Anaerobic utilization of phosphite and hypophosphite by Bacillus sp

AMP-dependent phosphite dehydrogenase, a phosphorylating enzyme in dissimilatory phosphite oxidation

Oxidation of phosphite (HPO32-) to phosphate (HPO42-) releases electrons at a very low redox potential (E0'= -690 mV) which renders phosphite an excellent electron donor for microbial energy metabolism. To date, two pure cultures of strictly anaerobic bacteria have been isolated that run their energy metabolism on the basis of phosphite oxidation, the Gram-negative Desulfotignum phosphitoxidans (DSM 13687) and the Gram-positive Phosphitispora fastidiosa (DSM 112739). Here, we describe the key enzyme for dissimilatory phosphite oxidation in these bacteria. The enzyme catalyzed phosphite oxidation in the presence of adenosine monophosphate (AMP) to form adenosine diphosphate (ADP), with concomitant reduction of oxidized nicotinamide adenine dinucleotide (NAD+) to reduced nicotinamide adenine dinucleotide (NADH). The enzyme of P. fastidiosa was heterologously expressed in Escherichia coli. It has a molecular mass of 35.2 kDa and a high affinity for phosphite and NAD+. Its activity was enhanced more than 100-fold by addition of ADP-consuming adenylate kinase (myokinase) to a maximal activity between 30 and 80 mU x mg protein-1. A similar NAD-dependent enzyme oxidizing phosphite to phosphate with concomitant phosphorylation of AMP to ADP is found in D. phosphitoxidans, but this enzyme could not be heterologously expressed. Based on sequence analysis, these phosphite-oxidizing enzymes are related to nucleotide-diphosphate-sugar epimerases and indeed represent AMP-dependent phosphite dehydrogenases (ApdA). A reaction mechanism is proposed for this unusual type of substrate-level phosphorylation reaction.

On the potential roles of phosphorus in the early evolution of energy metabolism

Energy metabolism in extant life is centered around phosphate and the energy-dense phosphoanhydride bonds of adenosine triphosphate (ATP), a deeply conserved and ancient bioenergetic system. Yet, ATP synthesis relies on numerous complex enzymes and has an autocatalytic requirement for ATP itself. This implies the existence of evolutionarily simpler bioenergetic pathways and potentially primordial alternatives to ATP. The centrality of phosphate in modern bioenergetics, coupled with the energetic properties of phosphorylated compounds, may suggest that primordial precursors to ATP also utilized phosphate in compounds such as pyrophosphate, acetyl phosphate and polyphosphate. However, bioavailable phosphate may have been notably scarce on the early Earth, raising doubts about the roles that phosphorylated molecules might have played in the early evolution of life. A largely overlooked phosphorus redox cycle on the ancient Earth might have provided phosphorus and energy, with reduced phosphorus compounds potentially playing a key role in the early evolution of energy metabolism. Here, we speculate on the biological phosphorus compounds that may have acted as primordial energy currencies, sources of environmental energy, or sources of phosphorus for the synthesis of phosphorylated energy currencies. This review encompasses discussions on the evolutionary history of modern bioenergetics, and specifically those pathways with primordial relevance, and the geochemistry of bioavailable phosphorus on the ancient Earth. We highlight the importance of phosphorus, not only in the form of phosphate, to early biology and suggest future directions of study that may improve our understanding of the early evolution of bioenergetics.

Role of phosphite in the environmental phosphorus cycle

In modern geochemistry, phosphorus (P) is considered synonymous with phosphate (Pi) because Pi controls the growth of organisms as a limiting nutrient in many ecosystems. The researchers therefore realised that a complete P cycle is essential. Limited by thermodynamic barriers, P was long believed to be incapable of redox reactions, and the role of the redox cycle of reduced P in the global P cycling system was thus not ascertained. Nevertheless, the phosphite (Phi) form of P is widely present in various environments and participates in the global P redox cycle. Herein, global quantitative evidences of Phi are enumerated and the early origin and modern biotic/abiotic sources of Phi are elaborated. Further, the Phi-based redox pathway for P reduction is analysed and global multienvironmental Phi redox cycle processes are proposed on the basis of this pathway. The possible role of Phi in controlling algae in eutrophic lakes and its ecological benefits to plants are proposed. In this manner, the important role of Phi in the P redox cycle and global P cycle is systematically and comprehensively identified and confirmed. This work will provide scientific guidance for the future production and use of Phi products and arouse attention and interest on clarifying the role of Phi in the environmental phosphorus cycle.

