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-bindingcassette (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 Ϸ6 nM, which is the range of the lowest Km 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. micronutrient transport ͉ molybdenum ͉ molybdenum cofactor ͉ nitrate assimilation M olybdenum is one of the least abundant elements in organisms and is essential for molybdenum cofactor (Moco) biosynthesis. Moco is present in almost all living beings, taking part, as a prosthetic group, in the active site of key enzymes such as nitrate reductase, aldehyde oxidase, xanthine dehydrogenase, and sulfite oxidase. These enzymes participate in crucial processes for life such as nitrate assimilation, phytohormone biosynthesis, purine metabolism, and sulfite detoxification in plants, animals, and microorganisms (1, 2).Moco biosynthesis is a conserved pathway that consists of four main steps (3), conversion of GTP into cyclic pyranopterin monophosphate, synthesis of the molybdopterin dithiolate, adenylation of molybdopterin, and molybdenum-insertion reaction. In the fourth and final step, mature Moco is formed by the ligation of a single Mo atom to adenylated molybdopterin; this process directly depends on the intracellular availability of molybdenum. Deficiency in Moco biosynthesis results in a pleiotropic loss of all Mo-enzymes activity, which leads in humans to a severe metabolic disorder, and affected patients die in early childhood (4).In prokaryotes, molybdenum is taken up in an energydependent process by high-affinity molybdate transporters belonging to the ABC family (5). This system is formed by a periplasmic molybdate-bindin...

Iron is critical for symbiotic nitrogen fixation (SNF) as a key component of multiple ferroproteins involved in this biological process. In the model legume Medicago truncatula, iron is delivered by the vasculature to the infection/maturation zone (zone II) of the nodule, where it is released to the apoplast. From there, plasma membrane iron transporters move it into rhizobia-containing cells, where iron is used as the cofactor of multiple plant and rhizobial proteins (e.g. plant leghemoglobin and bacterial nitrogenase). MtNramp1 (Medtr3g088460) is the M. truncatula Natural Resistance-Associated Macrophage Protein family member, with the highest expression levels in roots and nodules. Immunolocalization studies indicate that MtNramp1 is mainly targeted to the plasma membrane. A loss-of-function nramp1 mutant exhibited reduced growth compared with the wild type under symbiotic conditions, but not when fertilized with mineral nitrogen. Nitrogenase activity was low in the mutant, whereas exogenous iron and expression of wild-type MtNramp1 in mutant nodules increased nitrogen fixation to normal levels. These data are consistent with a model in which MtNramp1 is the main transporter responsible for apoplastic iron uptake by rhizobia-infected cells in zone II.

Transition metals such as iron, copper, zinc, or molybdenum are essential nutrients for plants. These elements are involved in almost every biological process, including photosynthesis, tolerance to biotic and abiotic stress, or symbiotic nitrogen fixation. However, plants often grow in soils with limiting metallic oligonutrient bioavailability. Consequently, to ensure the proper metal levels, plants have developed a complex metal uptake and distribution system, that not only involves the plant itself, but also its associated microorganisms. These microorganisms can simply increase metal solubility in soils and making them more accessible to the host plant, as well as induce the plant metal deficiency response, or directly deliver transition elements to cortical cells. Other, instead of providing metals, can act as metal sinks, such as endosymbiotic rhizobia in legume nodules that requires relatively large amounts to carry out nitrogen fixation. In this review, we propose to do an overview of metal transport mechanisms in the plant–microbe system, emphasizing the role of arbuscular mycorrhizal fungi and endosymbiotic rhizobia.

