S. cerevisiae possess two catalase isoforms, a peroxisomal/mitochondrial Cta1 [18] and an atypical cytosolic Ctt1 [19]. Hence, we next addressed the relative importance of each isoform in protecting cells against H2O2 challenge using the single catalase knockouts, ctt1∆ and cta1∆. Comparable catalase activity is induced in wild-type and cta1∆ YPH250 cells after H2O2 exposure ( Fig. 2A,B,D) but negligible catalase induction occurs in the ctt1∆ mutant strain ( Fig. 2C,D). Similar results were observed for the catalase knockouts in the BY4741 genetic background (data not shown). Combined, our data indicate that cytosolic Ctt1 activity is stimulated ~10-fold by exogenous H2O2 in YPD, consistent with the 15-fold increase in production of the Ctt1 protein on H2O2 challenge in SCD [10]. Furthermore, cta1∆ cells exhibit wild-type resistance to H2O2 ( Fig. 2E), whereas the ctt1∆ mutant exhibits H2O2 hypersensitivity with cell viability dropping by 30% on challenge with 0.2 mM H2O2 ( Fig. 2E). H2O2 challenge in YPD does not upregulate Cta1 activity ( Fig. 3C) and no increase in Cta1 protein levels were detected on H2O2 challenge in SCD [10]. Thus, we conclude that stimulation of cytosolic Ctt1 activity on H2O2 challenge in YPD is a critical factor in protecting cells against this ROS.