Production of metallothionein in copper- and cadmium-resistant strains of Saccharomyces cerevisiae

Genetic system underlying responses of Cryptococcus neoformans to cadmium

Cryptococcus neoformans, basidiomycetous pathogenic yeast, is basically an environmental fungus and, therefore, challenged by ever changing environments. In this study, we focused on how C. neoformans responds to stress caused by cadmium that is one of high-risk pollutants. By tracking phenotypes of the resistance or sensitivity to cadmium, we undertook forward and reverse genetic studies to identify genes involved in cadmium metabolism in C. neoformans. We found that the main route of Cd²⁺ influx is through Mn²⁺ ion transporter, Smf1, which is an ortholog of Nramp (natural resistance-associated macrophage protein 1) of mouse. We found that serotype A strains are generally more resistant to cadmium than serotype D strains and that cadmium resistance of H99, a representative of serotype A strains, was found to be due to a partial defect in SMF1. We found that calcium channel has a subsidiary role for cadmium uptake. We also showed that Pca1 (P-type-ATPase) functions as an extrusion pump for cadmium. We examined the effects of some metals on cadmium toxicity and suggested (i) that Ca²⁺ and Zn²⁺ could exert their protective function against Cd²⁺ via restoring cadmium-inhibited cellular processes and (ii) that Mg²⁺ and Mn²⁺ could have antagonistic roles in an unknown Smf1-independent Cd²⁺ uptake system. We proposed a model for Cd2⁺-response of C. neoformans, which will serve as a platform for understanding how this organism copes with the toxic metal.

Application of Pb(II) to probe the physiological responses of fungal intracellular vesicles

Vesicles (Ves) within fungal cells are the critical linkage between intracellular and extracellular systems. This study explored the application of Pb²⁺ to probe the physiology of intracellular Ves in Rhodotorula mucilaginosa (Rho). At low Pb²⁺ levels (0-500 mg/L), there was no evident change in the content of extracellular polymeric substances (EPS) or microbial activity. At medium-high levels (1000-2000 mg/L), the sizes of Ves within the Rho cells were significantly enlarged, with abundant lead nano-particles (Pb NPs) formed either on the cell surface or interior, whereas the EPS content and bioactivity were still stable. At a high level (2500 mg/L), the Rho cells were severely deformed, with cell counts reduced by more than 99%. However, the EPS contents and the respiration rate of the surviving cells dramatically increased to the maximum values (i.e., 1785 mg/10¹⁰ cells and 37 mg C 10^-10 cells h^-1, respectively). The Ves surface adsorbed Pb cations with higher density, compared with the cell membrane. Moreover, fusion of some Ves to the membrane (functioning in transport) was observed under transmission electron microscope (TEM). Three pathways of detoxification via intracellular Ves were finally proposed, i.e., Ve-mediated transport (from intracellular to extracellular) of EPS components, absorption of Pb NPs on the Ve surface, and accumulation of Pb NPs within Ves. This study sheds light on the possibility of exploring microbial physiology via Pb²⁺ cations.

The Yeast Atlas of Appalachia: Species and Phenotypic Diversity of Herbicide Resistance in Wild Yeast

Glyphosate and copper-based herbicides/fungicides affect non-target organisms, and these incidental exposures can impact microbial populations. In this study, glyphosate resistance was found in the historical collection of S. cerevisiae, which was collected over the last century, but only in yeast isolated after the introduction of glyphosate. Although herbicide application was not recorded, the highest glyphosate-resistant S. cerevisiae were isolated from agricultural sites. In an effort to assess glyphosate resistance and impact on non-target microorganisms, different yeast species were harvested from 15 areas with known herbicidal histories, including an organic farm, conventional farm, remediated coal mine, suburban locations, state park, and a national forest. Yeast representing 23 genera were isolated from 237 samples of plant, soil, spontaneous fermentation, nut, flower, fruit, feces, and tree material samples. Saccharomyces, Candida, Metschnikowia, Kluyveromyces, Hanseniaspora, and Pichia were other genera commonly found across our sampled environments. Managed areas had less species diversity, and at the brewery only Saccharomyces and Pichia were isolated. A conventional farm growing RoundUp Ready™ corn had the lowest phylogenetic diversity and the highest glyphosate resistance. The mine was sprayed with multiple herbicides including a commercial formulation of glyphosate; however, the S. cerevisiae did not have elevated glyphosate resistance. In contrast to the conventional farm, the mine was exposed to glyphosate only one year prior to sample isolation. Glyphosate resistance is an example of the anthropogenic selection of nontarget organisms.

