PROJECT SUMMARY MicroRNAs are ~22-nt-long RNAs that bind Argonaute proteins to function as guides to direct gene silencing of mRNAs containing partial sequence complementarity. MicroRNAs have critical roles throughout animal development. Disruption of microRNA activity can lead to a variety of diseases including cancer. Recently, microRNAs have emerged as having probable roles in drug resistance and in preventing bacterial and viral infections, yet their specific functions in these processes are not well understood. The goal of this proposal is to identify the genes and genetic networks under the control of microRNAs that affect development, drug resistance, and pathogen defense. We will address three specific challenges that have limited our understanding of microRNAs and their roles in disease and human health: (1) identify the roles of microRNAs in drug resistance and pathogen defense; (2) identify the genes and genetic networks under the control of microRNAs that affect development, drug resistance, and pathogen defense; and (3) identify how separate branches of the microRNA pathway become specialized for distinct mechanistic roles in drug resistance and pathogen defense. We will use a systems-level approach to unravel the genetic networks that affect the surveillance, transport, metabolism, and detoxification of foreign chemicals such as toxins produced by pathogens and drugs used for the treatment of cancer and infectious diseases. Drug resistance and pathogen defense are complex and dynamic processes involving cell-to-cell signaling making them difficult to study using human and mammalian cell cultures. We will establish the genetically tractable species Caenorhabditis elegans as a whole-animal model for understanding these processes, thereby paving the way for high- throughput chemical screens to identify compounds that can modulate drug efficacy and prevent infections. Understanding how microRNAs intersect genetic networks that affect drug resistance and pathogen defense will improve our fundamental understanding of diseases, infections, and acquired drug resistance. Knowledge of how microRNAs and their Argonaute protein partners are specialized for distinct functions will have important implications in the study of gene regulation and how its dysregulation contributes to health and disease.