Review of Phosphite as a Plant Nutrient and Fungicide

Phosphite (Phi)-containing products are marketed for their antifungal and nutritional value. Substantial evidence of the anti-fungal properties of Phi on a wide variety of plants has been documented. Although Phi is readily absorbed by plant leaves and/or roots, the plant response to Phi used as a phosphorus (P) source is variable. Negative effects of Phi on plant growth are commonly observed under P deficiency compared to near adequate plant P levels. Positive responses to Phi may be attributed to some level of fungal disease control. While only a few studies have provided evidence of Phi oxidation through cellular enzymes genetically controlled in plant cells, increasing evidence exists for the potential to manipulate plant genes to enhance oxidation of Phi to phosphate (Pi) in plants. Advances in genetic engineering to sustain growth and yield with Phi + Pi potentially provides a dual fertilization and weed control system. Further advances in genetic manipulation of plants to utilize Phi are warranted. Since Phi oxidation occurs slowly in soils, additional information is needed to characterize Phi oxidation kinetics under variable soil and environmental conditions.

Shifts in the Composition of the Microbiota of Stored Wheat Grains in Response to Fumigation

While the wheat-associated microbiome is of major agricultural importance, little is known about the alterations in wheat grain microbial community composition during storage. Characterization of the bacterial and fungal communities in stored wheat grains revealed the impact of phosphine fumigation, one of the most effective methods to eliminate insects in stored commodities, on the composition of the wheat grain microbiome. High-throughput amplicon sequencing of the bacterial 16S rRNA gene and fungal internal transcribed spacer (ITS) region was used to analyze the wheat grain microbiome at different times over as 6 months period of storage. Higher bacterial diversity was found across the samples during the first (immediately after harvest) and second (3 months later) time points, with a predominance of Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Planctomycetes. A two-fold decrease in the number of bacterial operational taxonomic units (OTUs) was observed in wheat grains at the last time point (6 months later), following phosphine treatment. In contrast to the effect of phosphine on bacteria, it did not affect fungal diversity in stored grains. The majority of fungal sequences were assigned to Ascomycota, followed by Basidiomycota, Glomeromycota, and unidentified fungi, which were evenly distributed throughout the storage period. Alpha and beta diversity analyses were confirmed by examination of the cultured microbial taxa obtained from the stored wheat grains. Mycotoxin analysis of wheat grains collected after phosphine fumigation revealed the presence of Fusarium toxins, primarily deoxynivalenol (DON). Several mycotoxigenic Fusarium spp. were also detected in the same samples. Results of the present study indicate that microbiome of stored, whole wheat grains was strongly affected by phosphine fumigation, which changed the structure of the microbial community leading to shifts in species composition toward mycotoxigenic strains. A better understanding of the complex interactions within the microbial communities of stored grains will assist in the development of novel biocontrol strategies to overcome mycotoxin contamination.

Metagenomics-guided analysis of microbial chemolithoautotrophic phosphite oxidation yields evidence of a seventh natural CO2 fixation pathway

Dissimilatory phosphite oxidation (DPO), a microbial metabolism by which phosphite (HPO32-) is oxidized to phosphate (PO43-), is the most energetically favorable chemotrophic electron-donating process known. Only one DPO organism has been described to date, and little is known about the environmental relevance of this metabolism. In this study, we used 16S rRNA gene community analysis and genome-resolved metagenomics to characterize anaerobic wastewater treatment sludge enrichments performing DPO coupled to CO2 reduction. We identified an uncultivated DPO bacterium, Candidatus Phosphitivorax (Ca. P.) anaerolimi strain Phox-21, that belongs to candidate order GW-28 within the Deltaproteobacteria, which has no known cultured isolates. Genes for phosphite oxidation and for CO2 reduction to formate were found in the genome of Ca. P. anaerolimi, but it appears to lack any of the known natural carbon fixation pathways. These observations led us to propose a metabolic model for autotrophic growth by Ca. P. anaerolimi whereby DPO drives CO2 reduction to formate, which is then assimilated into biomass via the reductive glycine pathway.

