Phosphate disruption and metal toxicity in Saccharomyces cerevisiae: effects of RAD23 and the histone chaperone HPC2

Coordination of Phosphate and Magnesium Metabolism in Bacteria

The majority of cellular phosphate (PO₄^-3; Pi) exists as nucleoside triphosphates, mainly adenosine triphosphate (ATP), and ribosomal RNA (rRNA). ATP and rRNA are also the largest cytoplasmic reservoirs of magnesium (Mg²⁺), the most abundant divalent cation in living cells. The co-occurrence of these ionic species in the cytoplasm is not coincidental. Decades of work in the Pi and Mg²⁺ starvation responses of two model enteric bacteria, Escherichia coli and Salmonella enterica, have led to the realization that the metabolisms of Pi and Mg²⁺ are interconnected. Bacteria must acquire these nutrients in a coordinated manner to achieve balanced growth and avoid loss of viability. In this chapter, we will review how bacteria sense and respond to fluctuations in environmental and intracellular Pi and Mg²⁺ levels. We will also discuss how these two compounds are functionally linked, and how cells elicit physiological responses to maintain their homeostasis.

Low phosphate mitigates cadmium-induced oxidative stress in Saccharomyces cerevisiae by enhancing endogenous antioxidant defence system

Cadmium is a highly toxic heavy metal that causes many harmful effects on human health and ecosystems. Metal chelation-based techniques have become a common approach for the treatment of metal poisoning and also for the remediation of metal contamination. Phosphate, an essential nutrient required for key cellular functions, has been supposed to be effective in reducing cadmium bioavailability, possibly through its chelating potential. In this study, we explored the effects of phosphate on cadmium toxicity and cellular response to cadmium stress in the eukaryotic model Saccharomyces cerevisiae. Our results reveal that cadmium toxicity is unexpectedly enhanced during phosphate repletion and optimal phosphate levels for yeast growth under cadmium stress conditions decline with increasing cadmium concentrations. The profound cadmium toxicity during phosphate repletion is unlikely to result from either elevated cadmium accumulation or dysregulated homeostasis of essential metals, but rather due to increased production of intracellular reactive oxygen species. We show that, under phosphate-depleted conditions, the activities of antioxidant enzymes, especially Mn-superoxide dismutase and catalase, are significantly promoted through transcriptional upregulation. Our findings highlight the important role of cellular response to phosphate limitation in mitigating cadmium toxicity and endogenous oxidative stress through the enhancement of antioxidant enzyme activity.

Limitation of phosphate assimilation maintains cytoplasmic magnesium homeostasis

Phosphorus (P) is essential for life. As the fifth-most-abundant element in living cells, P is required for the synthesis of an array of biological molecules including (d)NTPs, nucleic acids, and membranes. Organisms typically acquire environmental P as inorganic phosphate (Pi). While essential for growth and viability, excess intracellular Pi is toxic for both bacteria and eukaryotes. Using the bacterium Salmonella enterica serovar Typhimurium as a model, we establish that Pi cytotoxicity is manifested following its assimilation into adenosine triphosphate (ATP), which acts as a chelating agent for Mg²⁺ and other cations. Our findings identify physiological processes disrupted by excessive Pi and how bacteria tune P assimilation to cytoplasmic Mg²⁺ levels.

Phosphorus (P) is an essential component of core biological molecules. In bacteria, P is acquired mainly as inorganic orthophosphate (Pi) and assimilated into adenosine triphosphate (ATP) in the cytoplasm. Although P is essential, excess cytosolic Pi hinders growth. We now report that bacteria limit Pi uptake to avoid disruption of Mg²⁺-dependent processes that result, in part, from Mg²⁺ chelation by ATP. We establish that the MgtC protein inhibits uptake of the ATP precursor Pi when Salmonella enterica serovar Typhimurium experiences cytoplasmic Mg²⁺ starvation. This response prevents ATP accumulation and overproduction of ribosomal RNA that together ultimately hinder bacterial growth and result in loss of viability. Even when cytoplasmic Mg²⁺ is not limiting, excessive Pi uptake increases ATP synthesis, depletes free cytoplasmic Mg²⁺, inhibits protein synthesis, and hinders growth. Our results provide a framework to understand the molecular basis for Pi toxicity. Furthermore, they suggest a regulatory logic that governs P assimilation based on its intimate connection to cytoplasmic Mg²⁺ homeostasis.

The heat responsive wheat TaRAD23 rescues developmental and thermotolerant defects of the rad23b mutant in Arabidopsis thaliana

High temperature severely damage the growth and development of crops with climate change. To effectively screen heat responsive proteins in wheat (Triticum aestivum L.), the isobaric tandem mass tag (TMT)-labeled quantitative proteomic analysis and quantitative real-time PCR (qRT-PCR) were performed. Here, we found that a wheat RADIATION SENSITIVE 23 protein, TaRAD23, was up-regulated at both protein and RNA levels by exposing to heat stress. Sequence homology analysis indicated that the TaRAD23 is a conserved protein, which is closely related to the Arabidopsis thaliana proteins AtRAD23B and AtRAD23A. Genetic knockout of AtRAD23B, but not AtRAD23A, shows multiple developmental defects, as well as sensitivity to heat stress. Meanwhile, we observed that constitutive overexpression of TaRAD23 in rad23b fully rescued developmental and thermotolerant defects of the mutant. Furthermore, qRT-PCR analysis of heat responsive genes in rad23b and its complementary lines suggested that suppression of the heat shock transcription factor AtHSFA2 and heat responsive genes (HSP70, HSP90, HSP17.6 and HSA32) may be the cause of the weaker thermotolerance in rad23b. Taken together, the data suggest that the heat responsive TaRAD23 is a functionally highly conserved protein that plays an important role in development, as well as the regulation in heat stress response network.

