Application of the comprehensive set of heterozygous yeast deletion mutants to elucidate the molecular basis of cellular chromium toxicity.

BACKGROUND: The serious biological consequences of metal toxicity are well documented, but the key modes of action of most metals are unknown. To help unravel molecular mechanisms underlying the action of chromium, a metal of major toxicological importance, we grew over 6,000 heterozygous yeast mutants in competition in the presence of chromium. Microarray-based screens of these heterozygotes are truly genome-wide as they include both essential and non-essential genes. RESULTS: The screening data indicated that proteasomal (protein degradation) activity is crucial for cellular chromium (Cr) resistance. Further investigations showed that Cr causes the accumulation of insoluble and toxic protein aggregates, which predominantly arise from proteins synthesised during Cr exposure. A protein-synthesis defect provoked by Cr was identified as mRNA mistranslation, which was oxygen-dependent. Moreover, Cr exhibited synergistic toxicity with a ribosome-targeting drug (paromomycin) that is known to act via mistranslation, while manipulation of translational accuracy modulated Cr toxicity. CONCLUSION: The datasets from the heterozygote screen represent an important public resource that may be exploited to discover the toxic mechanisms of chromium. That potential was validated here with the demonstration that mRNA mistranslation is a primary cause of cellular Cr toxicity.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Role of Symbiotic Auxotrophy in the Rhizobium-Legume Symbioses

Symbiotic auxotrophy occurs in both determinate pea and indeterminate bean nodules demonstrating its importance for bacteroid formation and nodule function in legumes with different developmental programmes. However, only small quantities of branched chain amino acids are needed and symbiotic auxotrophy did not occur in the Sinorhizobium meliloti-alfalfa symbiosis under the conditions measured. The contrasting symbiotic phenotypes of aap bra mutants inoculated on different legumes probably reflects altered timing of amino acid availability, development of symbiotic auxotrophy and nodule developmental programmes

Strains, plasmids, primers.

<p>Str  =  streptinomycin; Tc  =  tetracycline; Sp  =  spectinomycin; Gm  =  gentamicin; <i>Bam</i>HI sites in primers are underlined.</p

Transport of amino acids and plant dry weights by strains of <i>R. leguminosarum</i> bv <i>phaseoli.</i>

<p>All strains were grown on AMS glucose/NH₄Cl minimal medium. Rates of uptake are expressed as nmol min⁻¹mg⁻¹ protein. Values are the mean ± SEM of determinations from three or more independent cultures. Dry weights (g plant⁻¹) are averages of >27 plants.</p

TEM pictures of bacteroids of wild-type (A and B) and RU1933 <i>aap bra</i> (C and D).

<p>A and C are pictures taken of 4 week old nodules, B and D of 6 week old nodules. Size bars are 500 µm.</p

Symbiotic properties of 4 week old bean plants.

<p>The values are the mean ± SEM of plants inoculated with RU2222 (n = 8) and RU1933 (n = 6). P-values were determined by ttest.</p

Transport of amino acids by strains of <i>Sinorhizobium meliloti.</i>

<p>All strains were grown in M9 modified glucose/NH₄Cl minimal medium. Rates of uptake are expressed as nmol min⁻¹mg⁻¹ protein. Values are the mean ± SEM of determinations from three independent cultures, except for strain 2011 alanine, leucine and valine transports, which just served as controls and represent single measurements. ND  =  not determined.</p

Transport rates of branched-chain amino acids and plant dry weights by strains of <i>R. leguminosarum</i> bv. <i>viciae.</i>

<p>Transport rates are the mean ± SEM of 3 independent cultures expressed as nmol min⁻¹mg⁻¹ protein. Dry weights (g plant⁻¹) are averages of 15 plants.</p