Microbial Phosphite Oxidation and Its Potential Role in the Global Phosphorus and Carbon Cycles

Phosphite [Formula: see text] is a highly soluble, reduced phosphorus compound that is often overlooked in biogeochemical analyses. Although the oxidation of phosphite to phosphate is a highly exergonic process (E^o'=-650mV), phosphite is kinetically stable and can account for 10-30% of the total dissolved P in various environments. There is also evidence that phosphite was more prevalent under the reducing conditions of the Archean period and may have been involved in the development of early life. Its role as a phosphorus source for a variety of extant microorganisms has been known since the 1950s, and the pathways involved in assimilatory phosphite oxidation have been well characterized. More recently, it was demonstrated that phosphite could also act as an electron donor for energy metabolism in a process known as dissimilatory phosphite oxidation (DPO). The bacterium described in this study, Desulfotignum phosphitoxidans strain FiPS-3, was isolated from brackish sediments and is capable of growing by coupling phosphite oxidation to the reduction of either sulfate or carbon dioxide. FiPS-3 remains the only isolated organism capable of DPO, and the prevalence of this metabolism in the environment is still unclear. Nonetheless, given the widespread presence of phosphite in the environment and the thermodynamic favorability of its oxidation, microbial phosphite oxidation may play an important and hitherto unrecognized role in the global phosphorus and carbon cycles.

Life based on phosphite: a genome-guided analysis of Desulfotignum phosphitoxidans

Background

The Delta-Proteobacterium Desulfotignum phosphitoxidans is a type strain of the genus Desulfotignum, which comprises to date only three species together with D. balticum and D. toluenicum. D. phosphitoxidans oxidizes phosphite to phosphate as its only source of electrons, with either sulfate or CO₂ as electron acceptor to gain its metabolic energy, which is of exclusive interest. Sequencing of the genome of this bacterium was undertaken to elucidate the genomic basis of this so far unique type of energy metabolism.

Results

The genome contains 4,998,761 base pairs and 4646 genes of which 3609 were assigned to a function, and 1037 are without function prediction. Metabolic reconstruction revealed that most biosynthetic pathways of Gram negative, autotrophic sulfate reducers were present. Autotrophic CO₂ assimilation proceeds through the Wood-Ljungdahl pathway. Additionally, we have found and confirmed the ability of the strain to couple phosphite oxidation to dissimilatory nitrate reduction to ammonia, which in itself is a new type of energy metabolism. Surprisingly, only two pathways for uptake, assimilation and utilization of inorganic and organic phosphonates were found in the genome. The unique for D. phosphitoxidans Ptx-Ptd cluster is involved in inorganic phosphite oxidation and an atypical C-P lyase-coding cluster (Phn) is involved in utilization of organophosphonates.

Conclusions

We present the whole genome sequence of the first bacterium able to gain metabolic energy via phosphite oxidation. The data obtained provide initial information on the composition and architecture of the phosphite–utilizing and energy-transducing systems needed to live with phosphite as an unusual electron donor.

Draft Genome Sequence of Desulfotignum phosphitoxidans DSM 13687 Strain FiPS-3

We report the 5.008-Mbp assembled draft genome sequence of Desulfotignum phosphitoxidans strain FiPS-3 (DSM 13687), which gains metabolic energy from the oxidation of phosphite to phosphate. Its genome provides insights into the composition and architecture of the phosphite-utilizing and energy-transducing systems required to live with phosphite as electron donor.

Identification and heterologous expression of genes involved in anaerobic dissimilatory phosphite oxidation by Desulfotignum phosphitoxidans

Desulfotignum phosphitoxidans is a strictly anaerobic, Gram-negative bacterium that utilizes phosphite as the sole electron source for homoacetogenic CO2 reduction or sulfate reduction. A genomic library of D. phosphitoxidans, constructed using the fosmid vector pJK050, was screened for clones harboring the genes involved in phosphite oxidation via PCR using primers developed based on the amino acid sequences of phosphite-induced proteins. Sequence analysis of two positive clones revealed a putative operon of seven genes predicted to be involved in phosphite oxidation. Four of these genes (ptxD-ptdFCG) were cloned and heterologously expressed in Desulfotignum balticum, a related strain that cannot use phosphite as either an electron donor or as a phosphorus source. The ptxD-ptdFCG gene cluster was sufficient to confer phosphite uptake and oxidation ability to the D. balticum host strain but did not allow use of phosphite as an electron donor for chemolithotrophic growth. Phosphite oxidation activity was measured in cell extracts of D. balticum transconjugants, suggesting that all genes required for phosphite oxidation were cloned. Genes of the phosphite gene cluster were assigned putative functions on the basis of sequence analysis and enzyme assays.

