Saccharomyces cerevisiae HSP70 heat shock elements are functionally distinct

Heat shock protein 70 (HmHsp70) from Hypsizygus marmoreus confers thermotolerance to tobacco

The 70-kD heat shock proteins (Hsp70s) have been proved to be important for stress tolerance and protein folding and unfolding in almost all organisms. However, the functions of Hsp70s in mushroom are not well understood. In the present study, a hsp70 gene from Hypsizygus marmoreus, hmhsp70, was cloned and transferred to tobacco (Nicotiana tabacum) to evaluate its function in thermotolerance. Sequence alignments and phylogenetic analysis revealed that HmHsp70 may be located in the mitochondria region. qPCR analysis revealed that the transcription level of hmhsp70 in H. marmoreus mycelia increased after heat shock treatment in high temperature (42 °C) compared with untreated mycelia (at 25 °C). Transgenic tobaccos expressing hmhsp70 gene showed enhanced resistance to lethal temperature compared with the wild type (WT) plants. Nearly 30% of the transgenic tobaccos survived after treated at a high temperature (50 °C and 52 °C for 4 h); however, almost all the WT tobaccos died after treated at 50 °C and no WT tobacco survived after heat shock at 52 °C. This study firstly showed the function of a hsp70 gene from H. marmoreus.

Identification of a Heat-Inducible Element of Cysteine Desulfurase Gene Promoter in Lentinula edodes

Volatile organosulfur compounds are the main components that contribute to the unique aroma of dried Lentinula edodes. They are mainly generated during the hot-air drying process, and cysteine desulfurase is the key enzyme in this process. Temperature may be an essential factor of volatile organosulfur compound production by influencing the expression of the cysteine desulfurase gene. In this study, the promoter sequence of the cysteine desulfurase gene (pCS) was cloned and analyzed using bioinformatics tools. A series of 5′deletion fragments and site-directed mutations of pCS were constructed to identify the element that responds to heat stress. Six heat shock transcription factor (HSTF) binding sites were predicted by SCPD (The Promoter Database of Saccharomyces cerevisiae) and three of the binding sites were predicted by Yeastract (Yeast Search for Transcriptional Regulators and Consensus Tracking) in pCS. The results indicated that pCS was able to drive the expression of the EGFP (Enhanced Green Fluorescent Protein) gene in L. edodes. Moreover, the fluorescence intensity increased after heat stress. The changes in fluorescence intensity of different 5′deletion fragments showed that the heat response region was located between −500 bp and −400 bp in pCS. The site-directed mutation analysis further showed that the heat-inducible element was between −490 bp and −500 bp (TTTCTAGAAT) in pCS. Our results provide molecular insight for studying the formation of volatile organosulfur compounds in dried L. edodes.

Xenobiotic biotransformation, oxidative stress and obesogenic molecular biomarker responses in Tilapia guineensis from Eleyele Lake, Nigeria

Despite the important ecosystem and hydrological roles of coastal and inland waters, there are no established biomonitoring protocols for evaluating environmental, wildlife and human health for these coastlines in Nigeria. In the present study, contaminants tissue burden and effects at molecular and physiological levels, were investigated in Tilapia guineensis from a man-made lake (Eleyele Lake) that is used for municipal domestic water supply and compared to a reference site (Igboho Lake). Gene expression of phase I and II biotransformation systems, oxidative stress and obesogen responses were analyzed using real-time PCR, and these results were related to general health index (condition factor: CF) and muscle burden of trace metals, aliphatic and polycyclic aromatic hydrocarbons (PAHs). We observed a significant increase in phase I and II biotransformation systems, oxidative stress and obesogen responses in male and female fish from Eleyele lake compared with the reference site. Overall, our data showed significant relationships between biological responses and tissue concentrations of metals and PAHs for the Eleyele lake compared with the reference site. Given that a positive influence on genes and pathways associated with metabolic status has been previously associated with peroxisome proliferator activated receptors (PPARs), xenobiotic compounds that activate PPARs may produce changes in energy and metabolic processes, leading to obesity. The high CF (>1 = good health condition) observed, coupled with the high muscle burden of inorganic and organic contaminants in fish from Eleyele lake, suggest a potential obesogenic effect in these fishes. These findings represent co-relational evidence that the Eleyele lake is contaminated and consequently affecting biological and physiological integrity of organisms inhabiting the lake. These findings also suggest potential health risks for humans, since the lake is extensively used for domestic water supply and fisheries.

