Cellular resistance to anti-cancer agents is the predominant reason why chemotherapy regimens fail to eradicate disseminated malignancies. Several mechanisms for drug resistance have been identified including the cell's endogenous stress response pathway. Moreover, the stress pathway that responds to reactive oxygen species (ROS) appears to be of singular importance for influencing drug sensitivity. However, the molecular mechanisms underlying the role of the ROS pathway in altering the drug response are largely unknown. The long-term goal of this proposal is to fully describe the ROS response system and determine how this pathway influences drug sensitivity in both normal and transformed cells. In the budding yeast S. cerevisiae, we found that the C-type cyclin-cyclin dependent kinase 8 (cyclin C- Cdk8p) represses the transcription of several genes involved in the stress response. To relieve this repression, cyclin C is destroyed in cells exposed to several stressors including reactive oxygen. We have demonstrated that the human cyclin C (Hscyclin C) is also down regulated in transformed cell lines subjected to stress indicating that this system is remarkably well conserved. In addition, prior to destruction, the yeast cyclin C exits the nucleus and associates with the mitochondria. Finally, modulating cyclin C levels in yeast dramatically affects ROS sensitivity. Specifically, deleting cyclin C makes cells more resistant to ROS exposure while protecting the cyclin from degradation makes cells hypersensitive. These changes in cell viability directly correlate with the presence of signature apoptotic signals indicating that cyclin C represents a previously undescribed regulator of programmed cell death. Given the important connection between apoptotic control and the mitochondria, our finding that cyclin C localizes to the mitochondria following stress suggests a novel regulatory strategy for controlling ROS-induced apoptosis. These results are consistent with a model that cyclin C represented a new type of tumor suppressor activity. Rather than preventing aberrant cell cycle progression such as p53 and Rb, cyclin C-Cdk8 prevents tumor progression by negatively regulating the stress response. This model is supported by the findings that the Hscyclin C locus (CCNC) is deleted in diverse tumor types including breast, mesothelioma and osteosarcoma. To elucidate the molecular role of cyclin C in regulating the oxidative stress response and drug resistance, the following aims are proposed: Aim 1. Elucidate the molecular role for cyclin C-Cdk8p in controlling the apoptotic response in yeast. Aim 2. Identify new components of yeast cyclin C apoptosis regulatory network. Aim 3. Determine the role of mammalian cyclin C degradation in the cellular response to stress and anticancer drugs.