Phosphate transport in membrane vesicles from Escherichia coli

Role of an archaeal PitA transporter in the copper and arsenic resistance of Metallosphaera sedula, an extreme thermoacidophile

Thermoacidophilic archaea, such as Metallosphaera sedula, are lithoautotrophs that occupy metal-rich environments. In previous studies, an M. sedula mutant lacking the primary copper efflux transporter, CopA, became copper sensitive. In contrast, the basis for supranormal copper resistance remained unclear in the spontaneous M. sedula mutant, CuR1. Here, transcriptomic analysis of copper-shocked cultures indicated that CuR1 had a unique regulatory response to metal challenge corresponding to the upregulation of 55 genes. Genome resequencing identified 17 confirmed mutations unique to CuR1 that were likely to change gene function. Of these, 12 mapped to genes with annotated function associated with transcription, metabolism, or transport. These mutations included 7 nonsynonymous substitutions, 4 insertions, and 1 deletion. One of the insertion mutations mapped to pseudogene Msed_1517 and extended its reading frame an additional 209 amino acids. The extended mutant allele was identified as a homolog of Pho4, a family of phosphate symporters that includes the bacterial PitA proteins. Orthologs of this allele were apparent in related extremely thermoacidophilic species, suggesting M. sedula naturally lacked this gene. Phosphate transport studies combined with physiologic analysis demonstrated M. sedula PitA was a low-affinity, high-velocity secondary transporter implicated in copper resistance and arsenate sensitivity. Genetic analysis demonstrated that spontaneous arsenate-resistant mutants derived from CuR1 all underwent mutation in pitA and nonselectively became copper sensitive. Taken together, these results point to archaeal PitA as a key requirement for the increased metal resistance of strain CuR1 and its accelerated capacity for copper bioleaching.

Biology of polyphosphate-accumulating bacteria involved in enhanced biological phosphorus removal

Recent research on the process of biological phosphorus removal in lab-scale treatment systems has indicated that: (i) the development of an actively polyP-accumulating bacterial community after the introduction of an anaerobic period may take at least 4 months; (ii) up to 80% of all aerobic bacteria isolated from these communities are able to accumulate polyP; (iii) polyP synthesized by the bacterial communities of lab-scale treatment systems is probably mainly low polymeric, not exceeding 20 P-residues, and this polyP is rapidly degraded during the anaerobic period; (iv) the enzymatic hydrolysis of polyP under anaerobic conditions is accompanied by PHB formation from exogenous acetate, reducing equivalents are provided by the degradation of carbohydrates; and (v) nitric oxide inhibits the release of phosphate under anaerobic conditions in Renpho and F&D sludges. Bacteria belonging to the genus Acinetobacter occur in a wide variety of activated sludges in which enhanced biological phosphate removal is observed. A. johnsonii 210A was studied in detail with respect to the elemental composition of polyP granules, the enzymatic synthesis and degradation of polyP, the regulation of polyP metabolism, and the transport of phosphate. A. johnsonii 210A reflects activated sludge in a number of ways as far as polyP metabolism is concerned but its polyP is highly polymeric and the phosphate efflux rate under anaerobic conditions is relatively low and not increased by exogenous acetate. In addition to Acinetobacter, other polyP-accumulating microorganisms may be involved in biological phosphorus removal. The isolation of polyP-accumulating denitrifying bacteria may well have interesting implications for a new process design in wastewater treatment systems. Further studies on the enzymes involved in polyP biosynthesis and on the uptake and efflux systems of phosphate, potassium, magnesium and lower fatty acids in pure cultures will enlarge our insight in the energetics of the metabolism of polyP. In addition, the regulation of the metabolism of polyP-accumulating organisms needs to be studied in more detail.

Molecular aspects of phosphate transport in Escherichia coli

Escherichia coli transports inorganic phosphate (Pi) by the low-affinity transport system, Pit. When the level of the external Pi is lower than 20 microM, another transport system, Pst, is induced with a Kt of 0.25 microM. An outer-membrane porin, PhoE, with a Km of about 1 microM is also induced. The outer membrane allows the intake of organic phosphates which are degraded to Pi by phosphatases in the periplasm. The Pi-binding protein will capture the free Pi produced in the periplasm and direct it to the transmembrane channel of the cytoplasmic membrane. The channel consists of two proteins, PstA and PstC, which have six and five transmembrane helices, respectively. On the cytoplasmic side of the membrane the channel is linked to the PstB protein, which carries a nucleotide (probably ATP)-binding site. PstB probably provides the energy required by the channel to free Pi. The Pst system has two functions in E. coli: (i) the transport of Pi, and (ii) the negative regulation of the phosphate regulon (a complex of 20 proteins mostly related to organic phosphate transport). It is remarkable that these two functions are not related, since the repressibility of the regulon depends on the integral structure of Pst (PiBP + PstA + PstC + PstB) and not on the Pi transported. Another gene of the pst operon, phoU, produces a protein involved in the negative regulation of the Pho regulon, but the mechanism of this function has not been explained. Thus the regulatory function of the Pst system remains obscure. Its basal level, present when Pi is abundant, is sufficient to repress the Pho regulon but the negative regulatory function is lost upon Pi starvation.

Initial characterization of the ferric iron transport system of Corynebacterium diphtheriae

Transport of ferric iron into Corynebacterium diphtheriae C7(beta) was shown to occur by a high-affinity, active transport system. Optimal rates were at pH 6.8 and 40 degrees C. Strong inhibition of uptake by carbonyl cyanide m-chlorophenylhydrazone was consistent with the electrochemical proton gradient as the major energy source for iron transport, and inhibition by Hg2+ indicated that sulfhydryl groups were also important. Evidence was obtained for stimulation of iron uptake at pH 8.0 by a dialyzable, extracellular factor present in conditioned medium from low-iron cultures of C. diphtheriae.