The cadmium tolerance in Saccharomyces cerevisiae depends on inorganic polyphosphate

The sensitivity to cadmium (Cd(II)), an important environmental pollutant, was studied in the cells of Saccharomyces cerevisiae strains with genetically altered polyphosphate metabolism. The strains overproducing polyphosphatases PPX1 or PPN1 were more sensitive to Cd(II) than the parent strain. The half maximal inhibitory concentrations were 0.02 and 0.05 mM for the transformants and the parent strain, respectively. Transformant strains cultivated in the presence of Cd(II) show a decrease in the content of short-chained cytosolic acid soluble polyphosphate. The role of this polyphosphate fraction in detoxification of heavy metal ions is discussed.

Plants as useful vectors to reduce environmental toxic arsenic content

Arsenic (As) toxicity in soil and water is an increasing menace around the globe. Its concentration both in soil and environment is due to natural and anthropogenic activities. Rising arsenic concentrations in groundwater is alarming due to the health risks to plants, animals, and human beings. Anthropogenic As contamination of soil may result from mining, milling, and smelting of copper, lead, zinc sulfide ores, hide tanning waste, dyes, chemical weapons, electroplating, gas exhaust, application of municipal sludge on land, combustion of fossil fuels, As additives to livestock feed, coal fly ash, and use of arsenical pesticides in agricultural sector. Phytoremediation can be viewed as biological, solar-driven, pump-and-treat system with an extensive, self-extending uptake network (the root system) that enhances the natural ecosystems for subsequent productive use. The present review presents recent scientific developments regarding phytoremediation of arsenic contaminated environments and its possible detoxification mechanisms in plants.

Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation

The study of the role that fungi have played and are playing in fundamental geological processes can be termed 'geomycology' and this article seeks to emphasize the fundamental importance of fungi in several key areas. These include organic and inorganic transformations and element cycling, rock and mineral transformations, bioweathering, mycogenic mineral formation, fungal-clay interactions, metal-fungal interactions, and the significance of such processes in the environment and their relevance to areas of environmental biotechnology such as bioremediation. Fungi are intimately involved in biogeochemical transformations at local and global scales, and although such transformations occur in both aquatic and terrestrial habitats, it is the latter environment where fungi probably have the greatest influence. Within terrestrial aerobic ecosystems, fungi may exert an especially profound influence on biogeochemical processes, particularly when considering soil, rock and mineral surfaces, and the plant root-soil interface. The geochemical transformations that take place can influence plant productivity and the mobility of toxic elements and substances, and are therefore of considerable socio-economic relevance, including human health. Of special significance are the mutualistic symbioses, lichens and mycorrhizas. Some of the fungal transformations discussed have beneficial applications in environmental biotechnology, e.g. in metal leaching, recovery and detoxification, and xenobiotic and organic pollutant degradation. They may also result in adverse effects when these processes are associated with the degradation of foodstuffs, natural products, and building materials, including wood, stone and concrete. It is clear that a multidisciplinary approach is essential to understand fully all the phenomena encompassed within geomycology, and it is hoped that this review will serve to catalyse further research, as well as stimulate interest in an area of mycology of global significance.

Cadmium and heat response of the fungus Heliscus lugdunensis isolated from highly polluted and unpolluted areas

Induction of heat shock protein (Hsp) 70 and distinct metallothionein-like proteins (MTLPs) in response to Cd and heat treatment were studied in two strains of the aquatic hyphomycete Heliscus lugdunensis: Hl-H4, isolated from a heavy metal polluted site, and Hl-BB taken from an unpolluted area. Upon Cd-exposure, Hsp70 was actively synthesized in the strain Hl-H4, and to a much lower degree in the strain Hl-BB. The Hsp70-expression was time- and dose-dependent, reaching a maximum after 24 h incubation with 80 microM Cd. Upon heat-stress, a similar response was observed: a strong Hsp70-expression in Hl-H4, and only a marginal one in Hl-BB. The strains reacted to Cd-exposure by a specific, environmentally related induction of MTLPs, as shown by the highly sensitive bimane derivatisation method of SH-rich proteins. In Hl-H4, a strong expression of 11 kDa MTLP was registered, which followed strictly the induction pattern of Hsp70. This suggests interdependence of the induction mechanisms and roles of these stress proteins in metal resistance. On the contrary, in Hl-BB a weak expression of MTLP of about 20 kDa was observed, exhibiting completely different induction pattern. The results suggest that the specific induction of Hsp70 and/or distinct MTLPs in the range of 11 kDa in H. lugdunensis strain Hl-H4 are essential adaptive mechanisms to continuous heavy metal exposure.

