Project Summary The ribosome is the macromolecular machine, conserved throughout evolution, that is responsible for the synthesis of proteins and is therefore a fundamental component of gene expression. Because of its central role in biochemistry, the ribosome has become the target of more than half of all antibiotics, and the evolution of antibiotic resistance due to mutations in the ribosome has become a major threat to human health. Understanding the molecular basis of antibiotic resistance therefore has significant implications for the fight against the threat of resistance. In deciphering the mechanism of antibiotic resistance, fundamental insights into the mechanism of ribosome function can also be acquired, leading to possible novel antimicrobial agents through rational drug design. Through an ongoing collaborative effort, this proposal capitalizes on the ability of the PIs to combine genetics and structural biology to address fundamental questions of ribosome structure and function and provide a framework for future development of novel antimicrobial agents. The Aims of this proposal are: (1) Dissect 30S subunit conformational dynamics during tRNA movement through the ribosome; (2) Define the role of two intersubunit bridges in 30S-50S rotation during translocation; (3) Determine the structural basis for signaling pathways through the ribosome. Achieving these aims will involve exploiting the expertise of the three PIs to generate and characterize functionally impactful mutants and determine their structures by X-ray crystallography or cryo-electron microscopy. The strengths of our proposal include: (1) extensive expertise in the experimental methods to be applied; (2) a long track record of effective collaboration between the PIs; (3) an extensive publication track record in the field of ribosome structure and function; (4) a large volume of preliminary data and ongoing studies, including multiple studies nearing completion. Accomplishing these goals will significantly advance the field by answering long-standing questions about antibiotic resistance mechanisms, while simultaneously expanding our understanding of the basic underlying mechanism of the ribosome.