Genetic analysis of the modABCD (molybdate transport) operon of Escherichia coli

Solution NMR chemical shift assignment of apo and molybdate-bound ModA at two pHs

ModA is a soluble periplasmic molybdate-binding protein found in most gram-negative bacteria. It is part of the ABC transporter complex ModABC that moves molybdenum into the cytoplasm, to be used by enzymes that carry out various redox reactions. Since there is no clear analog for ModA in humans, this protein could be a good target for antibacterial drug design. Backbone 1H, 13C and 15N chemical shifts of apo and molybdate-bound ModA from E. coli were assigned at pHs 6.0 and 4.5. In addition, side chain atoms were assigned for apo ModA at pH 6.0. When comparing apo and molybdate-bound ModA at pH 6.0, large chemical shift perturbations are observed, not only in areas near the bound metal, but also in regions that are distant from the metal-binding site. Given the significant conformational change between apo and holo ModA, we might expect the large chemical shift changes to be more widespread; however, since they are limited to specific regions, the residues with large perturbations may reveal allosteric sites that could ultimately be important for the design of antibiotics that target ModA.

First report on in-depth genome and comparative genome analysis of a metal-resistant bacterium Acinetobacter pittii S-30, isolated from environmental sample

A newly isolated bacterium Acinetobacter pittii S-30 was recovered from waste-contaminated soil in Ranchi, India. The isolated bacterium belongs to the ESKAPE organisms which represent the major nosocomial pathogens that exhibit high antibiotic resistance. Furthermore, average nucleotide identity (ANI) analysis also showed its closest match (>95%) to other A. pittii genomes. The isolate showed metal-resistant behavior and was able to survive up to 5 mM of ZnSO4. Whole genome sequencing and annotations revealed the occurrence of various genes involved in stress protection, motility, and metabolism of aromatic compounds. Moreover, genome annotation identified the gene clusters involved in secondary metabolite production (biosynthetic gene clusters) such as arylpolyene, acinetobactin like NRP-metallophore, betalactone, and hserlactone-NRPS cluster. The metabolic potential of A. pittii S-30 based on cluster of orthologous, and Kyoto Encyclopedia of Genes and Genomes indicated a high number of genes related to stress protection, metal resistance, and multiple drug-efflux systems etc., which is relatively rare in A. pittii strains. Additionally, the presence of various carbohydrate-active enzymes such as glycoside hydrolases (GHs), glycosyltransferases (GTs), and other genes associated with lignocellulose breakdown suggests that strain S-30 has strong biomass degradation potential. Furthermore, an analysis of genetic diversity and recombination in A. pittii strains was performed to understand the population expansion hypothesis of A. pittii strains. To our knowledge, this is the first report demonstrating the detailed genomic characterization of a heavy metal-resistant bacterium belonging to A. pittii. Therefore, the A. pittii S-30 could be a good candidate for the promotion of plant growth and other biotechnological applications.

[Construction and characterization of a modA gene mutant strain of Klebsiella pneumoniae]

OBJECTIVE

To construct a mutant strain of Klebsiella pneumoniae NTUH- K2044 with modA gene deletion and its complementary strain and explore the role of modA gene in modulating anaerobic nitrate respiratory growth and phenotypes of K. pneumoniae.

METHODS

The modA deletion mutant K. pneumoniae strain was constructed by homologous recombination using the suicide vector pKO3-Km. To obtain the complementary strain C-modA, the whole sequence fragment containing the promoter, open reading frame and terminator regions of modA was cloned into pGEM-T-easy and electrically transformed into the modA deletion mutant. The NTUH-K2044 wild-type strain, modA gene deletion mutant and complementary strain were compared by measuring in vitro anaerobic nitrate respiration growth, competitiveness index, biofilm quantification, mucoviscosity assay and morphological measurement using Image J.

RESULTS

The modA deletion mutant strain ΔmodA and the complementary strain C-modA were successfully constructed. The modA gene knockout strain showed inhibited anaerobic nitrate respiratory growth compared with the wild- type and C-modA strains with significantly weakened competitiveness, reduced capacity of biofilm synthesis during anaerobiosis, and lowered mucoviscosity under anaerobic conditions. The ΔmodA strain showed a spherical morphology in anaerobic conditions as compared with the normal short rod-like morphology of K. pneumoniae, with also distinctly shorter length than the wild-type and C-modA strains.

CONCLUSION

The molybdate transport system encoding gene modA is associated with the pathogenic capacity of K. pneumoniae by modulating its anaerobic nitrate respiration, competitiveness, biofilm formation, hypermucoviscous phenotype and morphology.

Mo than meets the eye: genomic insights into molybdoenzyme diversity of Seleniivibrio woodruffii strain S4T

Seleniivibrio woodruffii strain S4T is an obligate anaerobe belonging to the phylum Deferribacterota. It was isolated for its ability to respire selenate and was also found to respire arsenate. The high-quality draft genome of this bacterium is 2.9 Mbp, has a G+C content of 48%, 2762 predicted genes of which 2709 are protein-coding, and 53 RNA genes. An analysis of the genome focusing on the genes encoding for molybdenum-containing enzymes (molybdoenzymes) uncovered a remarkable number of genes encoding for members of the dimethylsulfoxide reductase family of proteins (DMSOR), including putative reductases for selenate and arsenate respiration, as well as genes for nitrogen fixation. Respiratory molybdoenzymes catalyze redox reactions that transfer electrons to a variety of substrates that can act as terminal electron acceptors for energy generation. Seleniivibrio woodruffii strain S4T also has essential genes for molybdate transporters and the biosynthesis of the molybdopterin guanine dinucleotide cofactors characteristic of the active centers of DMSORs. Phylogenetic analysis revealed candidate respiratory DMSORs spanning nine subfamilies encoded within the genome. Our analysis revealed the untapped potential of this interesting microorganism and expanded our knowledge of molybdoenzyme co-occurrence.