Synergistic cellular responses to heavy metal exposure: A minireview

BACKGROUND

Metal-responsive transcription factor 1 (MTF-1) induces the expression of metallothioneins (MTs) which bind and sequester labile metal ions. While MTF-1 primarily responds to excess metal exposure, additional stress response mechanisms are activated by excess metals. Evidence suggests potential crosstalk between responses mediated by MTF-1 and stress signaling enhances cellular tolerance to metal exposure.

SCOPE OF REVIEW

This review aims to summarize the current understanding of interaction between the stress response mediated by MTF-1 and other cellular mechanisms, notably the nuclear factor κB (NF-κB) and heat shock response (HSR).

MAJOR CONCLUSIONS

Crosstalk between MTF-1 mediated metal response and NF-κB signaling or HSR can modulate expression of stress proteins in response to metal exposure via effects on precursor signals or direct interaction of transcriptional activators. The interaction between stress signaling pathways can enhance cell survival and tolerance through a unified response system.

GENERAL SIGNIFICANCE

Elucidating the interactions between MTF-1 and cell stress response mechanisms is critical to a comprehensive understanding of metal-based cellular effects. Co-activation of HSR and NF-κB signaling allows the cell to detect metal contamination in the environment and improve survival outcomes.

Cell-to-cell variability in the propensity to transcribe explains correlated fluctuations in gene expression

Random fluctuations in gene expression lead to wide cell-to-cell differences in RNA and protein counts. Most efforts to understand stochastic gene expression focus on local (intrinisic) fluctuations, which have an exact theoretical representation. However, no framework exists to model global (extrinsic) mechanisms of stochasticity. We address this problem by dissecting the sources of stochasticity that influence the expression of a yeast heat shock gene, SSA1. Our observations suggest that extrinsic stochasticity does not influence every step of gene expression, but rather arises specifically from cell-to-cell differences in the propensity to transcribe RNA. This led us to propose a framework for stochastic gene expression where transcription rates vary globally in combination with local, gene-specific fluctuations in all steps of gene expression. The proposed model better explains total expression stochasticity than the prevailing ON-OFF model and offers transcription as the specific mechanism underlying correlated fluctuations in gene expression.

Quantitative proteomics and network analysis of SSA1 and SSB1 deletion mutants reveals robustness of chaperone HSP70 network in Saccharomyces cerevisiae

Molecular chaperones play an important role in protein homeostasis and the cellular response to stress. In particular, the HSP70 chaperones in yeast mediate a large volume of protein folding through transient associations with their substrates. This chaperone interaction network can be disturbed by various perturbations, such as environmental stress or a gene deletion. Here, we consider deletions of two major chaperone proteins, SSA1 and SSB1, from the chaperone network in Sacchromyces cerevisiae. We employ a SILAC-based approach to examine changes in global and local protein abundance and rationalise our results via network analysis and graph theoretical approaches. Although the deletions result in an overall increase in intracellular protein content, correlated with an increase in cell size, this is not matched by substantial changes in individual protein concentrations. Despite the phenotypic robustness to deletion of these major hub proteins, it cannot be simply explained by the presence of paralogues. Instead, network analysis and a theoretical consideration of folding workload suggest that the robustness to perturbation is a product of the overall network structure. This highlights how quantitative proteomics and systems modelling can be used to rationalise emergent network properties, and how the HSP70 system can accommodate the loss of major hubs.