Genetic diversity and horizontal transfer of genes involved in oxidation of reduced phosphorus compounds by Alcaligenes faecalis WM2072

Enrichment was performed to isolate organisms that could utilize reduced phosphorus compounds as their sole phosphorus sources. One isolate that grew well with either hypophosphite or phosphite was identified by 16S rRNA gene analysis as a strain of Alcaligenes faecalis. The genes required for oxidation of hypophosphite and phosphite by this organism were identified by using transposon mutagenesis and include homologs of the ptxD and htxA genes of Pseudomonas stutzeri WM88, which encode an NAD-dependent phosphite dehydrogenase (PtxD) and 2-oxoglutarate-dependent hypophosphite dioxygenase (HtxA). This organism also has the htxB, htxC, and htxD genes that comprise an ABC-type transporter, presumably for hypophosphite and phosphite transport. The role of these genes in reduced phosphorus metabolism was confirmed by analyzing the growth of mutants in which these genes were deleted. Sequencing data showed that htxA, htxB, htxC, and htxD are virtually identical to their homologs in P. stutzeri at the DNA level, indicating that horizontal gene transfer occurred. However, A. faecalis ptxD is very different from its P. stutzeri homolog and represents a new ptxD lineage. Therefore, this gene has ancient evolutionary roots in bacteria. These data suggest that there is strong evolutionary selection for the ability of microorganisms to oxidize hypophosphite and phosphite.

Mechanism and applications of phosphite dehydrogenase

Phosphite dehydrogenase catalyzes the NAD+-dependent oxidation of hydrogen phosphonate (common name phosphite) to phosphate in what amounts to a formal phosphoryl transfer reaction from hydride to hydroxide. This review places the enzyme in the context of phosphorus redox metabolism in nature and discusses the results of mechanistic investigations into its reaction mechanism. The potential of the enzyme as a NAD(P)H cofactor regeneration system is discussed as well as efforts to engineer the cofactor specificity of the protein.

The htx and ptx operons of Pseudomonas stutzeri WM88 are new members of the pho regulon

The htx and ptx operons of Pseudomonas stutzeri WM88 allow for the use of the inorganic reduced phosphorus (P) compounds hypophosphite (P valence, +1) and phosphite (P valence, +3) as sole P sources. To support the proposed in vivo role for the htx and ptx operons, namely the use of phosphite and hypophosphite as alternative P sources, we used reporter gene fusions to examine their expression levels with respect to various P conditions. Expression of the htx and ptx operons was induced up to 17- and 22-fold, respectively, in cultures grown under phosphate starvation conditions relative to expression in medium with excess phosphate (Pi). However, the presence of the reduced P substrate hypophosphite, phosphite, or methylphosphonate, in addition to excess Pi, did not result in an increase in the expression of either operon. To provide further support for a role of the htx and ptx operons in Pi acquisition, we identified P. stutzeri phoBR homologs and constructed deletion mutants. Induction of the htx and ptx reporter gene fusions in response to growth on limiting Pi was abolished in DeltaphoB, DeltaphoR, and DeltaphoBR mutants, demonstrating that htx and ptx expression is phoBR dependent. The putative LysR-type regulator encoded by ptxE has no apparent role in the expression of the htx and ptx operons, as no effect was observed on the level of induction of either operon in a DeltaptxE mutant.

Two C-P lyase operons in Pseudomonas stutzeri and their roles in the oxidation of phosphonates, phosphite, and hypophosphite