Characterization of ampicillin-resistant genes in Vibrio parahaemolyticus

Vibrio parahaemolyticus is strongly resistant to ampicillin (AMP). In this study, AMP-resistant genes in V. parahaemolyticus ATCC33846 were characterized. Transcriptomic analysis of V. parahaemolyticus exposed to AMP revealed 4608 differentially transcribed genes, including 670 significantly up-regulated genes and 655 significantly down-regulated genes. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses, significantly modulated genes in ATCC33846 under AMP stimulation were observed in the following categories: microbial metabolism in diverse environments, metabolic pathways, bacterial secretion system, citrate cycle, biofilm formation, oxidative phosphorylation, ribosome, citrate cycle, pyruvate metabolism, carbon metabolism, nitrogen metabolism, fatty acid metabolism and tryptophan metabolism. The genes VPA0510, VPA0252, VPA0699, VPA0768, VPA0320, VP0636, VPA1096, VPA0947 and VP1775 were significantly up-regulated at the similar level to blaA in V. parahaemolyticus under AMP stimulation, and their overexpression in V. parahaemolyticus could increase its resistance to AMP. These results indicate that AMP has a global influence on V. parahaemolyticus cells. The findings would provide new insights into the resistant mechanism of V. parahaemolyticus to AMP, which would be helpful for developing novel drugs for treating V. parahaemolyticus infection.

The Tol-Pal system of Escherichia coli plays an unexpected role in the import of the oxyanions chromate and phosphate

Chromate is a toxic metal that enters bacteria by using oxyanion importers. Here, we show that each mutant of the Tol-Pal system of Escherichia coli exhibited increased chromate resistance. This system, which spans the cell envelope, plays a major role in envelope integrity and septation. The ΔtolQR mutant accumulated three-fold less chromate than the wild-type. Addition of phosphate but not sulfate to rich medium drastically reduced chromate toxicity and import in the wild-type strain. Furthermore, the intracellular concentration of free inorganic phosphate was significantly reduced for the ΔtolR mutant in comparison to the wild-type strain. Moreover, extracellular labelled phosphate was significantly less incorporated into the ΔtolR mutant. Finally, two distinct TolQR mutant complexes, specifically affected in Tol-Pal energization without affecting the TolQRA complex structure, did not complement the ΔtolQR mutant for inorganic phosphate accumulation. We thus propose that, while the Pst system is well known to import inorganic phosphate, the Tol-Pal system participates to phosphate uptake in particular at medium to high extracellular phosphate concentrations. Since mutations disabling the Tol-Pal system lead to pleiotropic effects, chromate resistance and reduced inorganic phosphate import could occur from an indirect effect of mutations in components of the Tol-Pal system.

uvrY deletion and acetate reduce gut colonization of Crohn's disease-associated adherent-invasive Escherichia coli by decreasing expression of type 1 fimbriae

Adherent-invasive Escherichia coli (AIEC) is involved in onset and/or exacerbation of Crohn's disease. AIEC adapts to the gut environment by altering gene-expression programs, leading to successful gut-lumen colonization. However, the underlying mechanism of gut colonization is still far from clarified. Here, we show the role of UvrY, a response regulator of bacterial two-component signal transduction systems, in AIEC gut colonization. An AIEC mutant lacking the uvrY gene exhibited impairment of competitive colonization in the murine intestinal tract. UvrY contributes to functional expression of type 1 fimbriae by activating expression of small RNA CsrB, which confers adherence and invasion into epithelial cells on AIEC. In contrast, acetate suppresses the UvrY-dependent expression of type 1 fimbriae, resulting in less efficient cell invasion and attenuated gut colonization. Our findings might lead to therapeutic interventions for CD, in which inhibitions of UvrY activation and acetate supplementation reduce the colonization levels of AIEC by decreasing type-1 fimbriae expression.

The repertoire of ABC proteins in Clostridioides difficile

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ABC transporters  transport substrates across membranes driven by ATP hydrolysis.

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ABC proteins of C. difficile 630 can be classified into 12 sub-families.

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Most NPs are found within sub-families involving in drug export.

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Most core NPs in C. difficile are associated with drug efflux system.

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ABC proteins in sub-families 3, 6, 7, and 9 may participate in drug resistance.

ATP-binding cassette (ABC) transporters belong to one of the largest membrane protein superfamilies, which function in translocating substrates across biological membranes using energy from ATP hydrolysis. Currently, the classification of ABC transporters in Clostridioides difficile is not complete. Therefore, the sequence-function relationship of all ABC proteins encoded within the C. difficile genome was analyzed. Identification of protein domains associated with the ABC system in the C. difficile 630 reference genome revealed 226 domains: 97 nucleotide-binding domains (NBDs), 98 transmembrane domains (TMDs), 30 substrate-binding domains (SBDs), and one domain with features of an adaptor protein. Gene organization and transcriptional unit analyses indicated the presence of 78 ABC systems comprising 28 importers and 50 exporters. Based on NBD sequence similarity, ABC transporters were classified into 12 sub-families according to their substrates. Interestingly, all ABC exporters, accounting for 64% of the total ABC systems, are involved in antibiotic resistance. Based on analysis of ABC proteins from 49 C. difficile strains, the majority of core NBDs are predicted to be involved in multidrug resistance systems, consistent with the ability of this organism to survive exposure to an array of antibiotics. Our findings herein provide another step toward a better understanding of the function and evolutionary relationships of ABC proteins in this pathogen.

Transition metal transporters in rhizobia: tuning the inorganic micronutrient requirements to different living styles

A group of bacteria known as rhizobia are key players in symbiotic nitrogen fixation (SNF) in partnership with legumes. After a molecular exchange, the bacteria end surrounded by a plant membrane forming symbiosomes, organelle-like structures, where they differentiate to bacteroids and fix nitrogen. This symbiotic process is highly dependent on dynamic nutrient exchanges between the partners. Among these are transition metals (TM) participating as inorganic and organic cofactors of fundamental enzymes. While the understanding of how plant transporters facilitate TMs to the very near environment of the bacteroid is expanding, our knowledge on how bacteroid transporters integrate to TM homeostasis mechanisms in the plant host is still limited. This is significantly relevant considering the low solubility and scarcity of TMs in soils, and the in crescendo gradient of TM bioavailability rhizobia faces during the infection and bacteroid differentiation processes. In the present work, we review the main metal transporter families found in rhizobia, their role in free-living conditions and, when known, in symbiosis. We focus on discussing those transporters which could play a significant role in TM-dependent biochemical and physiological processes in the bacteroid, thus paving the way towards an optimized SNF.