Interaction between heat shock transcription factors (HSFs) and divergent binding sequences: binding specificities of yeast HSFs and human HSF1

The target genes of the heat shock transcription factor (HSF) contain a cis-acting sequence, the heat shock element (HSE), which consists of multiple inverted repeats of the sequence 5'-nGAAn-3'. Using data acquired in this and a previous study, we have identified the HSEs in 59 of 62 target genes of Saccharomyces cerevisiae Hsf1. The Hsf1 protein recognizes continuous and discontinuous repeats of the nGAAn unit; the nucleotide sequences and configuration of the units diverge slightly among functional HSEs. When Schizosaccharomyces pombe HSF was expressed in S. cerevisiae cells, heat shock induced S. pombe HSF to bind to various HSE types, which properly activated transcription from almost all target genes, suggesting that the S. pombe genome also contains divergent HSEs. Human HSF1 induced the heat shock response via HSEs with continuous units in S. cerevisiae cells but failed to do so via HSEs with discontinuous units. Binding of human HSF1 to the discontinuous type of HSE was observed in vitro but was significantly inhibited in vivo. These results show that human HSF1 recognizes HSEs in a slightly different way than yeast HSFs and suggest that the configuration of the unit is an important determinant for HSF-HSE interactions.

Identification of a novel class of target genes and a novel type of binding sequence of heat shock transcription factor in Saccharomyces cerevisiae

In response to hyperthermia, heat shock transcription factor (HSF) activates transcription of a set of genes encoding heat shock proteins (HSPs). The promoter regions of HSP genes contain the HSF binding sequence called the heat shock element (HSE), which consists of contiguous inverted repeats of the sequence 5'-nGAAn-3' (where n is any nucleotide). We have constructed an hsf1 mutant of Saccharomyces cerevisiae and analyzed genome-wide changes in heat shock response in the mutant cells. The results have revealed that Hsf1 is necessary for heat-induced transcription of not only HSP but also genes encoding proteins involved in diverse cellular processes such as protein degradation, detoxification, energy generation, carbohydrate metabolism, and maintenance of cell wall integrity. Approximately half of the Hsf1-regulated genes lacked the typical HSE in their promoter regions. Instead, several of these genes have a novel Hsf1 binding sequence that contains three direct repeats of nTTCn (or nGAAn) interrupted by 5 bp. The number and spacing of the repeating units are critical determinants for heat-induced transcription as well as for recognition by Hsf1. In the yeast genome, the presence of the sequence is enriched in Hsf1-regulated genes, suggesting that it is generally used as an HSE in the Hsf1 regulon.

Carboxy-terminal region of the yeast heat shock factor contains two domains that make transcription independent of the TFIIH protein kinase

BACKGROUND

Phosphorylation of the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II is implicated in transition from initiation to elongation in the transcription cycle. In yeast cells, Kin28, a subunit of the general transcription factor TFIIH, is responsible for the CTD phosphorylation. Although Kin28 is indispensable for transcription of many genes, its requirement is bypassed in certain genes such as SSA4 or CUP1, whose transcription is activated by the heat shock factor Hsf1.

RESULTS

We show that C-terminal region of Hsf1, which consists of an activation domain AR2 and a regulatory domain CTM, mediates the Kin28-independent transcription. The AR2 domain, when fused to the DNA-binding domain of Gal4 and recruited to the GAL7 gene via the Gal4-binding sequence, is sufficient for activating GAL7 in the absence of Kin28. We have further found that AR2 has an ability to recruit TATA box-binding protein-associated factors (TAFs) to the promoter. Consistently, transcription from promoters occupied naturally or artificially with TAFs is sustained in the absence of Kin28 function.

CONCLUSIONS

These results show that CTM modulates activation function of AR2 in the Hsf1 molecule. We also suggest that recruitment of TAFs to a promoter is involved in the Kin28-independent transcription.