Mode of action of pamamycin in Staphylococcus aureus

Pamamycin was previously identified as a stimulator of aerial mycelium formation in Streptomyces alboniger and as a new antibiotic. Studies in Staphylococcus aureus grown in brain heart infusion broth showed that this antibiotic was bacteriostatic at 0.1 to 0.3 U/ml and bactericidal at 0.5 U/ml or higher. At concentrations which inhibited growth ca. 40%, pamamycin inhibited the uptake of nucleosides and Pi, as well as purine and pyrimidine bases, and the incorporation of these precursors into nucleic acids. Under the same conditions, the antibiotic had no effect on protein and cell wall synthesis, glucose utilization, or the uptake of amino acids, 2-deoxyglucose, or Mn2+. Further studies suggested that the primary action of pamamycin was on transport processes rather than de novo ribonucleic acid and deoxyribonucleic acid synthesis. Direct in vitro studies of nucleoside transport with isolated membrane vesicles confirmed this conclusion. Pamamycin bound tightly to bacterial membranes and induced significant release of ultraviolet-absorbing material at bactericidal concentrations. It was concluded that the mechanism of growth inhibition involves alteration of membrane-associated cellular functions. Inhibition of Pi transport is a likely primary target for this effect.

The mechanism of proton translocation driven by the respiratory nitrate reductase complex of Escherichia coli

Low concentrations (1-50mum) of ubiquinol(1) were rapidly oxidized by spheroplasts of Escherichia coli derepressed for synthesis of nitrate reductase using either nitrate or oxygen as electron acceptor. Oxidation of ubiquinol(1) drove an outward translocation of protons with a corrected -->H(+)/2e(-) stoichiometry [Scholes & Mitchell (1970) J. Bioenerg.1, 309-323] of 1.49 when nitrate was the acceptor and 2.28 when oxygen was the acceptor. Proton translocation driven by the oxidation of added ubiquinol(1) was also observed in spheroplasts from a double quinone-deficient mutant strain AN384 (ubiA(-)menA(-)), whereas a haem-deficient mutant, strain A1004a, did not oxidize ubiquinol(1). Proton translocation was not observed if either the protonophore carbonyl cyanide m-chlorophenylhydrazone or the respiratory inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide was present. When spheroplasts oxidized Diquat radical (DQ(+)) to the oxidized species (DQ(++)) with nitrate as acceptor, nitrate was reduced to nitrite according to the reaction: [Formula: see text] and nitrite was further reduced in the reaction: [Formula: see text] Nitrite reductase activity (2) was inhibited by CO, leaving nitrate reductase activity (1) unaffected. Benzyl Viologen radical (BV(+)) is able to cross the cytoplasmic membrane and is oxidized directly by nitrate reductase to the divalent cation, BV(++). In the presence of CO, this reaction consumes two protons: [Formula: see text] The consumption of these protons could not be detected by a pH electrode in the extra-cellular bulk phase of a suspension of spheroplasts unless the cytoplasmic membrane was made permeable to protons by the addition of nigericin or tetrachlorosalicylanilide. It is concluded that the protons of eqn. (3) are consumed at the cytoplasmic aspect of the cytoplasmic membrane. Diquat radical, reduced N-methylphenazonium methosulphate and its sulphonated analogue N-methylphenazonium-3-sulphonate (PMSH) and ubiquinol(1) are all oxidized by nitrate reductase via a haem-dependent, endogenous quinone-independent, 2-n-heptyl-4-hydroxyquinoline N-oxide-sensitive pathway. Approximate-->H(+)/2e(-) stoichiometries were zero with Diquat radical, an electron donor, 1.0 with reduced N-methylphenazonium methosulphate or its sulphonated analogue, both hydride donors, and 2.0 with ubiquinol(1) (QH(2)), a hydrogen donor. It is concluded that the protons appearing in the medium are derived from the reductant and the observed-->H(+)/2e(-) stoichiometries are accounted for by the following reactions occurring at the periplasmic aspect of the cytoplasmic membrane.: [Formula: see text]

Measurements of membrane potentials in Escherichia coli K-12 inner membrane vesicles with the safranine method

The use of safranine, a positively-charged dye, as a probe for the determination of membrane potentials in Escherichia coli vesicles has been studied. 1. Shifts in the spectrum of safranine were observed during induction of potassium ion diffusion potentials with valinomycin or during oxidation of formate by vesicles prepared from cells of E. coli K-12 or ML 308-225 subjected to anaerobic growth with nitrate. The extent of the valinomycin-dependent spectral change correlated linearly with the magnitude of the K+ equilibrium potential, as calculated from the Nernst equation, from 50 to 160 mV (interior negative). The formate-induced changes could also be calibrated by increasing the concentration of potassium in the presence of valinomycin, after the formation of formate-dependent responses. In this case, results identical to those obtained with the first method were obtained. 2. O2 or nitrate-dependent oxidation of formate resulted in a membrane potential of the order of 170 mV. The oxidation of ascorbate-reduced N-methylphenazonium methosulphate resulted in a potential of similar magnitude, but anaerobically with nitrate only a small but definite potential was formed. 3. The water-soluble quinones, duroquinone and menadione, could produce membrane potentials when used in their oxidized or reduced forms in the presence of formate or nitrate (or oxygen). 2-Hydroxy-1,4-naphthoquinone was not only ineffective but was found to be inhibitory. 4. N,N'-dicyclohexylcarbodiimide at suitable concentrations increased the rate of formation and the extent of membrane potentials induced by respiration or by artificial means.