DNA damage tolerance pathway choice in Drosophila Project Summary When replication forks encounter DNA lesions, they must bypass these roadblocks through DNA damage tolerance mechanisms or DNA synthesis will stop, leading to gaps in the genome, genomic instability, and eventual cell death. Strategies that cells use for bypass include the recruitment of error-prone translesion synthesis polymerases, template switching at the replication fork, and repriming downstream of the lesion followed by gap filling. The choice of damage tolerance mechanism is important, as some pathways are more mutagenic than others. However, the control of lesion bypass pathway choice in metazoans is not well understood, particularly in the context of different tissues. Such knowledge is critical in order to understand how both normal and cancerous cells deal with replication-blocking lesions that result from treatment with chemotherapeutic agents. We have recently discovered that the REV1 protein is a key mediator of DNA damage tolerance in Drosophila. REV1 promotes the recruitment of translesion polymerases to bypass damaged bases, which we have shown is the preferred tolerance mechanism in rapidly dividing tissues in the developing fly. In addition, REV1 appears to mediate template switching through an unknown mechanism. In the experiments described here, we will use domain-specific mutants, genetic analysis, and cellular assays in larval imaginal discs to characterize how REV1 coordinates various damage tolerance pathways to promote continuance of DNA replication. In addition, we will use both genetic and biochemical methods to identify new proteins involved in damage tolerance. Our investigations will be aided by novel techniques that we have developed to assess DNA repair and mutagenesis in both cells and tissues and by a rich collection of DNA repair and replication mutants. The use of whole Drosophila in our experimental design provides us with an opportunity to study damage tolerance in the context of tissue specificity and development. Together, our proposed studies will advance our long-term goal to understand why different DNA damage tolerance mechanisms are preferentially used in different contexts and how this choice of bypass strategy impacts cellular survival and genome stability.