The regulation of Moco biosynthesis and molybdoenzyme gene expression by molybdenum and iron in bacteria

The regulation of the operons involved in Moco biosynthesis is dependent on the availability of Fe–S clusters in the cell.

Characterization of the ModABC Molybdate Transport System of Pseudomonas putida in Nicotine Degradation

Pseudomonas putida J5 is an efficient nicotine-degrading bacterial strain that catabolizes nicotine through the pyrrolidine pathway. In our previous study, we used Tn5 transposon mutagenesis to investigate nicotine metabolism-associated genes, and 18 nicotine degradation-deficient mutants were isolated from 16,324 Tn5-transformants. Three of the mutants were Tn5 inserts into the modABC gene cluster that encoded an ABC-type, high-affinity, molybdate transporter. In-frame deletion of the modABC genes abolished the nicotine-degrading ability of strain J5, and complementation with modABC either from P. putida or Arthrobacter oxidans restored the degrading activity of the mutant to wild-type level. Nicotine degradation of J5 was inhibited markedly by addition of tungstate, a specific antagonist of molybdate. Molybdate at a non-physiologically high concentration (100 μM) fully restored nicotine-degrading activity and recovered growth of the modABC mutant in a nicotine minimal medium. Transcriptional analysis revealed that the expression of modABC was up-regulated at low molybdate concentrations and down-regulated at high moybdate concentrations, which indicated that at least one other system was able to transport molybdate, but with lower affinity. These results suggested that the molybdate transport system was essential to nicotine metabolism in P. putida J5.

Sulfate Transporters in Dissimilatory Sulfate Reducing Microorganisms: A Comparative Genomics Analysis

The first step in the sulfate reduction pathway is the transport of sulfate across the cell membrane. This uptake has a major effect on sulfate reduction rates. Much of the information available on sulfate transport was obtained by studies on assimilatory sulfate reduction, where sulfate transporters were identified among several types of protein families. Despite our growing knowledge on the physiology of dissimilatory sulfate-reducing microorganisms (SRM) there are no studies identifying the proteins involved in sulfate uptake in members of this ecologically important group of anaerobes. We surveyed the complete genomes of 44 sulfate-reducing bacteria and archaea across six phyla and identified putative sulfate transporter encoding genes from four out of the five surveyed protein families based on homology. We did not find evidence that ABC-type transporters (SulT) are involved in the uptake of sulfate in SRM. We speculate that members of the CysP sulfate transporters could play a key role in the uptake of sulfate in thermophilic SRM. Putative CysZ-type sulfate transporters were present in all genomes examined suggesting that this overlooked group of sulfate transporters might play a role in sulfate transport in dissimilatory sulfate reducers alongside SulP. Our in silico analysis highlights several targets for further molecular studies in order to understand this key step in the metabolism of SRMs.

Medicago truncatula Molybdate Transporter type 1 (MtMOT1.3) is a plasma membrane molybdenum transporter required for nitrogenase activity in root nodules under molybdenum deficiency

Molybdenum, as a component of the iron-molybdenum cofactor of nitrogenase, is essential for symbiotic nitrogen fixation. This nutrient has to be provided by the host plant through molybdate transporters. Members of the molybdate transporter family Molybdate Transporter type 1 (MOT1) were identified in the model legume Medicago truncatula and their expression in nodules was determined. Yeast toxicity assays, confocal microscopy, and phenotypical characterization of a Transposable Element from Nicotiana tabacum (Tnt1) insertional mutant line were carried out in the one M. truncatula MOT1 family member specifically expressed in nodules. Among the five MOT1 members present in the M. truncatula genome, MtMOT1.3 is the only one uniquely expressed in nodules. MtMOT1.3 shows molybdate transport capabilities when expressed in yeast. Immunolocalization studies revealed that MtMOT1.3 is located in the plasma membrane of nodule cells. A mot1.3-1 knockout mutant showed impaired growth concomitant with a reduction of nitrogenase activity. This phenotype was rescued by increasing molybdate concentrations in the nutritive solution, or upon addition of an assimilable nitrogen source. Furthermore, mot1.3-1 plants transformed with a functional copy of MtMOT1.3 showed a wild-type-like phenotype. These data are consistent with a model in which MtMOT1.3 is responsible for introducing molybdate into nodule cells, which is later used to synthesize functional nitrogenase.

A putative molybdate transporter LjMOT1 is required for molybdenum transport in Lotus japonicus

Molybdenum (Mo) is an essential micronutrient that is required for plant growth and development, and it affects the formation of root nodules and nitrogen fixation in legumes. In this study, Lotus japonicus was grown on MS solid media containing 0 nmol l^-1 (-Mo), 103 nmol l^-1 (+Mo) and 1030 nmol l^-1 (10 × Mo) of Mo. The phenotypes of plants growing on the three different media showed no obvious differences after 15 days, but the plants growing on -Mo for 45 days presented typical symptoms of Mo depletion, such as a short taproot, few lateral roots and yellowing leaves. A Mo transporter gene, LjMOT1, was isolated from L. japonicus. It encoded 468 amino acids, including two conserved motifs, and was predicted to locate to chromosome 3 of the L. japonicus genome. A homology comparison indicated that LjMOT1 had high similarities to other MOT1 proteins and was closely related to GmMOT1. Subcellular localization indicated that LjMOT1 is localized to the plasma membrane. qRT-PCR analyses showed that increasing Mo concentrations regulated the relative expression level of LjMOT1. Moreover, the Mo concentration in shoots was positively correlated to the expression of LjMOT1, but there was no such evident correlation in the roots. In addition, changes in the nitrate reductase activity were coincident with changes in the Mo concentration. These results suggest that LjMOT1 may be involved in the transport of Mo and provide a theoretical basis for further understanding of the mechanism of Mo transport in higher plants.

