PROJECT SUMMARY Our long-term goal is to understand post-transcriptional mechanisms that spatially and temporally control gene activity during animal development. We focus on intracellular mRNA localization, translational control, and degradation, which play crucial roles in regulating production of proteins from maternally supplied transcripts. These transcripts, which underpin the initial developmental program of nearly all animals, are pre-loaded in the egg. Thus, control over where and when they are deployed to produce protein must be exerted post- transcriptionally. Our research capitalizes on the highly manipulable Drosophila model, which relies heavily on maternal transcripts, to investigate mechanisms that generate asymmetric maternal mRNA and protein distributions needed for embryonic body patterning and germline development. Our previous work revealed that numerous mRNAs accumulate at the posterior of the Drosophila oocyte through their incorporation into large, non-membrane-bound ribonucleoprotein (RNP) assemblies called germ granules. During embryogenesis, germ granules deliver this cohort of mRNAs to the primordial germ cells, where they support germline development. Germ granules are a hallmark of primordial germ cells throughout the animal kingdom. They contain conserved components that indicate a common function in RNA regulation to promote the differentiation, proliferation, and maintenance of the germline. Germ granules are just one of a plethora of cellular RNP assemblies that have been characterized for their phase-transitioned behavior. How mRNA regulation occurs in the context of RNA granules more generally and its biological significance remain, however, poorly understood. As complex and biologically relevant RNP assemblies, germ granules are ideal for studying RNA localization, translational control, and degradation and, in particular, their biological significance. Our studies lead to a model for germ granule "client" RNA accumulation by self-recruitment and organization into homotypic clusters. This model provides a framework to investigate how mRNAs self-organize in granules and how this organization imparts selective control over the translation and stability of different mRNAs within shared granules. We will combine quantitative high resolution imaging with powerful biochemical and genetic/genomic strategies, as well as novel methods that disrupt granules, to investigate fundamental principles governing the organization of granule client RNAs. These studies will enable us to decipher the relationship between granule association, translational activity, and germ granule function. Given the widespread use of post-transcriptional mechanisms during development, and the prevalence of RNA compartmentalization in granules, this work will have broad impact. Mechanistic insights gained from our studies will also provide a foundation for understanding defects in RNA metabolism associated with numerous disorders including infert...