FIGURE 3: Non-inducible nitrosative stress tolerance mechanism mediated by Cup1. (A) The CUP1 mRNA was determined by RT-qPCR in WT strain with or without the exposure to 20 μM CuSO₄ or 2 mM NaNO₂ for 1 hour. The relative mRNA level of the CUP1 gene was calculated with the CUP1 expression level in the untreated sample as 1.0 using ACT1 as a reference gene. The values are the means and standard deviations of at least three independent experiments. Statistic significances of differences were analyzed by Student's t-test (*p < 0.05, vs the untreated sample). (B) The model of nitrosative stress tolerance mechanism by Cup1 in yeast cells. The expression of Cup1 is not induced by NO stimuli. The constitutively or basically produced Cup1 traps NO with its cysteine residue(S) under nitrosative stress conditions. Cup1 modified with NO can be reduced to its unmodified form through Trx/Trr or Grx/Grr reduction system using NADPH as a reducing force.