TupA: a tungstate binding protein in the periplasm of Desulfovibrio alaskensis G20

The TupABC system is involved in the cellular uptake of tungsten and belongs to the ABC (ATP binding cassette)-type transporter systems. The TupA component is a periplasmic protein that binds tungstate anions, which are then transported through the membrane by the TupB component using ATP hydrolysis as the energy source (the reaction catalyzed by the ModC component). We report the heterologous expression, purification, determination of affinity binding constants and crystallization of the Desulfovibrio alaskensis G20 TupA. The tupA gene (locus tag Dde_0234) was cloned in the pET46 Enterokinase/Ligation-Independent Cloning (LIC) expression vector, and the construct was used to transform BL21 (DE3) cells. TupA expression and purification were optimized to a final yield of 10 mg of soluble pure protein per liter of culture medium. Native polyacrylamide gel electrophoresis was carried out showing that TupA binds both tungstate and molybdate ions and has no significant interaction with sulfate, phosphate or perchlorate. Quantitative analysis of metal binding by isothermal titration calorimetry was in agreement with these results, but in addition, shows that TupA has higher affinity to tungstate than molybdate. The protein crystallizes in the presence of 30% (w/v) polyethylene glycol 3350 using the hanging-drop vapor diffusion method. The crystals diffract X-rays beyond 1.4 Å resolution and belong to the P2₁ space group, with cell parameters a = 52.25 Å, b = 42.50 Å, c = 54.71 Å, β = 95.43°. A molecular replacement solution was found, and the structure is currently under refinement.

The Biosynthesis of the Molybdenum Cofactor in Escherichia coli and Its Connection to FeS Cluster Assembly and the Thiolation of tRNA

The thiolation of biomolecules is a complex process that involves the activation of sulfur. The L-cysteine desulfurase IscS is the main sulfur mobilizing protein in Escherichia coli that provides the sulfur from L-cysteine to several important biomolecules in the cell such as iron sulfur (FeS) clusters, molybdopterin (MPT), thiamine, and thionucleosides of tRNA. Various proteins mediate the transfer of sulfur from IscS to various biomolecules using different interaction partners. A direct connection between the sulfur-containing molecules FeS clusters, thiolated tRNA, and the molybdenum cofactor (Moco) has been identified. The first step of Moco biosynthesis involves the conversion of 5′GTP to cyclic pyranopterin monophosphate (cPMP), a reaction catalyzed by a FeS cluster containing protein. Formed cPMP is further converted to MPT by insertion of two sulfur atoms. The sulfur for this reaction is provided by the L-cysteine desulfurase IscS in addition to the involvement of the TusA protein. TusA is also involved in the sulfur transfer for the thiolation of tRNA. This review will describe the biosynthesis of Moco in E. coli in detail and dissects the sulfur transfer pathways for Moco and tRNA and their connection to FeS cluster biosynthesis.

Molybdenum cofactor: a key component of Mycobacterium tuberculosis pathogenesis?

Mycobacterium tuberculosis (Mtb) and other members of the Mtb complex possess an expanded complement of genes for the biosynthesis of molybdenum cofactor (MoCo), a tricyclic pterin molecule that is covalently attached to molybdate. This cofactor allows the redox properties of molybdenum to be harnessed by enzymes in order to catalyze redox reactions in carbon, nitrogen and sulfur metabolism. In this article, we summarize recent advances in elucidating the MoCo biosynthetic pathway in Mtb and highlight the evidence implicating the biosynthesis of this cofactor, as well as the enzymes that depend upon it for activity, in Mtb pathogenesis.

Comparative genomics analysis of the metallomes

Biological trace metals are needed in small quantities, but used by all living organisms. They are employed in key cellular functions in a variety of biological processes, resulting in the various degree of dependence of organisms on metals. Most effort in the field has been placed on experimental studies of metal utilization pathways and metal-dependent proteins. On the other hand, systemic level analyses of metalloproteomes (or metallomes) have been limited for most metals. In this chapter, we focus on the recent advances in comparative genomics, which provides many insights into evolution and function of metal utilization. These studies suggested that iron and zinc are widely used in biology (presumably by all organisms), whereas some other metals such as copper, molybdenum, nickel, and cobalt, show scattered occurrence in various groups of organisms. For these metals, most user proteins are well characterized and their dependence on a specific element is evolutionarily conserved. We also discuss evolutionary dynamics of the dependence of user proteins on different metals. Overall, comparative genomics analysis of metallomes provides a foundation for the systemic level understanding of metal utilization as well as for investigating the general features, functions, and evolutionary dynamics of metal use in the three domains of life.

Polyoxometalates active against tumors, viruses, and bacteria

Polyoxometalates (PMs) as discrete metal-oxide cluster anions with high solubility in water and photochemically and electrochemically active property have a wide variety of structures not only in molecular size from sub-nano to sub-micrometers with a various combination of metals but also in symmetry and highly negative charge. One of the reasons for such a structural variety originates from their conformation change (due to the condensed aggregation and the structural assembly) which strongly depends on environmental parameters such as solution pH, concentration, and coexistent foreign inorganic and/or organic substances. In the course of the application of the physicochemical properties of such PMs to the medical fields, antitumoral, antiviral, and antibacterial activities have been developed for realization of a novel inorganic medicine which provides a biologically excellent activity never replaced by other approved medicines. Several PMs as a candidate for clinical uses have been licensed toward the chemotherapy of solid tumors (such as human gastric cancer and pancreatic cancer), DNA and RNA viruses (such as HSV, HIV, influenza, and SARS), and drug-resistant bacteria (such as MRSA and VRSA) in recent years: [NH3Pr(i)]6[Mo7O24]∙3H2O (PM-8) and [Me3NH]6[H2Mo(V) 12O28(OH)12(Mo(VI)O3)4]∙2H2O (PM-17) for solid tumors; K7[PTi2W10O40]∙6H2O (PM-19), [Pr(i)NH3]6H[PTi2W10O38(O2)2]∙H2O (PM-523), and K11H[(VO)3(SbW9O33)2]∙27H2O (PM-1002) for viruses; and K6[P2W18O62]∙14H2O (PM-27), K4[SiMo12O40]∙3H2O (SiMo12), and PM-19 for MRSA and VRSA. The results are discussed from a point of view of the chemotherapeutic clarification in this review.