DNA sequencing and analysis of two distinct C-P lyase operons in Pseudomonas stutzeri WM88 were completed. The htxABCDEFGHIJKLMN operon encodes a hypophosphite-2-oxoglutarate dioxygenase (HtxA), whereas the predicted amino acid sequences of HtxB to HtxN are each homologous to the components of the Escherichia coli phn operon, which encodes C-P lyase, although homologs of E. coli phnF and phnO are absent. The genes in the htx operon are cotranscribed based on gene organization, and the presence of the intergenic sequences is verified by reverse transcription-PCR with total RNA. Deletion of the htx locus does not affect the ability of P. stutzeri to grow on phosphonates, indicating the presence of an additional C-P lyase pathway in this organism. To identify the genes comprising this pathway, a Deltahtx strain was mutagenized and one mutant lacking the ability to grow on methylphosphonate as the sole P source was isolated. A ca.-10.6-kbp region surrounding the transposon insertion site of this mutant was sequenced, revealing 13 open reading frames, designated phnCDEFGHIJKLMNP, which were homologous to the E. coli phn genes. Deletion of both the htx and phn operons of P. stutzeri abolishes all growth on methylphosphonate and aminoethylphosphonate. Both operons individually support growth on methylphosphonate; however, the phn operon supports growth on aminoethylphosphonate and phosphite, as well. The substrate ranges of both C-P lyases are limited, as growth on other phosphonate compounds, including glyphosate and phenylphosphonate, was not observed.

A new activity for an old enzyme: Escherichia coli bacterial alkaline phosphatase is a phosphite-dependent hydrogenase

Genetic analysis indicates that Escherichia coli possesses two independent pathways for oxidation of phosphite (Pt) to phosphate. One pathway depends on the 14-gene phn operon, which encodes the enzyme C-P lyase. The other pathway depends on the phoA locus, which encodes bacterial alkaline phosphatase (BAP). Transposon mutagenesis studies strongly suggest that BAP is the only enzyme involved in the phoA-dependent pathway. This conclusion is supported by purification and biochemical characterization of the Pt-oxidizing enzyme, which was proven to be BAP by N terminus protein sequencing. Highly purified BAP catalyzed Pt oxidation with specific activities of 62-242 milliunits/mg and phosphate ester hydrolysis with specific activities of 41-61 units/mg. Surprisingly, BAP catalyzes the oxidation of Pt to phosphate and molecular H2. Thus, BAP is a unique Pt-dependent, H2-evolving hydrogenase. This reaction is unprecedented in both P and H biochemistry, and it is likely to involve direct transfer of hydride from the substrate to water-derived protons.

Isolation and biochemical characterization of hypophosphite/2-oxoglutarate dioxygenase. A novel phosphorus-oxidizing enzyme from Psuedomonas stutzeri WM88

The htxA gene is required for the oxidation of hypophosphite in Pseudomonas stutzeri WM88 (Metcalf, W. W., and Wolfe, R. S. (1998) J. Bacteriol. 180, 5547-5558). Amino acid sequence comparisons suggest that hypophosphite:2-oxoglutarate dioxygenase (HtxA) is a novel member of the 2-oxoglutarate-dependent dioxygenase enzyme family. To provide experimental support for this hypothesis, HtxA was overproduced in Escherichia coli and purified to apparent homogeneity. Recombinant HtxA is identical to the native enzyme based on amino terminus sequencing and mass spectral analysis, and it catalyzes the oxidation of hypophosphite to phosphite in a process strictly dependent on 2-oxoglutarate, ferrous ions, and oxygen. Succinate and phosphite are stoichiometrically produced, indicating a strict coupling of the reaction. Size exclusion analysis suggests that HtxA is active as a homodimer, and maximal activity is observed at pH 7.0 and at 27 degrees C. The apparent K(m) values for hypophosphite and 2-oxoglutarate were 0.58 +/- 0.04 mm and 10.6 +/- 1.4 microm, respectively. V(max) and k(cat) values were determined to be 10.9 +/- 0.30 micromol min(-1) mg(-1) and 355 min(-1), respectively. 2-Oxoadipate and pyruvate substitute poorly for 2-oxoglutarate as a cosubstrate. The highest specific activity is observed with hypophosphite as substrate, but HtxA is also able to oxidize formate and arsenite at significant rates. The substrate analog inhibitors, formate and nitrate, significantly reduce HtxA activity.

