Tellurite increases the oxidation of cytoplasmic proteins and membrane lipids in <i>E. coli.</i>

<p>Effects of K₂TeO₃ (0.5 µg/ml) and H₂O₂ (100 µM) on protein carbonyl (A) and TBARs content (B) of <i>E. coli</i> BW25113 cells exposed to these compounds for 30 min. A, protein oxidation was determined by a chemical protein carbonyl assay by derivatizing total cellular proteins with DNPH and reading specific carbonyls absorbance at 370 nm. B, membrane peroxidation products were determined as thiobarbituric acid-reactive substances present in crude extracts of <i>E. coli</i> BW25113 by the method described by Rice-Evans et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000211#pone.0000211-RiceEvans1" target="_blank">[19]</a>.</p

Minimal inhibitory concentrations (MIC) of K₂TeO₃for <i>E. coli</i> BW25113 strains deficient in ROS-responsive genes.

<p>Numbers are the mean of 4 independent determinations.</p

<i>In vitro</i> tellurite reduction generates superoxide in <i>E. coli.</i>

<p>Superoxide generation was evaluated using an <i>in vitro</i> tellurite reduction assay previously developed in our laboratory <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000211#pone.0000211-Caldern1" target="_blank">[34]</a>. The system makes use of the O₂⁻ specific reactive compound WST-1. WST-1 reduction was determined in the presence of catalase and NADPH (Cat); catalase, tellurite and NADPH (Cat/Te); catalase, tellurite, NADPH and purified superoxide dismutase (Cat/Te/SOD); catalase, tellurite, NADPH and β-amylase (Cat/Te/amylase).</p

Tellurite-induction of β-galactosidase activity in <i>E. coli</i> reporter strains.

<p> <i>E. coli</i> reporter strains ADA100 [AB734 λΦ(<i>ibp</i>::<i>lacZ</i>)], ADA310 [AB734λΦ (<i>cspA</i>::<i>lacZ</i>)], ADA410 [AB734 λΦ(p3<i>RpoH</i>::<i>lacZ</i>)] and ADA510 [AB734 λΦ(<i>sulA</i>::<i>lacZ</i>)] containing the stress-responsive promoters <i>ibpA, cspA, p3RpoH</i> and <i>sulA</i> fused to the <i>lacZ</i> gene respectively, were used to study transcription induction in cells treated or untreated with K₂TeO₃ (0.5 µg/ml). β-galactosidase activity was evaluated at time 0 and after 3 h with or without tellurite treatment. The fold induction was calculated dividing the value obtained at 3 h by the value at time 0. Results are the average of at least 4 determinations.</p

Tellurite induces <i>katG</i> and <i>soxS</i> mRNA synthesis in <i>E. coli.</i>

<p>DNA fragments (300 bp) from <i>E. coli sodA, sodB, katG, soxS</i> and <i>gapA</i> genes were amplified by RT-PCR and fractionated by electrophoresis on agarose gels (1.5%). Total RNA from cells grown with (K₂TeO₃) or without (control) 0.5 µg/ml potassium tellurite was used as template for the RT-PCR. The estimated DNA (ng) content determined for each band is shown (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000211#s4" target="_blank">Material and Methods</a> for details).</p

Effect of potassium tellurite on <i>E. coli</i> catalase and superoxide dismutase.

<p>A, activity of catalases in crude protein extracts of <i>E. coli</i> BW25113 treated (•) or untreated (▪) with 0.5 µg/ml of K₂TeO₃. Cells were collected at 15 min intervals and catalase activity (µmol hydrogen peroxide/min/mg protein) was determined. B, activity of superoxide dismutases in crude protein extracts of <i>E. coli</i> BW25113 treated (•) and untreated (▪) with 0.5 µg/ml K₂TeO₃. Cells were harvested and SOD activity (U/mg protein) was determined.</p

The tellurite reductase activity in crude extracts prepared from <i>S. epidermidis</i> CH is dependent on NADH as cofactor.

