Effect of Selenite on Growth and Protein Synthesis in the Phototrophic Bacterium Rhodobacter sphaeroides

The effect of selenite on the growth rate and protein synthesis has been investigated in Rhodobacter sphaeroides. This photosynthetic bacterium efficiently reduces selenite with intracellular accumulation under both dark aerobic and anaerobic photosynthetic conditions. Addition of 1 mM selenite under these two growth conditions does not affect the final cell density, although a marked slowdown in growth rate is observed under aerobic growth. The proteome analysis of selenite response by two-dimensional gel electrophoresis shows an enhanced synthesis of some chaperones, an elongation factor, and enzymes associated to oxidative stress. The induction of these antioxidant proteins confirms that the major toxic effect of selenite is the formation of reactive oxygen species during its metabolism. In addition, we show that one mutant unable to precipitate selenite, selected from a transposon library, is affected in the smoK gene. This encodes a constituent of a putative ABC transporter implicated in the uptake of polyols. This mutant is less sensitive to selenite and does not express stress proteins identified in the wild type in response to selenite. This suggests that the entry of selenite into the cytoplasm is mediated by a polyol transporter in R. sphaeroides

Sulfate Assimilation Mediates Tellurite Reduction and Toxicity in Saccharomyces cerevisiae▿†

Despite a century of research and increasing environmental and human health concerns, the mechanistic basis of the toxicity of derivatives of the metalloid tellurium, Te, in particular the oxyanion tellurite, Te(IV), remains unsolved. Here, we provide an unbiased view of the mechanisms of tellurium metabolism in the yeast Saccharomyces cerevisiae by measuring deviations in Te-related traits of a complete collection of gene knockout mutants. Reduction of Te(IV) and intracellular accumulation as metallic tellurium strongly correlated with loss of cellular fitness, suggesting that Te(IV) reduction and toxicity are causally linked. The sulfate assimilation pathway upstream of Met17, in particular, the sulfite reductase and its cofactor siroheme, was shown to be central to tellurite toxicity and its reduction to elemental tellurium. Gene knockout mutants with altered Te(IV) tolerance also showed a similar deviation in tolerance to both selenite and, interestingly, selenomethionine, suggesting that the toxicity of these agents stems from a common mechanism. We also show that Te(IV) reduction and toxicity in yeast is partially mediated via a mitochondrial respiratory mechanism that does not encompass the generation of substantial oxidative stress. The results reported here represent a robust base from which to attack the mechanistic details of Te(IV) toxicity and reduction in a eukaryotic organism

Anti-bacterial TeNPs biosynthesized by haloarcheaon Halococcus salifodinae BK3

Microbial synthesis of highly structured metal sulfide and metallic nanoparticles is a benign approach of nanomaterial synthesis. Various microbes have been exploited for nanoparticle synthesis, but nanofabrication using haloarchaea is still in nascent stages. Here, we report the intracellular synthesis of hexagonal needle-shaped tellurium nanoparticles with an aspect ratio of 1:4.4, by the haloarcheon Halococcus salifodinae BK3. The isolate was able to tolerate up to 5.5 mM K2TeO3. The yield of tellurium nanoparticles was highest when the culture was exposed to 3 mM K2TeO3, even though the isolate exhibited slightly decreased growth rate as compared to the culture growing in the absence of K2TeO3. The enzyme tellurite reductase was responsible for tellurite resistance and nanoparticle synthesis in H. salifodinae BK3. These tellurium nanoparticles exhibited anti-bacterial activities against both Gram-positive and Gram-negative bacteria, with higher antibacterial activity towards Gram-negative bacteria. This is the first report on the synthesis of tellurium nanoparticles by Halophilic archaea