Biological formation of volatile phosphorus compounds

Phosphine and phosphides are reported to occur at numerous environmental sites such as fresh and marine sediments, landfills, faecal matter, biogas digesters and soils. The concentrations are several log units lower than the time-weighted average exposure standard, i.e. in the order of ng per m3 of gas or ng per kg material. Research about the biological formation of highly reduced gaseous phosphorus compounds dates back more than a hundred years. The early reports had to deal with a lot of scepticism. Thanks to new analytical tools (gas chromatography) it has become clear, during the last decade, that phosphine is a global constituent of the atmosphere. Pure strains of micro-organisms cultivated under highly anaerobic conditions were shown to produce phosphine. Thermodynamic considerations indicate that it is very improbable that the reduction of phosphate to phosphine is endergonic. Therefore the generation of phosphine cannot be compared with sulphidogenesis and methanogenesis. There seems to be a link between the existence of highly reactive gaseous phosphorus compounds and increased levels of metal corrosion. The reactive compounds could be formed by micro-organisms or they are liberated from phosphorus-containing impurities in the iron by the action of bacterial metabolites. The biochemical pathways responsible for the production of gaseous phosphorus compounds have not been characterised yet.

Purification and characterization of a novel phosphorus-oxidizing enzyme from Pseudomonas stutzeri WM88

The ptxD gene from Pseudomonas stutzeri WM88 encoding the novel phosphorus oxidizing enzyme NAD:phosphite oxidoreductase (trivial name phosphite dehydrogenase, PtxD) was cloned into an expression vector and overproduced in Escherichia coli. The heterologously produced enzyme is indistinguishable from the native enzyme based on mass spectrometry, amino-terminal sequencing, and specific activity analyses. Recombinant PtxD was purified to homogeneity via a two-step affinity protocol and characterized. The enzyme stoichiometrically produces NADH and phosphate from NAD and phosphite. The reverse reaction was not observed. Gel filtration analysis of the purified protein is consistent with PtxD acting as a homodimer. PtxD has a high affinity for its substrates with Km values of 53.1 +/- 6.7 microm and 54.6 +/- 6.7 microm, for phosphite and NAD, respectively. Vmax and kcat were determined to be 12.2 +/- 0.3 micromol x min(-1) x mg(-1) and 440 min(-1). NADP can substitute poorly for NAD; however, none of the numerous compounds examined were able to substitute for phosphite. Initial rate studies in the absence or presence of products and in the presence of the dead end inhibitor sulfite are most consistent with a sequential ordered mechanism for the PtxD reaction, with NAD binding first and NADH being released last. Amino acid sequence comparisons place PtxD as a new member of the d-2-hydroxyacid NAD-dependent dehydrogenases, the only one to have an inorganic substrate. To our knowledge, this is the first detailed biochemical study on an enzyme capable of direct oxidation of a reduced phosphorus compound.

Molecular genetic analysis of phosphite and hypophosphite oxidation by Pseudomonas stutzeri WM88

The first molecular and genetic characterization of a biochemical pathway for oxidation of the reduced phosphorus (P) compounds phosphite and hypophosphite is reported. The pathway was identified in Pseudomonas stutzeri WM88, which was chosen for detailed studies from a group of organisms isolated based on their ability to oxidize hypophosphite (+1 valence) and phosphite (+3 valence) to phosphate (+5 valence). The genes required for oxidation of both compounds by P. stutzeri WM88 were cloned on a single ca. 30-kbp DNA fragment by screening for expression in Escherichia coli and Pseudomonas aeruginosa. Two lines of evidence suggest that hypophosphite is oxidized to phosphate via a phosphite intermediate. First, plasmid subclones that conferred oxidation of phosphite, but not hypophosphite, upon heterologous hosts were readily obtained. All plasmid subclones that failed to confer phosphite oxidation also failed to confer hypophosphite oxidation. No subclones that conferred only hypophosphite expression were obtained. Second, various deletion derivatives of the cloned genes were made in vitro and recombined onto the chromosome of P. stutzeri WM88. Two phenotypes were displayed by individual mutants. Mutants with the region encoding phosphite oxidation deleted (based upon the subcloning results) lost the ability to oxidize either phosphite or hypophosphite. Mutants with the region encoding hypophosphite oxidation deleted lost only the ability to oxidize hypophosphite. The phenotypes displayed by these mutants also demonstrate that the cloned genes are responsible for the P oxidation phenotypes displayed by the original P. stutzeri WM88 isolate. The DNA sequences of the minimal regions implicated in oxidation of each compound were determined. The region required for oxidation of phosphite to phosphate putatively encodes a binding-protein-dependent phosphite transporter, an NAD+-dependent phosphite dehydrogenase, and a transcriptional activator of the lysR family. The region required for oxidation of hypophosphite to phosphite putatively encodes a binding-protein-dependent hypophosphite transporter and an alpha-ketoglutarate-dependent hypophosphite dioxygenase. The finding of genes dedicated to oxidation of reduced P compounds provides further evidence that a redox cycle for P may be important in the metabolism of this essential, and often growth-limiting, nutrient.