<div><p>The tellurite reductase activity present in crude extracts of <i>S. epidermidis</i> CH cells was followed over time as the increase in absorbance at 500 nm due to the conversion of tellurite to tellurium in the presence (▪) and absence (•) of NADH under standard assay conditions.</p> <p>The values shown represent the means of three independent determinations made with crude extracts containing 1.2 mg/ml total protein.</p></div

Catalases have tellurite reductase activity <b><i>in situ.</i></b>

<div><p>Assays for the H₂O₂ dismutase and TeO₃²⁻ reductase activities of catalase <i>in situ</i> were carried out by resolving proteins on polyacrylamide gels, incubating gel strips in the presence of substrate, and developing gel strips to reveal the presence of specific products of each reaction (Woodbury et al. 1971; Gregory and Fridovich 1974).</p> <p>For dismutase assays, 25 µg of protein were loaded in each gel lane; 250 µg of protein was used for tellurite reductase assays.</p> <p> <b>(A)</b> The results in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000070#pone-0000070-g003" target="_blank">Fig. 3A</a> show that bovine liver catalase (a tetramer of 255 kDa) migrates as a single band on native gels (lane 2) with an apparent molecular mass similar to that of a 244 kD standard (lane 1), and has both hydrogen peroxide dismutase (lane 3) and tellurite reductase (lane 4) activities.</p> <p> <b>(B)</b> The activities of catalase present in extracts made from <i>E. coli</i> Top10 cells carrying the plasmid vector pBAD display lower H₂O₂ dismutase and TeO₃²⁻ reductase activities (lanes 7 and 9) than does an otherwise isogenic strain with plasmid pCAT, which expresses the <i>S. epidermidis</i> CH <i>katA</i> (catalase) gene (lanes 8 and 10); these activities most likely correspond to the predominant tetrameric form of catalase.</p> <p>In crude extracts of <i>S. epidermidis</i>, two different bands of each activity are observed (lanes 5 and 6).</p></div

Figure 4. Expression of the <i>S. epidermidis katA</i> gene in <i>E. coli</i> confers increased resistance to both tellurite and hydrogen peroxide.

<div><p> <b>(A)</b> Aliquots of exponentially growing cultures of <i>E. coli</i> strain JWK3914 (<i>katG</i>) carrying plasmids pBAD (<b>a</b> and <b>c</b>) or pCAT (<b>b</b> and <b>d</b>) were spread onto the surface of plates with LB medium, 15.5 µl K₂TeO₃ (4 mM)(<b>a</b> and <b>b</b>) or hydrogen peroxide (3%) (<b>c</b> and <b>d</b>) were spotted onto the centers of the bacterial lawns, and cells were grown to reveal zones of inhibition (Fuentes et al. 2005).</p> <p> <b>(B)</b> The mean areas of growth inhibition zones (cm²) were determined from three independent experiments; thin bars represent the standard deviations.</p></div

The product of tellurite reduction by catalase is elemental tellurium.

<div><p>Absorption spectra of products formed upon the reduction of K₂TeO₃<i>in vitro</i> by <b>(A)</b> 2-mercaptoethanol, <b>(B)</b> a crude extract prepared from <i>S. epidermidis</i> CH cells prior to chromatographic enrichment, and <b>(C)</b> purified bovine liver catalase, were resolved by Induced Coupled Plasma-Optical Emission spectroscopy <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000070#pone.0000070-Grotti1" target="_blank">[30]</a>.</p> <p>All products show an absorption maximum at 214.281 nm, the peak wavelength of the Te° standard (triangle in panel A).</p> <p>We note that the scale used to measure absorbance in the crude extract differs by a factor of ten from those used to measure absorbance in chemically prepared tellurium (A) and in the product of tellurite reduction by bovine liver catalase (C).</p> <p>This is because the crude extract includes a plethora of components with absorption maxima at or near this wavelength.</p></div