reduction of nitrate to nitrite, mediated by nitrate reductase (NarGHJI) of M. tuberculosis, is induced during the dormant stage, and the enzyme has a respiratory function in the absence of oxygen. Nitrite reductase (NirBD) is also functional during aerobic growth when nitrite is the sole nitrogen source. However, the role of NirBD-mediated nitrite reduction during the dormancy is not yet characterized. Here, we analyzed nitrite reduction during aerobic growth as well as in a hypoxic dormancy model of M. tuberculosis in vitro. When nitrite was used as the sole nitrogen source in the medium, the organism grew and the reduction of nitrite was evident in both hypoxic and aerobic cultures of M. tuberculosis. Remarkably, the hypoxic culture of M. tuberculosis, compared to the aerobic culture, showed 32- and 4-fold-increased expression of nitrite reductase (NirBD) at the transcription and protein levels, respectively. More importantly, a nirBD mutant of M. tuberculosis was unable to reduce nitrite and compared to the wild-type (WT) strain had a >2-log reduction in viability after 240 h in the Wayne model of hypoxic dormancy. Dependence of M. tuberculosis on nitrite reductase (NirBD) was also seen in a human macrophage-based dormancy model where the nirBD mutant was impaired for survival compared to the WT strain. Overall, the increased expression and essentiality of nitrite reductase in the in vitro dormancy models suggested that NirBD-mediated nitrite reduction could be critical during the persistent stage of M. tuberculosis.

KEGG orthology-based annotation of the predicted proteome of Acropora digitifera: ZoophyteBase - an open access and searchable database of a coral genome

BACKGROUND

Contemporary coral reef research has firmly established that a genomic approach is urgently needed to better understand the effects of anthropogenic environmental stress and global climate change on coral holobiont interactions. Here we present KEGG orthology-based annotation of the complete genome sequence of the scleractinian coral Acropora digitifera and provide the first comprehensive view of the genome of a reef-building coral by applying advanced bioinformatics.

DESCRIPTION

Sequences from the KEGG database of protein function were used to construct hidden Markov models. These models were used to search the predicted proteome of A. digitifera to establish complete genomic annotation. The annotated dataset is published in ZoophyteBase, an open access format with different options for searching the data. A particularly useful feature is the ability to use a Google-like search engine that links query words to protein attributes. We present features of the annotation that underpin the molecular structure of key processes of coral physiology that include (1) regulatory proteins of symbiosis, (2) planula and early developmental proteins, (3) neural messengers, receptors and sensory proteins, (4) calcification and Ca2+-signalling proteins, (5) plant-derived proteins, (6) proteins of nitrogen metabolism, (7) DNA repair proteins, (8) stress response proteins, (9) antioxidant and redox-protective proteins, (10) proteins of cellular apoptosis, (11) microbial symbioses and pathogenicity proteins, (12) proteins of viral pathogenicity, (13) toxins and venom, (14) proteins of the chemical defensome and (15) coral epigenetics.

CONCLUSIONS

We advocate that providing annotation in an open-access searchable database available to the public domain will give an unprecedented foundation to interrogate the fundamental molecular structure and interactions of coral symbiosis and allow critical questions to be addressed at the genomic level based on combined aspects of evolutionary, developmental, metabolic, and environmental perspectives.

Characterization of Melioribacter roseus gen. nov., sp. nov., a novel facultatively anaerobic thermophilic cellulolytic bacterium from the class Ignavibacteria, and a proposal of a novel bacterial phylum Ignavibacteriae