Involvement of the Escherichia coli phn (psiD) gene cluster in assimilation of phosphorus in the form of phosphonates, phosphite, Pi esters, and Pi

The phn (psiD) gene cluster is induced during Pi limitation and is required for the use of phosphonates (Pn) as a phosphorus (P) source. Twelve independent Pn-negative (Pn-) mutants have lesions in the phn gene cluster which, as determined on the basis of recombination frequencies, is larger than 10 kbp. This distance formed the basis for determining the complete DNA sequence of a 15.6-kbp BamHI fragment, the sequences of which suggested an operon with 17 open reading frames, denoted (in alphabetical order) the phnA to phnQ genes (C.-M. Chen, Q.-Z. Ye, Z. Zhu, B. L. Wanner, and C. T. Walsh, J. Biol. Chem. 265:4461-4471, 1990) Ten Pn- lesions lie in the phnD, phnE, phnH, phnJ, phnK, phnO, and phnP genes. We propose a smaller gene cluster with 14 open reading frames, phnC to phnP, which probably encode transporter and regulatory functions, in addition to proteins needed in Pn biodegradation. On the basis of the effects on phosphite (Pt), Pi ester, and Pi use, we propose that PhnC, PhnD, and PhnE constitute a binding protein-dependent Pn transporter which also transports Pt, Pi esters, and Pi. We propose that PhnO has a regulatory role because a phnO lesion affects no biochemical function, except for those due to polarity. Presumably, the 10 other phn gene products mostly act in an enzyme complex needed for breaking the stable carbon-phosphorus bond. Interestingly, all Pn- mutations abolish the use not only of Pn but also of Pt, in which P is in the +3 oxidation state. Therefore, Pn metabolism and Pt metabolism are related, supporting a biochemical mechanism for carbon-phosphorus bond cleavage which involves redox chemistry at the P center. Furthermore, our discovery of Pi-regulated genes for the assimilation of reduced P suggests that a P redox cycle may be important in biology.

Sodium hypophosphite inhibition of the growth of selected gram-positive foodborne pathogenic bacteria

Sodium hypophosphite (SHP) was evaluated for inhibition of growth of selected Gram-positive foodborne pathogenic bacteria in Trypticase Soy Broth. In addition, the effects of pH and sodium chloride (NaCl) alone and in combination with (SHP) were also examined. All inhibition studies were performed with optimal or nearly optimal growth conditions for each bacterium. Growth was monitored by determining culture optical density at 600 nm, and a time to significant growth determined for each test media. Ratios of time to significant growth for each control over that in test variables were used to evaluate the effect of SHP and other variables on growth. SHP was effective in inhibiting growth of Clostridium perfringens and Clostridium botulinum strains 62A 52A and Lamanna B, but generally ineffective against Staphylococcus aureus and Bacillus cereus. Results from this investigation show that SHP has potential as a food ingredient for the inhibition of certain Gram-positive foodborne pathogens.

The utilization of inorganic and organic phosphorous compounds as nutrients by eukaryotic microalgae: a multidisciplinary perspective: part 1

This comprehensive literature review of the phosphorus nutrition and metabolism of eukaryotic microalgae deals sequentially with (1) extracellular P-compounds available for algal utilization and growth; (2) orthophosphate uptake mechanisms, kinetics, and influence from environmental variables; (3) phosphatase-mediated utilization of organic phosphates involving multiple enzymes, induction and cellular location of repressible and irrepressible phosphatases, and their role in growth physiological processes; (4) intracellular phosphate metabolism covering diversity of phosphometabolites. ATP-linked energy regulation, polyphosphate pools and storage roles, phospholipids and phospholipases; (5) steady-state and transient-state models relating phosphate utilization to growth; (6) ecological aspects covering manifestations of phosphorus limitation, interspecific competition for phosphonutrients among microorganisms, and current views on phosphorus cycling and turnover in aquatic ecosystems. Although concentrating on the microalgae, the review often points out sounder conclusions drawn from bacteria and fungi, and includes specific macroalgae in considering certain subtopics where such algae were better investigated and provided a good basis for comparison with the microalgae.