Extracytoplasmic storage as the nickel resistance mechanism in a natural isolate of Pseudomonas putida S4

Metal resistances in microbes are important to study not only to understand metal homeostasis but also to use such organisms further in environmental bioremediation. Nickel (Ni2+) is an important micronutrient, which at higher concentration becomes toxic. Many Ni2+-resistant organisms are known, which resist metal by active efflux. Pseudomonas putida S4, a natural isolate from India, is reported to show a multi-metal resistance profile. In the present study, the Ni2+-resistance mechanism in strain S4 was examined. Wild-type cells gradually accumulated Ni2+ but kept it preferentially in the periplasmic space in a bound form. In Ni2+-sensitive mutants, periplasmic storage was disturbed and more metal accumulated cytoplasmically, producing toxicity. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis of periplasmic proteins revealed a band of approximately 18 kDa, which appeared only in Ni2+-exposed wild-type cells, and which was absent from cells not exposed to Ni2+ as well as from Ni2+-sensitive mutants. On the basis of these observations, we propose a Ni2+-resistance mechanism in P. putida S4 based on sequestration of metal in the periplasmic space. This is the first study of sequestration-based Ni2+ resistance.

Biochemical characterization of N-methyl N’-nitro-N-nitrosoguanidine-induced cadmium resistant mutants ofAspergillus niger

Two cadmium resistant mutants (Cd1 and Cd2) of Aspergillus niger, among the six isolated by mutagenization with N-methyl N'-nitro-N-nitrosoguanidine (MNNG) at pH 6.4 were selected for the study. Analysis of lipid composition of the mutants and the wildtype indicated that total lipid as well as individual lipids of the cadmium resistant mutants were changed as compared with that of the wildtype. The increased activities of metal-lothionein and reduced activities of D-xylose isomerase and L-phenylalanine ammonia lyase in cell free extract of the cadmium resistant mutants suggested that mutants could allow high concentration of cadmium salt as compared with that of the wildtype. The respiratory activity and intracellular as well as extracellular Cd2+ concentration of the mutants reflected the high tolerance of the Cd mutants to cadmium ion.

Synergistic fungicidal activity of Cu(2+) and allicin, an allyl sulfur compound from garlic, and its relation to the role of alkyl hydroperoxide reductase 1 as a cell surface defense in Saccharomyces cerevisiae

Cu(2+) showed a dose-dependent fungicidal activity against Saccharomyces cerevisiae cells, and its lethal effect was extremely enhanced in the presence of allicin, an allyl sulfur compound from garlic. The fungicidal activity of Cu(2+) was unaffected or rather attenuated by other sulfur-containing compounds such as N-acetyl-cysteine, l-cysteine or dithiothreitol. Ca(2+) could absolutely protect against the lethal effect of Cu(2+) itself, but showed no protection against the fungicidal activity of Cu(2+) newly generated in combination with allicin. Cu(2+) accelerated an endogenous generation of reactive oxygen species (ROS) in S. cerevisiae cells at a lethal concentration, but such intracellular oxidative stress induction was not observed during cell death progression upon treatment with Cu(2+) and allicin. A surfactant, sodium N-lauroyl sarcosinate (SLS), enhanced the solubilization of a few proteins including alkyl hydroperoxide reductase 1 (AHP1) in intact cells, accounting for the absence of this protein in the extract from allicin-treated cells. Allicin-treated cells were rendered extremely sensitive to the subsequent Cu(2+) treatment as in the case of SLS-treated cells. Allicin-treated cells and SLS-treated cells similarly showed an increased sensitivity to exogenously added tert-butyl hydroperoxide (t-BOOH), an organic peroxide that is detoxified by the action of AHP1. Our study suggests that allicin influences the mode of cell surface localization or the related function of AHP1 as a defense against phospholipid peroxidation by the external action of Cu(2+).

