Project Summary Protein synthesis, the translation of an mRNA into a polypeptide, is a defining feature of all cellular life, and is subject to complex regulation that integrates multiple aspects of the metabolic status of the cell, primarily nutrient and energy signaling. Recent publications from this laboratory have also led to an understanding how external signals interact with the cell's endogenous timekeeper, the circadian clock, to regulate protein synthesis. Central to the regulation of protein synthesis is the TOR kinase signaling axis, which coordinates protein synthesis and cell proliferation. Because the TOR pathway has been implicated in the pathology of diverse diseases, from cancer to metabolic and neurological disorders, it continues to be a prime target for the development of new drugs. Therefore, it is essential to develop a thorough understanding of the spectrum of biochemical outputs of this pathway. Once activated, a substrate and target of TOR signaling, S6 kinase, phosphorylates the RPS6 protein (eS6), a component of the small subunit of the ribosome. Surprisingly, the biochemical role of RPS6 phosphorylation remains unknown nearly fifty years after its discovery, although research from yeast to mice has demonstrated an evolutionary fitness benefit for this pan-eukaryotic event. This project presents an innovative and comprehensive strategy to identify the biochemical consequence of RPS6 phosphorylation, by combining genetic, genomic, and biochemical experimentation in a new, tractable model organism, Arabidopsis thaliana, which recapitulates the central features of the human pathway. Preliminary research in this lab and others has established that Arabidopsis RPS6 integrates metabolic signals similar to metazoans, including humans, in the context of circadian timing signals issued by the circadian clock. This project will test a series of related hypotheses regarding the role of RPS6 phosphorylation in cellular biochemistry. It will test whether RPS6 phosphorylation regulates the translation of specific mRNAs or whether it affects the entire transcriptome globally. It will examine whether RPS6 phosphorylation affects ribosome biogenesis or ribosome turnover, or whether RPS6 phosphorylation serves as a cellular gauge of metabolic signals that is perceived by specific ?reader? proteins from beyond the translation apparatus.