Dual posttranscriptional regulation via a cofactor-responsive mRNA leader

Riboswitches are cis-acting mRNA elements that regulate gene expression in response to ligand binding. Recently, a class of riboswitches was proposed to respond to the molybdenum cofactor (Moco), which serves as a redox center for metabolic enzymes. The 5' leader of the Escherichia coli moaABCDE transcript exemplifies this candidate riboswitch class. This mRNA encodes enzymes for Moco biosynthesis, and moaA expression is feedback inhibited by Moco. Previous RNA-seq analyses showed that moaA mRNA copurified with the RNA binding protein CsrA (carbon storage regulator), suggesting that CsrA binds to this RNA in vivo. Among its global regulatory roles, CsrA represses stationary phase metabolism and activates central carbon metabolism. Here, we used gel mobility shift analysis to determine that CsrA binds specifically and with high affinity to the moaA 5' mRNA leader. Northern blotting and studies with a series of chromosomal lacZ reporter fusions showed that CsrA posttranscriptionally activates moaA expression without altering moaA mRNA levels, indicative of translation control. Deletion analyses, nucleotide replacement studies and footprinting with CsrA-FeBABE identified two sites for CsrA binding. Toeprinting assays suggested that CsrA binding causes changes in moaA RNA structure. We propose that the moaA mRNA leader forms an aptamer, which serves as a target of posttranscriptional regulation by at least two different factors, Moco and the protein CsrA. While we are not aware of similar dual posttranscriptional regulatory mechanisms, additional examples are likely to emerge.

Molybdenum enzymes, their maturation and molybdenum cofactor biosynthesis in Escherichia coli

Molybdenum cofactor (Moco) biosynthesis is an ancient, ubiquitous, and highly conserved pathway leading to the biochemical activation of molybdenum. Moco is the essential component of a group of redox enzymes, which are diverse in terms of their phylogenetic distribution and their architectures, both at the overall level and in their catalytic geometry. A wide variety of transformations are catalyzed by these enzymes at carbon, sulfur and nitrogen atoms, which include the transfer of an oxo group or two electrons to or from the substrate. More than 50 molybdoenzymes were identified in bacteria to date. In molybdoenzymes Mo is coordinated to a dithiolene group on the 6-alkyl side chain of a pterin called molybdopterin (MPT). The biosynthesis of Moco can be divided into four general steps in bacteria: 1) formation of the cyclic pyranopterin monophosphate, 2) formation of MPT, 3) insertion of molybdenum into molybdopterin to form Moco, and 4) additional modification of Moco with the attachment of GMP or CMP to the phosphate group of MPT, forming the dinucleotide variant of Moco. This review will focus on molybdoenzymes, the biosynthesis of Moco, and its incorporation into specific target proteins focusing on Escherichia coli. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Comparative proteome analysis of Helicobacter pylori clinical strains by two-dimensional gel electrophoresis

OBJECTIVE

To investigate the pathogenic properties of Helicobacter pylori by comparing the proteome map of H. pylori clinical strains.

METHODS

Two wild-type H. pylori strains, YN8 (isolated from biopsy tissue of a gastric cancer patient) and YN14 (isolated from biopsy tissue of a gastritis and duodenal ulcer patient), were used. Proteomic analysis, using a pH range of 3-10 and 5-8, was performed. The individual proteins were identified by quadrupole time-of-flight (Q-TOF) mass spectrometer and protein database search.

RESULTS

Variation in spot patterns directed towards differential protein expression levels was observed between the strains. The gel revealed prominent proteins with several protein "families". The comparison of protein expressions of the two strains reveals a high variability. Differentially present or absent spots were observed. Nine differentially expressed protein spots identified by Q-TOF included adenosine triphosphate (ATP)-binding protein, disulfide oxidoreductase B (DsbB)-like protein, N utilization substance A (NusA), ATP-dependent protease binding subunit/heat shock protein, hydantoin utilization protein A, seryl-tRNA synthetase, molybdenum ABC transporter ModD, and hypothetical proteins.

CONCLUSIONS

This study suggests that H. pylori strains express/repress protein variation, not only in terms of the virulence proteins, but also in terms of physiological proteins, when they infect a human host. The difference of protein expression levels between H. pylori strains isolated from gastric cancer and gastritis may be the initiator of inflammation, and result in the different clinical presentation. In this preliminary study, we report seven differential proteins between strains, with molecule weights from approximately 10 kDa to approximately 40 kDa. Further studies are needed to investigate those proteins and their function associated with H. pylori colonization and adaptation to host environment stress.

Classification of a Haemophilus influenzae ABC transporter HI1470/71 through its cognate molybdate periplasmic binding protein, MolA

molA (HI1472) from H. influenzae encodes a periplasmic binding protein (PBP) that delivers substrate to the ABC transporter MolB(2)C(2) (formerly HI1470/71). The structures of MolA with molybdate and tungstate in the binding pocket were solved to 1.6 and 1.7 Å resolution, respectively. The MolA-binding protein binds molybdate and tungstate, but not other oxyanions such as sulfate and phosphate, making it the first class III molybdate-binding protein structurally solved. The ∼100 μM binding affinity for tungstate and molybdate is significantly lower than observed for the class II ModA molybdate-binding proteins that have nanomolar to low micromolar affinity for molybdate. The presence of two molybdate loci in H. influenzae suggests multiple transport systems for one substrate, with molABC constituting a low-affinity molybdate locus.

Molybdenum metabolism in the alga Chlamydomonas stands at the crossroad of those in Arabidopsis and humans

Molybdenum (Mo) is a very scarce element whose function is fundamental in living beings within the active site of Mo-oxidoreductases, playing key roles in the metabolism of N, S, purines, hormone biosynthesis, transformation of drugs and xenobiotics, etc. In eukaryotes, each step from Mo acquisition until its incorporation into a biologically active molybdenum cofactor (Moco) together with the assembly of this Moco in Mo-enzymes is almost understood. The deficiency in function of a particular molybdoenzyme can be critical for the survival of the organism dependent on the pathway involved. However, incapacity in forming a functional Moco has a pleiotropic effect in the different processes involving this cofactor. A detailed overview of Mo metabolism: (a) specific transporters for molybdate, (b) the universal biosynthesis pathway for Moco from GTP, (c) Moco-carrier and Moco-binding proteins for Moco transfer and (d) Mo-enzymes, is analyzed in light of recent findings and three systems are compared, the unicellular microalga Chlamydomonas, the plant Arabidopsis and humans.

