PROJECT SUMMARY/ABSTRACT Humans face many types of environmental genotoxic agents, such as various pollutants and radiation, that can cause deleterious mutations and diseases. To survive and adapt to genotoxic conditions, cells rely on many highly conserved and complex molecular strategies. One strategy to withstand genotoxic stress is the use of error-prone repair mechanisms to increase mutation rates, which can potentiate new functions for better adaptation to toxic environments. Another mechanism is the DNA damage checkpoint (DDC) system, which controls the arrest and resumption of the cell cycle in accordance with the burden of genomic lesions. While the frameworks of both strategies are largely established, important mechanistic details are lacking. To address these gaps in knowledge, I will capitalize on the budding yeast model organism’s highly conserved DDC and repair systems. Aim 1 will build on the recent observation that the desumoylase Ulp1 is sumoylated upon genotoxin treatment and that mutating its two sumoylation sites results in a phenotype indicative of increased homologous recombination (HR). I thus hypothesize that Ulp1 sumoylation suppresses HR to favor error-prone repair. I will test this hypothesis and define the broader SUMO-based control of DNA repair pathway usage in response to genotoxins. Aim 2 will address the recent findings that RPA is downregulated after genotoxin exposure. I hypothesize that RPA downregulation can induce DDC recovery. To test this, I will elucidate how RPA degradation is regulated and how this impacts DDC recovery. Defects in either of the DNA repair pathway choices and DDC responses can lead to genome instability syndromes, characterized by increased sensitivity to environmental genotoxins, developmental abnormalities, and increased tumorigenesis. This work will lay the foundation for elucidating cellular adaptation strategies and can inform disease prevention and treatment strategies. It will also provide important training for me to reach my long-term career goal of becoming a principle investigator studying how human cells response to environmental genotoxins.