Fluorescence-based sensing system for copper using genetically engineered living yeast cells

A whole cell-based optical sensing system for copper was developed based on Saccharomyces cerevisiae cells harboring plasmid pYEX-GFPuv. The basis of this system was the ability of the transcriptional activator protein Ace1 present in S. cerevisiae to control the expression of the reporter protein, GFPuv. When copper ions are present in the sample, the Ace1 protein activates the cup1 promoter located upstream from the gfpuv gene in plasmid pYEX-GFPuv, thus inducing the production of GFPuv. The concentration of copper ions in the sample can then be related to the GFPuv expressed in the yeast. The amount of GFPuv produced in the system was determined by monitoring the fluorescence emitted at 507 nm after excitation at 397 nm. This system can detect copper at concentrations as low as 5 x 10(-7) M, and is selective for copper over a variety of metal ions, with the exception of silver. The applicability of this sensing system to different analytical platforms and in real samples is demonstrated.

The effect of superoxide dismutase deficiency on cadmium stress

Saccharomyces cerevisiae mutant strains deficient in superoxide dismutase (Sod), an antioxidant enzyme, were used to analyze cadmium absorption and the oxidation produced by it. Cells lacking the cytosolic Sod1 removed twice as much cadmium as the control strain, while those deficient in the mitochondrial Sod2 exhibited poor metal absorption. Interestingly, the sod1 mutant did not become more oxidized after exposure to cadmium, as opposed to the control strain. We observed that the deficiency of Sod1 increases the expression of both Cup1 (a metallothionein) and Ycf1 (a vacuolar glutathione S-conjugate pump), proteins involved with protection against cadmium. Furthermore, when sod1 cells were exposed to cadmium, the ratio glutathione oxidized/glutathione reduced did not increase as expected. We propose that a high level of metallothionein expression would relieve glutathione under cadmium stress, while an increased level of Ycf1 expression would favor compartmentalization of this metal into the vacuole. Both conditions would reduce the level of glutathione-cadmium complex in cytosol, contributing to the high capacity of absorbing cadmium by the sod1 strain. Previous results showed that the glutathione-cadmium complex regulates cadmium uptake. These results indicate that, even indirectly, metallothionein also regulates cadmium transport.

Alkyl hydroperoxide reductase 1 protects Saccharomyces cerevisiae against metal ion toxicity and glutathione depletion

Alkyl hydroperoxide reductase 1 (Ahp1p) is a thioredoxin peroxidase of the peroxiredoxin family expressed by Saccharomyces cerevisiae. Recently, disruption of the AHP1 gene has shown that the gene is not essential for yeast growth on glucose medium but revealed a high sensitivity of null mutants to organic peroxides, suggesting that Ahp1p is an important enzyme implicated in oxidative stress protection in S. cerevisiae. To gain insight into antioxidant enzymatic mechanisms involved in cell protection against metal toxicity and glutathione depletion, we investigated the resistance of S. cerevisiae, in which the AHP1 gene was disrupted, against several metals and diethyl maleate, a glutathione depleting agent. We report that Ahp1p protects yeast against toxicity induced by copper, cobalt, chromium, arsenite, arsenate, mercury, zinc and diethyl maleate, suggesting that Ahp1p plays an important role in S. cerevisiae in the protection against metals possibly by reducing peroxides generated in cells by these compounds.

Decapping of stabilized, polyadenylated mRNA in yeast pab1 mutants

Interaction of the poly(A) binding protein, Pab1p, with mRNA plays an important role in gene expression. This work describes an analysis of pab1 mutants in Saccharomyces cerevisiae. Yeast pab1 mutants were found to be sensitive to elevated concentrations of copper (Cu) and 3-aminotriazole (3-AT) in the growth medium. This phenotype arises because these pab1 mutants underaccumulate mRNA, including the CUP1 and HIS3 mRNAs, the products of which are required for Cu and 3-AT resistance, respectively. To determine the cause of the mRNA underaccumulation, mRNA turnover and production were examined in the pab1-53 mutant. It was found that although the pattern of mRNA decay was altered, and substantial decapping of polyadenylated mRNA could be detected, mRNA was not destabilized in this strain. It was also found that the pab1 mutant was impaired in the production of mRNA. These data show that the decreased level of mRNA in the pab1-53 mutant arises from poor production, and they suggest that yeast Pab1p is involved in an aspect of nuclear mRNA metabolism. They also indicate that deadenylation can be uncoupled from decapping without significant changes in an mRNA's stability, and that this uncoupling can be tolerated by yeast.