The molybdenum cofactor (Moco) forms the catalytic site in all eukaryotic molybdenum enzymes and is synthesized by a multistep biosynthetic pathway. The mechanism of transfer, storage, and insertion of Moco into the appropriate apo-enzyme is poorly understood. In Chlamydomonas reinhardtii, a Moco carrier protein (MCP) has been identified and characterized recently. Here we show biochemical evidence that MCP binds Moco as well as the tungstate-substituted form of the cofactor (Wco) with high affinity, whereas molybdopterin, the ultimate cofactor precursor, is not bound. This binding selectivity points to a specific metal-mediated interaction with MCP, which protects Moco and Wco from oxidation with t1 ⁄ 2 of 24 and 96 h, respectively. UV-visible spectroscopy showed defined absorption bands at 393, 470, and 570 nm pointing to ene-diothiolate and protein side-chain charge transfer bonds with molybdenum. We have determined the crystal structure of MCP at 1.6 Å resolution using seleno-methionated and native protein. The monomer constitutes a Rossmann fold with two homodimers forming a symmetrical tetramer in solution. Based on conserved surface residues, charge distribution, shape, in silico docking studies, structural comparisons, and identification of an anionbinding site, a prominent surface depression was proposed as a Moco-binding site, which was confirmed by structure-guided mutagenesis coupled to substrate binding studies.Molybdenum is used in the active center of all molybdenum enzymes (1), catalyzing key metabolic reactions in the global sulfur, nitrogen, and carbon cycles in organisms ranging from bacteria to human. With the exception of nitrogenase, in all molybdenum enzymes, molybdenum is found as molybdenum cofactor (Moco) 3 (2) that consists of molybdenum covalently bound to the dithiolate moiety of a tricyclic pterin referred to as molybdopterin or metal-binding pterin (MPT), whose structure is conserved in eukaryotes, eubacteria, and archaea (3). This pterin molecule is also responsible for metal chelation in all tungsten-containing enzymes analyzed so far. Molybdenum enzymes are essential for diverse metabolic processes such as nitrate assimilation in autotrophs and phytohormone synthesis in plants (4) or sulfur detoxification and purine catabolism in mammals (5). Loss of Moco results in the pleiotropic loss of all molybdenum enzymes. Human Moco deficiency is a severe hereditary metabolic disorder (6), and affected patients die in early childhood.In all organisms studied so far, Moco is synthesized by a conserved pathway that can be divided into four major steps (3), according to the biosynthetic intermediates cyclic pyranopterin monophosphate, (formerly described as precursor Z) (7), MPT, and adenylated MPT. In the final and most diverse step of Moco biosynthesis, a single molybdenum atom is ligated to one (in pro-and eukaryotes) or two MPT dithiolates (in prokaryotes).After completion of biosynthesis, mature cofactor has to be inserted into molybdenum enzymes. In prokaryotes, a complex of proteins ...

Almost all living organisms need to obtain molybdenum from the external medium to achieve essential processes for life. Activity of important enzymes such as sulfite oxidase, aldehyde oxidase, xanthine dehydrogenase, and nitrate reductase is strictly dependent on the presence of Mo in its active site. Cells take up Mo in the form of the oxianion molybdate, but the molecular nature of the transporters is still not well known in eukaryotes. MOT1 is the first molybdate transporter identified in plant-type eukaryotic organisms, but it is absent in animal genomes. Here we report a molybdate transporter different from the MOT1 family, encoded by the Chlamydomonas reinhardtii gene MoT2 , that is also present in animals including humans. The knockdown of CrMoT2 transcription leads to the deficiency of molybdate uptake activity in Chlamydomonas . In addition, heterologous expression in Saccharomyces cerevisiae of MoT2 genes from Chlamydomonas and humans support the functionality of both proteins as molybdate transporters. Characterization of CrMOT2 and HsMOT2 activities showed an apparent Km of about 550 nM that, though higher than the Km reported for MOT1, still corresponds to high affinity systems. CrMoT2 transcription is activated when extracellular molybdate concentration is low but in contrast to MoT1 is not activated by nitrate. Analysis of protein databases revealed the presence of four motifs present in all the proteins with high similarity to MOT2, that label a previously undescribed family of proteins probably related to molybdate transport. Our results open the way toward the understanding of molybdate transport as part of molybdenum homeostasis and Moco biosynthesis in animals.

The viability of plants relies on molybdenum, which after binding to the organic moiety of molybdopterin forms the molybdenum cofactor (Moco) and acquires remarkable redox properties. Moco is in the active site of critical molybdoenzymes, which use to work as small electron transport chains and participate in N and S metabolism, hormone biosynthesis, toxic compound transformations and other important processes not only in plants but also in all the other kingdoms of life. Molybdate metabolism in plants is reviewed here, with special attention to two main aspects, the different molybdate transporters that with a very high affinity participate in molybdenum acquisition and the recently discovered Moco enzyme amidoxime-reducing component. Their functionality is starting to be understood.

Iron (Fe) is an essential micronutrient for symbiotic nitrogen fixation in legume nodules, where it is required for the activity of bacterial nitrogenase, plant leghemoglobin, respiratory oxidases, and other Fe proteins in both organisms. Fe solubility and transport within and between plant tissues is facilitated by organic chelators, such as nicotianamine and citrate. We have characterized a nodule-specific citrate transporter of the multidrug and toxic compound extrusion family, MtMATE67 of The MtMATE67 gene was induced early during nodule development and expressed primarily in the invasion zone of mature nodules. The MtMATE67 protein was localized to the plasma membrane of nodule cells and also the symbiosome membrane surrounding bacteroids in infected cells. In oocytes, MtMATE67 transported citrate out of cells in an Fe-activated manner. Loss of gene function resulted in accumulation of Fe in the apoplasm of nodule cells and a substantial decrease in symbiotic nitrogen fixation and plant growth. Taken together, the results point to a primary role of MtMATE67 in citrate efflux from nodule cells in response to an Fe signal. This efflux is necessary to ensure Fe(III) solubility and mobility in the apoplasm and uptake into nodule cells. Likewise, MtMATE67-mediated citrate transport into the symbiosome space would increase the solubility and availability of Fe(III) for rhizobial bacteroids.

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.