The galactose-induced decrease in phosphate levels leads to toxicity in yeast models of galactosemia

Classic galactosemia is an inborn error of metabolism caused by deleterious mutations in the GALT gene. A number of evidences indicate that the galactose-1-phosphate accumulation observed in patient cells is a cause of toxicity in this disease. Nevertheless, the consequent molecular events caused by the galactose-1-phosphate accumulation remain elusive. Here we show that intracellular inorganic phosphate levels decreased when yeast models of classic galactosemia were exposed to galactose. The decrease in phosphate levels is probably due to the trapping of phosphate in the accumulated galactose-1-phosphate since the deletion of the galactokinase encoding gene GAL1 suppressed this phenotype. Galactose-induced phosphate depletion caused an increase in glycogen content, an expected result since glycogen breakdown by the enzyme glycogen phosphorylase is dependent on inorganic phosphate. Accordingly, an increase in intracellular phosphate levels suppressed the galactose effect on glycogen content and conferred galactose tolerance to yeast models of galactosemia. These results support the hypothesis that the galactose-induced decrease in phosphate levels leads to toxicity in galactosemia and opens new possibilities for the development of better treatments for this disease.

Isolation and Characterization of a Variant Manganese Resistant Strain of Saccharomyces cerevisiae

　Manganese contamination in water is one of the most serious problems in Southeast Asian countries, including Vietnam. Bioremediation using microorganisms, especially from the brewing yeast Saccharomyces cerevisiae, is expected to be a useful technique to remove manganese from contaminated water. Yeast strain S. cerevisiae BY4741 as the wild-type strain and some manganese-accumulating mutants bred from BY4741 were examined for cell growth and manganese accumulation in YPD liquid medium containing various concentrations of Mn²⁺.　Variants accumulating larger concentrations of manganese were isolated by the repeated screening of survivors in YPD media containing10mM Mn²⁺. Manganese was accumulated by the yeast cells during growth, but the growth of BY4741 was retarded with increasing Mn²⁺ concentrations and almost inhibited at 15mM Mn²⁺. One variant isolate, named IM3, showed no retardation of growth up to 15 mM Mn²⁺ and could absorb over 4-fold more manganese than the BY4741 strain. Effects of culture temperature and pH on the growth and manganese accumulation were analyzed for IM3. Maximum accumulation was shown at 30℃, pH 6.0 while the optimal growth was shown at 37℃, pH 5.0 - 7.0. Interestingly, IM3 could grow a little at pH 9.0 when manganese was added to the culture media, while it could not grow without the addition of manganese.

Manganese acquisition and homeostasis at the host-pathogen interface

Pathogenic bacteria acquire transition metals for cell viability and persistence of infection in competition with host nutritional defenses. The human host employs a variety of mechanisms to stress the invading pathogen with both cytotoxic metal ions and oxidative and nitrosative insults while withholding essential transition metals from the bacterium. For example, the S100 family protein calprotectin (CP) found in neutrophils is a calcium-activated chelator of extracellular Mn and Zn and is found in tissue abscesses at sites of infection by Staphylococcus aureus. In an adaptive response, bacteria have evolved systems to acquire the metals in the face of this competition while effluxing excess or toxic metals to maintain a bioavailability of transition metals that is consistent with a particular inorganic "fingerprint" under the prevailing conditions. This review highlights recent biological, chemical and structural studies focused on manganese (Mn) acquisition and homeostasis and connects this process to oxidative stress resistance and iron (Fe) availability that operates at the human host-pathogen interface.

A manganese-rich environment supports superoxide dismutase activity in a Lyme disease pathogen, Borrelia burgdorferi

The Lyme disease pathogen Borrelia burgdorferi represents a novel organism in which to study metalloprotein biology in that this spirochete has uniquely evolved with no requirement for iron. Not only is iron low, but we show here that B. burgdorferi has the capacity to accumulate remarkably high levels of manganese. This high manganese is necessary to activate the SodA superoxide dismutase (SOD) essential for virulence. Using a metalloproteomic approach, we demonstrate that a bulk of B. burgdorferi SodA directly associates with manganese, and a smaller pool of inactive enzyme accumulates as apoprotein. Other metalloproteins may have similarly adapted to using manganese as co-factor, including the BB0366 aminopeptidase. Whereas B. burgdorferi SodA has evolved in a manganese-rich, iron-poor environment, the opposite is true for Mn-SODs of organisms such as Escherichia coli and bakers' yeast. These Mn-SODs still capture manganese in an iron-rich cell, and we tested whether the same is true for Borrelia SodA. When expressed in the iron-rich mitochondria of Saccharomyces cerevisiae, B. burgdorferi SodA was inactive. Activity was only possible when cells accumulated extremely high levels of manganese that exceeded cellular iron. Moreover, there was no evidence for iron inactivation of the SOD. B. burgdorferi SodA shows strong overall homology with other members of the Mn-SOD family, but computer-assisted modeling revealed some unusual features of the hydrogen bonding network near the enzyme's active site. The unique properties of B. burgdorferi SodA may represent adaptation to expression in the manganese-rich and iron-poor environment of the spirochete.