Evolution of thermotolerance and the heat-shock response: evidence from inter/intraspecific comparison and interspecific hybridization in the virilis species group of Drosophila. I. Thermal phenotype

Species in the virilis group of Drosophila (fruit flies), which overlap or replace one another along climatic gradients, exhibit corresponding differences in basal thermotolerance, inducible thermotolerance and the heat-shock response. The low-latitude species D. virilis exceeds the high-latitude species D. lummei in these measures of thermotolerance, the temperature threshold for heat-shock factor (HSF) activation and the ability to express hsp70 mRNA and diverse heat-shock proteins (e.g. Hsp70, Hsp83 and small Hsps) after intense heat shock (e.g. 40-41 degrees C). The xeric species D. novamexicana differs from the mesic species D. texana in much the same way for many of these traits. By contrast, intraspecific variation in these traits is small. Because D. virilis and D. lummei can readily be crossed to yield partially fertile progeny, genetic analysis of interspecific differences is possible. Interspecific hybrids are intermediate to the parental species in basal thermotolerance and inducible thermotolerance and resemble D. virilis in Hsp concentrations after intense heat shock and Hsp70 protein electromorphs.

Dynamic chromatin alterations triggered by natural and synthetic activation domains

The activation domains (ADs) of transcription activators recruit a multiplicity of enzymatic activities to gene promoters. The mechanisms by which such recruitment takes place are not well understood. Using chromatin immunoprecipitation, we demonstrate dynamic alterations in the abundance of histones H2A, H3, and H4 at promoters of genes regulated by the HSF and Gal4 activators of Saccharomyces cerevisiae. Transcriptional activation of these genes, particularly those regulated by HSF, is accompanied by a significant reduction in both acetylated and unacetylated histones at promoters and may involve the transient displacement of histone octamers. To gain insight into the function of ADs, we conducted a genetic screen to identify polypeptides that could substitute for the 340-residue C-terminal activator of HSF and rescue the temperature sensitivity caused by its deletion. We found that the ts(-) phenotype of HSF(1-493) could be complemented by peptides as short as 11 amino acids. Such peptides are enriched in acidic and hydrophobic residues, and exhibit both trans-activating and chromatin-modifying activities when fused to the Gal4 DNA-binding domain. We also demonstrate that a previously identified 14-amino acid histone H3-binding module of human CTF1/NF1, which is similar to synthetic ADs, can substitute for the HSF C-terminal activator in conferring temperature resistance and can mediate the modification of promoter chromatin structure. Possible mechanisms of AD function, including one involving direct interactions with histones, are discussed.

A novel non-conventional heat shock element regulates expression of MDJ1 encoding a DnaJ homolog in Saccharomyces cerevisiae

The heat shock factor (HSF) is a pivotal transcriptional factor that regulates the expression of genes encoding heat shock proteins (HSPs) via heat shock elements (HSEs). nGAAnnTTCnnGAAn functions as the minimum consensus HSE (cHSE) in vivo. Here we show that the expression of Saccharomyces cerevisiae MDJ1 encoding a mitochondrial DnaJ homolog is regulated by HSF via a novel non-consensus HSE (ncHSE(MDJ1)), which consists of three separated pentameric nGAAn motifs, nTTCn-(11 bp)-nGAAn-(5 bp)-nGAAn. This is the first evidence to show that the immediate contact of nGAAn motifs is dispensable for regulation by HSF in vivo. ncHSE(MDJ1) confers different heat shock responses versus cHSE and, unlike cHSE, definitively requires a carboxyl-terminal activation domain of HSF in the expression. ncHSE(MDJ1)-like elements are found in promoter regions of some other DnaJ-related genes. The highly conserved HSF/HSE system suggests that similar ncHSEs may be used for the expression of HSP genes in other eukaryotes including humans.