Effects of molybdate and tungstate on expression levels and biochemical characteristics of formate dehydrogenases produced by Desulfovibrio alaskensis NCIMB 13491

Formate dehydrogenases (FDHs) are enzymes that catalyze the formate oxidation to carbon dioxide and that contain either Mo or W in a mononuclear form in the active site. In the present work, the influence of Mo and W salts on the production of FDH by Desulfovibrio alaskensis NCIMB 13491 was studied. Two different FDHs, one containing W (W-FDH) and a second incorporating either Mo or W (Mo/W-FDH), were purified. Both enzymes were isolated from cells grown in a medium supplemented with 1 μM molybdate, whereas only the W-FDH was purified from cells cultured in medium supplemented with 10 μM tungstate. We demonstrated that the genes encoding the Mo/W-FDH are strongly downregulated by W and slightly upregulated by Mo. Metal effects on the expression level of the genes encoding the W-FDH were less significant. Furthermore, the expression levels of the genes encoding proteins involved in molybdate and tungstate transport are downregulated under the experimental conditions evaluated in this work. The molecular and biochemical properties of these enzymes and the selective incorporation of either Mo or W are discussed.

Molybdenum and tungsten in Campylobacter jejuni: their physiological role and identification of separate transporters regulated by a single ModE-like protein

Campylobacter jejuni is an important human pathogen that causes millions of cases of food-borne enteritis each year. The C. jejuni respiratory chain is highly branched and contains at least four enzymes predicted to contain a metal binding pterin (MPT), with the metal being either molybdenum or tungsten. Also predicted are two separate transport systems, one for molybdenum encoded by modABC and a second for tungsten encoded by tupABC. Both transport systems were mutated and the activities of the four predicted MPT-containing enzymes were assayed in the presence of molybdenum and tungsten in wild-type and mod and tup backgrounds. Results indicate that mod is primarily a molybdenum transporter that can also transport tungsten, while tup is a tungsten-specific transporter. The MPT containing enzymes nitrate reductase, sulphite oxidase, and SN oxide reductase are strict molybdoenzymes while formate dehydrogenase prefers tungsten. A ModE-like protein regulates both transporters, repressing mod in the presence of both molybdenum and tungsten and tup only in the presence of tungsten. Like other ModE proteins, the C. jejuni ModE binds DNA through a helix-turn-helix DNA binding domain, but unlike other members of the ModE family it does not have a metal binding domain.

Apo and ligand-bound structures of ModA from the archaeon Methanosarcina acetivorans

The trace-element oxyanion molybdate, which is required for the growth of many bacterial and archaeal species, is transported into the cell by an ATP-binding cassette (ABC) transporter superfamily uptake system called ModABC. ModABC consists of the ModA periplasmic solute-binding protein, the integral membrane-transport protein ModB and the ATP-binding and hydrolysis cassette protein ModC. In this study, X-ray crystal structures of ModA from the archaeon Methanosarcina acetivorans (MaModA) have been determined in the apoprotein conformation at 1.95 and 1.69 A resolution and in the molybdate-bound conformation at 2.25 and 2.45 A resolution. The overall domain structure of MaModA is similar to other ModA proteins in that it has a bilobal structure in which two mixed alpha/beta domains are linked by a hinge region. The apo MaModA is the first unliganded archaeal ModA structure to be determined: it exhibits a deep cleft between the two domains and confirms that upon binding ligand one domain is rotated towards the other by a hinge-bending motion, which is consistent with the 'Venus flytrap' model seen for bacterial-type periplasmic binding proteins. In contrast to the bacterial ModA structures, which have tetrahedral coordination of their metal substrates, molybdate-bound MaModA employs octahedral coordination of its substrate like other archaeal ModA proteins.

Biogenesis of membrane bound respiratory complexes in Escherichia coli

Escherichia coli is one of the preferred bacteria for studies on the energetics and regulation of respiration. Respiratory chains consist of primary dehydrogenases and terminal reductases or oxidases linked by quinones. In order to assemble this complex arrangement of protein complexes, synthesis of the subunits occurs in the cytoplasm followed by assembly in the cytoplasm and/or membrane, the incorporation of metal or organic cofactors and the anchoring of the complex to the membrane. In the case of exported metalloproteins, synthesis, assembly and incorporation of metal cofactors must be completed before translocation across the cytoplasmic membrane. Coordination data on these processes is, however, scarce. In this review, we discuss the various processes that respiratory proteins must undergo for correct assembly and functional coupling to the electron transport chain in E. coli. Targeting to and translocation across the membrane together with cofactor synthesis and insertion are discussed in a general manner followed by a review of the coordinated biogenesis of individual respiratory enzyme complexes. Lastly, we address the supramolecular organization of respiratory enzymes into supercomplexes and their localization to specialized domains in the membrane.

Molybdenum trafficking for nitrogen fixation

The molybdenum nitrogenase is responsible for most biological nitrogen fixation, a prokaryotic metabolic process that determines the global biogeochemical cycles of nitrogen and carbon. Here we describe the trafficking of molybdenum for nitrogen fixation in the model diazotrophic bacterium Azotobacter vinelandii. The genes and proteins involved in molybdenum uptake, homeostasis, storage, regulation, and nitrogenase cofactor biosynthesis are reviewed. Molybdenum biochemistry in A. vinelandii reveals unexpected mechanisms and a new role for iron-sulfur clusters in the sequestration and delivery of molybdenum.

General trends in trace element utilization revealed by comparative genomic analyses of Co, Cu, Mo, Ni, and Se

Trace elements are used by all organisms and provide proteins with unique coordination and catalytic and electron transfer properties. Although many trace element-containing proteins are well characterized, little is known about the general trends in trace element utilization. We carried out comparative genomic analyses of copper, molybdenum, nickel, cobalt (in the form of vitamin B(12)), and selenium (in the form of selenocysteine) in 747 sequenced organisms at the following levels: (i) transporters and transport-related proteins, (ii) cofactor biosynthesis traits, and (iii) trace element-dependent proteins. Few organisms were found to utilize all five trace elements, whereas many symbionts, parasites, and yeasts used only one or none of these elements. Investigation of metalloproteomes and selenoproteomes revealed examples of increased utilization of proteins that use copper in land plants, cobalt in Dehalococcoides and Dictyostelium, and selenium in fish and algae, whereas nematodes were found to have great diversity of copper transporters. These analyses also characterized trace element metabolism in common model organisms and suggested new model organisms for experimental studies of individual trace elements. Mismatches in the occurrence of user proteins and corresponding transport systems revealed deficiencies in our understanding of trace element biology. Biological interactions among some trace elements were observed; however, such links were limited, and trace elements generally had unique utilization patterns. Finally, environmental factors, such as oxygen requirement and habitat, correlated with the utilization of certain trace elements. These data provide insights into the general features of utilization and evolution of trace elements in the three domains of life.

