In our previous study, we established that inhibition of apoptosis by the general caspase inhibitor is associated with an increase in the level of oxidized proteins in a multicellular eukaryotic system. To gain further insight into a potential link between oxidative stress and apoptosis, we carried out studies with Saccharomyces cerevisiae, which contains a gene (YCA1) that encodes synthesis of metacaspase, a homologue of the mammalian caspase, and is known to play a crucial role in the regulation of yeast apoptosis. We show that upon exposure to H(2)O(2), the accumulation of protein carbonyls is much greater in a Delta yca1 strain lacking the YCA1 gene than in the wild type and that apoptosis was abrogated in the Delta yca1 strain, whereas wild type underwent apoptosis as measured by externalization of phosphatidylserine and the display of TUNEL-positive nuclei. We also show that H(2)O(2)-mediated stress leads to up-regulation of the 20S proteasome and suppression of ubiquitinylation activities. These findings suggest that deletion of the apoptotic-related caspase-like gene leads to a large H(2)O(2)-dependent accumulation of oxidized proteins and up-regulation of 20S proteasome activity.