A novel moderately thermophilic, facultatively anaerobic chemoorganotrophic bacterium strain P3M-2(T) was isolated from a microbial mat developing on the wooden surface of a chute under the flow of hot water (46°C) coming out of a 2775-m-deep oil exploration well (Tomsk region, Russia). Strain P3M-2(T) is a moderate thermophile and facultative anaerobe growing on mono-, di- or polysaccharides by aerobic respiration, fermentation or by reducing diverse electron acceptors [nitrite, Fe(III), As(V)]. Its closest cultivated relative (90.8% rRNA gene sequence identity) is Ignavibacterium album, the only chemoorganotrophic member of the phylum Chlorobi. New genus and species Melioribacter roseus are proposed for isolate P3M-2(T) . Together with I. album, the new organism represents the class Ignavibacteria assigned to the phylum Chlorobi. The revealed group includes a variety of uncultured environmental clones, the 16S rRNA gene sequences of some of which have been previously attributed to the candidate division ZB1. Phylogenetic analysis of M. roseus and I. album based on their 23S rRNA and RecA sequences confirmed that these two organisms could represent an even deeper, phylum-level lineage. Hence, we propose a new phylum Ignavibacteriae within the Bacteroidetes-Chlorobi group with a sole class Ignavibacteria, two families Ignavibacteriaceae and Melioribacteraceae and two species I. album and M. roseus. This proposal correlates with chemotaxonomic data and phenotypic differences of both organisms from other cultured representatives of Chlorobi. The most essential differences, supported by the analyses of complete genomes of both organisms, are motility, facultatively anaerobic and obligately organotrophic mode of life, the absence of chlorosomes and the apparent inability to grow phototrophically.

Enzyme or electrode?

The process of dissimilatory metal reduction shapes our environment on a global scale by using minerals as terminal acceptors in a biological electron transport chain employed by bacteria under anaerobic conditions. In this issue of Structure, Edwards et al. present the structure of an extracellular undecaheme cytochrome involved in the step of electron transfer to metal oxides.

Redundancy and modularity in membrane-associated dissimilatory nitrate reduction in Bacillus

The genomes of two phenotypically denitrifying type strains of the genus Bacillus were sequenced and the pathways for dissimilatory nitrate reduction were reconstructed. Results suggest that denitrification proceeds in the periplasmic space and in an analogous fashion as in Gram-negative organisms, yet with the participation of proteins that tend to be membrane-bound or membrane-associated. A considerable degree of functional redundancy was observed with marked differences between B. azotoformans LMG 9581(T) and B. bataviensis LMG 21833(T). In addition to the already characterized menaquinol/cyt c-dependent nitric oxide reductase (Suharti et al., 2001, 2004) of which the encoding genes could be identified now, evidence for another novel nitric oxide reductase (NOR) was found. Also, our analyses confirm earlier findings on branched electron transfer with both menaquinol and cytochrome c as reductants. Quite unexpectedly, both bacilli have the disposal of two parallel pathways for nitrite reduction enabling a life style as a denitrifier and as an ammonifying bacterium.

Redox properties of lysine- and methionine-coordinated hemes ensure downhill electron transfer in NrfH2A4 nitrite reductase

The multiheme NrfHA nitrite reductase is a menaquinol:nitrite oxidoreductase that catalyzes the 6-electron reduction of nitrite to ammonia in a reaction that involves eight protons. X-ray crystallography of the enzyme from Desulfovibrio vulgaris revealed that the biological unit, NrfH2A4, houses 28 c-type heme groups, 22 of them with low spin and 6 with pentacoordinated high spin configuration. The high spin hemes, which are the electron entry and exit points of the complex, carry a highly unusual coordination for c-type hemes, lysine and methionine as proximal ligands in NrfA and NrfH, respectively. Employing redox titrations followed by X-band EPR spectroscopy and surface-enhanced resonance Raman spectroelectrochemistry, we provide the first experimental evidence for the midpoint redox potential of the NrfH menaquinol-interacting methionine-coordinated heme (-270 ± 10 mV, z = 0.96), identified by the use of the inhibitor HQNO, a structural analogue of the physiological electron donor. The redox potential of the catalytic lysine-coordinated high spin heme of NrfA is -50 ± 10 mV, z = 0.9. These values determined for the integral NrfH2A4 complex indicate that a driving force for a downhill electron transfer is ensured in this complex.