Translesion DNA polymerases are required for spontaneous deletion formation in Salmonella typhimurium

How spontaneous deletions form in bacteria is still a partly unresolved problem. Here, we show that deletion formation in Salmonella typhimurium requires the presence of functional translesion polymerases. First, in wild-type bacteria, removal of the known translesion DNA polymerases, PolII (polB), PolIV (dinB), PolV (umuDC), and SamAB (samAB), resulted in a 10-fold decrease in the deletion rate, indicating that 90% of all spontaneous deletions require these polymerases for their formation. Second, overexpression of these polymerases by derepression of the DNA damage-inducible LexA regulon caused a 25-fold increase in deletion rate that depended on the presence of functional translesion polymerases. Third, overexpression of the polymerases PolII and PolIV from a plasmid increased the deletion rate 12- to 30-fold, respectively. Last, in a recBC(-) mutant where dsDNA ends are stabilized due to the lack of the end-processing nuclease RecBC, the deletion rate was increased 20-fold. This increase depended on the translesion polymerases. In lexA(def) mutant cells with constitutive SOS expression, a 10-fold increase in DNA breaks was observed. Inactivation of all 4 translesion polymerases in the lexA(def) mutant reduced the deletion rate 250-fold without any concomitant reduction in the amount of DNA breaks. Mutational inactivation of 3 endonucleases under LexA control reduced the number of DNA breaks to the wild-type level in a lexA(def) mutant with a concomitant 50-fold reduction in deletion rate. These findings suggest that the translesion polymerases are not involved in forming the DNA breaks, but that they require them to stimulate deletion formation.

A widespread riboswitch candidate that controls bacterial genes involved in molybdenum cofactor and tungsten cofactor metabolism

We have identified a highly conserved RNA motif located upstream of genes encoding molybdate transporters, molybdenum cofactor (Moco) biosynthesis enzymes, and proteins that utilize Moco as a coenzyme. Bioinformatics searches have identified 176 representatives in γ-Proteobacteria, δ-Proteobacteria, Clostridia, Actinobacteria, Deinococcus-Thermus species and DNAs from environmental samples. Using genetic assays, we demonstrate that a Moco RNA in Escherichia coli associated with the Moco biosynthetic operon controls gene expression in response to Moco production. In addition, we provide evidence indicating that this conserved RNA discriminates against closely related analogues of Moco. These results, together with extensive phylogenetic conservation and typical gene control structures near some examples, indicate that representatives of this structured RNA represent a novel class of riboswitches that sense Moco. Furthermore, we identify variants of this RNA that are likely to be triggered by the related tungsten cofactor (Tuco), which carries tungsten in place of molybdenum as the metal constituent.

A high-affinity molybdate transporter in eukaryotes

Molybdenum is an essential element for almost all living beings, which, in the form of a molybdopterin-cofactor, participates in the active site of enzymes involved in key reactions of carbon, nitrogen, and sulfur metabolism. This metal is taken up by cells in form of the oxyanion molybdate. Bacteria acquire molybdate by an ATP-binding-cassette (ABC) transport system in a widely studied process, but how eukaryotic cells take up molybdenum is unknown because molybdate transporters have not been identified so far. Here, we report a eukaryotic high-affinity molybdate transporter, encoded by the green alga Chlamydomonas reinhardtii gene MoT1. An antisense RNA strategy over the MoT1 gene showed that interference of the expression of this gene leads to the inhibition of molybdate transport activity and, in turn, of the Mo-containing enzyme nitrate reductase, indicating a function of MoT1 in molybdate transport. MOT1 functionality was also shown by heterologous expression in Saccharomyces cerevisiae. Molybdate uptake mediated by MOT1 showed a K(m) of approximately 6 nM, which is the range of the lowest K(m) values reported and was activated in the presence of nitrate. Analysis of deduced sequence from the putative protein coded by MoT1 showed motifs specifically conserved in similar proteins present in the databases, and defines a family of membrane proteins in both eukaryotes and prokaryotes probably involved in molybdate transport and distantly related to plant sulfate transporters SULTR. These findings represent an important step in the understanding of molybdate transport, a crucial process in eukaryotic cells.

Chaperone protection of immature molybdoenzyme during molybdenum cofactor limitation

Maturation of molybdoenzyme TorA involves chaperone TorD. This study shows that TorD is also required to protect apoTorA against proteolysis when the molybdenum cofactor is limiting in Escherichia coli. The absence of TorD leads to a complete loss of apoTorA during molybdenum cofactor deficiency whereas the presence of TorD maintains a significant amount of apoTorA that can be matured when the molybdenum cofactor becomes available.

The molybdate-binding protein (ModA) of the plant pathogen Xanthomonas axonopodis pv. citri

The modABC operon of phytopathogen Xanthomonas axonopodis pv. citri (X. citri) encodes a putative ABC transporter involved in the uptake of the molybdate and tungstate anions. Sequence analyses showed high similarity values of ModA orthologs found in X. campestris pv. campestris (X. campestris) and Escherichia coli. The X. citri modA gene was cloned in pET28a and the recombinant protein, expressed in the E. coli BL21 (DE3) strain, purified by immobilized metal affinity chromatography. The purified protein remained soluble and specifically bound molybdate and tungstate with K(d) 0.29+/-0.12 microM and 0.58+/-0.14 microM, respectively. Additionally binding of molybdate drastically enhanced the thermal stability of the recombinant ModA as compared to the apoprotein. This is the first characterization of a ModA ortholog expressed by a phytopathogen and represents an important tool for functional, biochemical and structural analyses of molybdate transport in Xanthomonas species.

