Stress-activated genomic expression changes serve a preparative role for impending stress in yeast

Abstract

Yeast cells respond to stress by mediating condition-specific gene expression changes and by mounting a common response to many stresses, called the environmental stress response (ESR). Giaever et al. previously revealed poor correlation between genes whose expression changes in response to acute stress and genes required to survive that stress, raising question about the role of stress-activated gene expression. Here we show that gene expression changes triggered by a single dose of stress are not required to survive that stimulus but rather serve a protective role against future stress. We characterized the increased resistance to severe stress in yeast preexposed to mild stress. This acquired stress resistance is dependent on protein synthesis during mild-stress treatment and requires the "general-stress" transcription factors Msn2p and/or Msn4p that regulate induction of many ESR genes. However, neither protein synthesis nor Msn2/4p is required for basal tolerance of a single dose of stress, despite the substantial expression changes triggered by each condition. Using microarrays, we show that Msn2p and Msn4p play nonredundant and condition-specific roles in gene-expression regulation, arguing against a generic general-stress function. This work highlights the importance of condition-specific responses in acquired stress resistance and provides new insights into the role of the ESR.

Figure 1.

Wild-type cells acquire resistance to severe stress after exposure to mild stress. Cell survival of severe (secondary) stress was scored at various times after exposure to a mild (primary) dose of stress. The fold-increase in maximum secondary-stress dose survived compared with a mock-treated culture is shown at 15, 30, 60, and 120 min of primary stress exposure. Primary stresses included (A) 0.7M NaCl, (B) 0.2 mM H₂O₂, (C) a 30°C to 37°C heat shock, or (D) growth to different phases. Increased thermotolerance is indicated as maximum °C survived. Plots represent the average and SE of the mean (SEM) of at least triplicate experiments, except for growth-phase experiments, which show the average and range of duplicates. Asterisks represent resistance that was significantly different from mock-treated cells in ≥2 time points (p < 0.01, t test).

Figure 2.

Protein synthesis is required for acquired stress resistance. Acquired stress resistance was measured as described in Figure 1 after 60 min treatment with (A) NaCl or (B) H₂O₂ primary stress, in the presence or absence of cycloheximide. Cycloheximide-treated cells were exposed to 10 μg/ml inhibitor for 20 min before and throughout primary-stress and/or secondary-stress exposures. Each plot shows the average and SEM of triplicate experiments. Conditions that provided acquired stress resistance compared with mock-treated cells are marked with an asterisk (p < 0.01).

Figure 3.

Strains lacking Msn2p or Msn4p show defects in acquired stress resistance. Cells were exposed to primary and secondary stresses as described in Figure 1. The fold-increase in maximum dose of secondary stress survived 60 min after exposure to 30–37°C heat shift, 0.7 M NaCl, or 0.2 mM H₂O₂ primary stress is shown for wild-type and mutant strains, as indicated by the key. Resistance to (A) NaCl or (B) H₂O₂ secondary stress is shown after each primary-stress treatment indicated on the x-axis. Plots show the average and SEM of at least triplicate experiments.

Figure 4.

Genes show different dependencies on Msn2p versus Msn4p, depending on the stress. Gene expression was characterized using DNA microarrays in wild-type and mutant cells 45 min after treatment with 0.7M NaCl or 15 min after a 30–37°C heat shock. The average log2 expression change is shown for genes dependent on Msn2p and/or Msn4p in response to NaCl (left) or heat shock (right): (A) 21 genes and 16 genes that showed a greater expression defect in the msn2Δmsn4Δ strain than either single mutant responding to NaCl treatment or heat shock, respectively; (B) 60 genes and 18 genes that showed an induction defect in both msn2Δ and msn4Δ strains responding to NaCl treatment or heat shock, respectively, but for which no additive defect was observed in the double mutant; and (C) 59 genes and 48 genes that showed a significant defect only in the msn2Δ and msn2Δmsn4Δ strains responding to NaCl treatment and heat shock, respectively.

Figure 5.

Cells with acquired stress resistance show an altered expression response to stress. Cells were exposed to 0.5 mM H₂O₂, alone or after 60 min pretreatment with 0.7M NaCl. Microarrays were conducted at 10, 20, 30, and 40 min after H₂O₂ treatments or 15, 30, 45, and 60 min after NaCl addition. The average log2 expression is shown for (A) 316 genes with acclimated H₂O₂-responsive repression in wild-type cells pretreated with NaCl; (B) 288 genes with acclimated H₂O₂-responsive induction in wild-type cells pretreated with NaCl; and (C) 64 Yap1p targets (Gasch et al., 2000) in wild-type (top panels) and msn2Δ cells (bottom panels).