A novel domain of the yeast heat shock factor that regulates its activation function

Heat shock factor Hsf1 of the yeast Saccharomyces cerevisiae binds to the heat shock element (HSE) of a subset of genes and activates their transcription in response to various environmental stresses. Hsf1 protein contains discrete domains respectively involved in DNA-binding, trimerization, transcription activation, and transcription repression. Here we have identified a novel domain rich in basic amino acids at the extreme C-terminus of Hsf1. Deletion or point mutations of the C-terminal basic region caused an inefficient heat shock response of genes containing noncanonical HSEs such as CUP1 and HSP26. The basic region is also essential for oxidative stress-inducible transcription of CUP1 by Hsf1. By contrast, it was dispensable for heat induction through the canonical HSE. We suggest that the basic region is a modulator involved in regulation of the Hsf1-mediated activation depending on the architecture of its binding site.

Heat shock and light activation of a Chlamydomonas HSP70 gene are mediated by independent regulatory pathways

Induction of HSP70 heat shock genes by light has been demonstrated in Chlamydomonas. Our aim was to establish whether this induction by light is mediated by the heat stress sensing pathway or by an independent signal chain. Inhibitors of cytoplasmic protein synthesis revealed an initial difference. Cycloheximide and other inhibitors of protein synthesis prevented HSP70A induction upon illumination but not during heat stress. Analysis of HSP70A induction in cells that had differentiated into gametes revealed a second difference. While heat shock resulted in elevated HSP70A mRNA levels, light was no longer able to serve as an inducer in gametes. To identify the regulatory sequences that mediate the response of the HSP70A gene to either heat stress or light we introduced a series of progressive 5' truncations into its promoter sequence. Analyses of the levels of mRNA transcribed from these deletion constructs showed that in most of them the responses to heat shock and light were similar, suggesting that light induction is mediated by a light-activated heat shock factor. However, we show that the HSP70A promoter also contains cis-acting sequences involved in light induction that do not participate in induction by heat stress. Together, these results provide evidence for a regulation of HSP70A gene expression by light through a heat shock-independent signal pathway.

Transcriptional regulation of flocculation genes in Saccharomyces cerevisiae

Northern analysis showed that DNA from the flocculation gene FLO1 hybridized to mRNA molecules of 4.8 kb. This transcript was specific for the FLO1 gene at the right end of chromosome I since disruption of this gene resulted in the disappearance of the transcript. We further found an absolute correlation between flocculation and the presence of transcripts hybridizing to FLO1 DNA, both in various flocculent and non-flocculent strains and in cells from the non-flocculating and flocculating stages of growth. In all cases transcripts were present in flocculating and absent from non-flocculating cultures. From these results we conclude that the FLO1 gene is transcriptionally regulated. Mutations in TUP1 or SSN6 cause flocculation. Several transcripts hybridizing to FLO1 DNA were present in the mutants but not in the corresponding wild-type strains. Disruption of the FLO1 gene in the tup1 and ssn6 strains showed that one of the transcripts corresponded to the FLO1 gene. Disruption of FLO1 did not abolish flocculation completely but only reduced it, indicating that at least two flocculation genes, including FLO1, are activated or derepressed by mutations in the TUP1/SSN6 regulatory cascade.

Mdj1p, a novel chaperone of the DnaJ family, is involved in mitochondrial biogenesis and protein folding

Mdj1p, a novel member of the DnaJ family, is a heat shock protein that is associated with the inner membrane of mitochondria of Saccharomyces cerevisiae. Disruption of the MDJ1 gene resulted in a petite phenotype, loss of mitochondrial DNA, and inviability at 37 degrees C. Import of precursor proteins was not affected by a lack of Mdj1p, but folding of newly imported proteins was markedly impaired. The efficiency of refolding of a tester protein, dihydrofolate reductase, was significantly reduced in mitochondria lacking Mdj1p after incubation at elevated temperature. We conclude that Mdj1p is an important mitochondrial chaperone that participates in the folding of newly imported proteins and in the protection of proteins against heat denaturation and aggregation.