PROJECT ABSTRACT: The mechanism and regulation of protein synthesis determines the diversity and capacity of the cellular proteome. At the center of this regulation is the ribosome - a megadalton RNA-protein complex composed of two-subunits – which integrates a wide variety of cellular signals. The ribosome’s exquisite sensitivity to regulatory cues is underscored by the fact that the majority of clinically used antibiotics exert their effect by either dysregulating or blocking specific aspects of the protein synthesis mechanism. Understanding the kinetic and structural basis of protein synthesis promises to elucidate core paradigms of gene expression control and to inform strategies for addressing the global threat posed by drug-resistant and emerging pathogens. Moreover, given that loss of translational control is a hallmark of cancer, a mechanistic understanding of ribosome function holds significant promise for developing novel small-molecule therapies, which are currently lacking in the treatment of human disease. Historically, investigations into structure-function relationships governing the protein synthesis mechanism have focused on bacterial systems approaches. Comparable studies in human systems have been hindered by the demand for large amounts of homogeneous protein synthesis machinery. As a result, the molecular distinctions between bacterial and mammalian protein synthesis - which underpin antibiotic specificity and potential therapeutic windows – remain obscure. Current evidence suggests that the elongation phase of protein synthesis, during which messenger RNA (mRNA) is decoded into protein, is the most time consuming, physiologically regulated and small-molecule sensitive. We and others hypothesize that elongation is particularly susceptible to regulation because it involves transient, repetitive interactions of the ribosome with auxiliary factors, coordinated through finely tuned conformational transitions that are acutely sensitive to perturbatio