Functional characterization of the Bradyrhizobium japonicum modA and modB genes involved in molybdenum transport

A modABC gene cluster that encodes an ABC-type, high-affinity molybdate transporter from Bradyrhizobium japonicum has been isolated and characterized. B. japonicum modA and modB mutant strains were unable to grow aerobically or anaerobically with nitrate as nitrogen source or as respiratory substrate, respectively, and lacked nitrate reductase activity. The nitrogen-fixing ability of the mod mutants in symbiotic association with soybean plants grown in a Mo-deficient mineral solution was severely impaired. Addition of molybdate to the bacterial growth medium or to the plant mineral solution fully restored the wild-type phenotype. Because the amount of molybdate required for suppression of the mutant phenotype either under free-living or under symbiotic conditions was dependent on sulphate concentration, it is likely that a sulphate transporter is also involved in Mo uptake in B. japonicum. The promoter region of the modABC genes has been characterized by primer extension. Reverse transcription and expression of a transcriptional fusion, P(modA)-lacZ, was detected only in a B. japonicum modA mutant grown in a medium without molybdate supplementation. These findings indicate that transcription of the B. japonicum modABC genes is repressed by molybdate.

The role of molybdenum in agricultural plant production

BACKGROUND

The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions. Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase.

SCOPE

Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields.

CONCLUSIONS

Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.

Molybdate transport and its effect on nitrogen utilization in the cyanobacterium Anabaena variabilis ATCC 29413

Molybdenum is an essential component of the cofactors of many metalloenzymes including nitrate reductase and Mo-nitrogenase. The cyanobacterium Anabaena variabilis ATCC 29413 uses nitrate and atmospheric N2 as sources of nitrogen for growth. Two of the three nitrogenases in this strain are Mo-dependent enzymes, as is nitrate reductase; thus, transport of molybdate is important for growth of this strain. High-affinity transport of molybdate in A. variabilis was mediated by an ABC-type transport system encoded by the products of modA and modBC. The modBC gene comprised a fused orf including components corresponding to modB and modC of Escherichia coli. The deduced ModC part of the fused gene lacked a recognizable molybdate-binding domain. Expression of modA and modBC was induced by starvation for molybdate. Mutants in modA or modBC were unable to grow using nitrate or Mo-nitrogenase. Growth using the alternative V-nitrogenase was not impaired in the mutants. A high concentration of molybdate (10 microM) supported normal growth of the modBC mutant using the Nif1 Mo-nitrogenase, indicating that there was a low-affinity molybdate transport system in this strain. The modBC mutant did not detectably transport low concentrations of 99Mo (molybdate), but did transport high concentrations. However, such transport was observed only after cells were starved for sulphate, suggesting that an inducible sulphate transport system might also serve as a low-affinity molybdate transport system in this strain.

Heavy metal tolerance and metal homeostasis inPseudomonas putidaas revealed by complete genome analysis

The genome of Pseudomonas putida KT2440 encodes an unexpected capacity to tolerate heavy metals and metalloids. The availability of the complete chromosomal sequence allowed the categorization of 61 open reading frames likely to be involved in metal tolerance or homeostasis, plus seven more possibly involved in metal resistance mechanisms. Some systems appeared to be duplicated. These might perform redundant functions or be involved in tolerance to different metals. In total, P. putida was found to bear two systems for arsenic (arsRBCH), one for chromate (chrA), four to six systems for divalent cations (two cadA and two to four czc chemiosmotic antiporters), two systems for monovalent cations: pacS, cusCBA (plus one cryptic silP gene containing a frameshift mutation), two operons for Cu chelation (copAB), one metallothionein for metal(loid) binding, one system for Te/Se methylation (tpmT) and four ABC transporters for the uptake of essential Zn, Mn, Mo and Ni (one nikABCDE, two znuACB and one mobABC). Some of the metal-related clusters are located in gene islands with atypical genome signatures. The predicted capacity of P. putida to endure exposure to heavy metals is discussed from an evolutionary perspective.

In vivo detection of molybdate-binding proteins using a competition assay with ModE in Escherichia coli

Molybdenum is an important trace element as it forms the essential part of the active site in all molybdenum-containing enzymes. We have designed an assay for the in vivo detection of molybdate binding to proteins in Escherichia coli. The assay is based on (i). the molybdate-dependent transcriptional regulation of the moa operon by the ModE protein, and (ii). the competition for molybdate between ModE and other molybdate-binding proteins in the cytoplasm of E. coli. We were able to verify in vivo molybdate binding to three different bacterial proteins that are known to bind molybdate. This sensitive in vivo system allows the testing of different proteins for molybdate binding under in vivo conditions and will facilitate the identification of other cellular factors needed for molybdate binding. As a first example, we examined the eukaryotic protein Cnx1 that is involved in the last step of molybdenum cofactor biosynthesis in plants, and show that it is able to compete with ModE for molybdate in a molybdopterin-dependent fashion.

Transport of molybdate in the cyanobacterium Anabaena variabilis ATCC 29413

Heterocyst-forming filamentous cyanobacteria, such as Anabaena variabilis ATCC 29413, require molybdenum as a component of two essential cofactors for the enzymes nitrate reductase and nitrogenase. A. variabilis efficiently transported (99)Mo (molybdate) at concentrations less than 10(-9) M. Competition experiments with other oxyanions suggested that the molybdate-transport system of A. variabilis also transported tungstate but not vanadate or sulfate. Although tungstate was probably transported, tungsten did not function in place of molybdenum in the Mo-nitrogenase. Transport of (99)Mo required prior starvation of the cells for molybdate, suggesting that the Mo-transport system was repressed by molybdate. Starvation, which required several generations of growth for depletion of molybdate, was enhanced by growth under conditions that required synthesis of nitrate reductase or nitrogenase. These data provide evidence for a molybdate storage system in A. variabilis. NtcA, a regulatory protein that is essential for synthesis of nitrate reductase and nitrogenase, was not required for transport of molybdate. The closely related strain Anabaena sp. PCC 7120 transported (99)Mo in a very similar way to A. variabilis.

Metals and the rhizobial-legume symbiosis--uptake, utilization and signalling

In this review, we consider how the nitrogen-fixing root nodule bacteria, the 'rhizobia', acquire various metals, paying particular attention to the uptake of iron. We also review the literature pertaining to the roles of molybdenum and nickel in the symbiosis with legumes. We highlight some gaps in our knowledge, for example the lack of information on how rhizobia acquire molybdenum. We examine the means whereby different metals affect rhizobial physiology and the role of metals as signals for gene regulation. We describe the ways in which genetics has shown (or not) if, and how, particular metal uptake and/or metal-mediated signalling pathways are required for the symbiotic interaction with legumes.