PROJECT SUMMARY/ABSTRACT Legionella pneumophila and Coxiella burnetii are intracellular bacterial pathogens capable of causing human disease. A notable feature of these two pathogens is that they retain a common virulence determinant that is essential for their ability to replicate intracellularly, which is the specialized type IV secretion system (T4SS) called Dot/Icm. The Dot/Icm is an incredibly versatile secretion apparatus that has the capacity to translocate into host cells a repertoire of over 300 different proteins with different biochemical functions and diverse structural properties. The goal of this project is to determine the structure and assembly of the Dot/Icm machine and elucidate how the individual Dot and Icm proteins contribute to machine function at the molecular level. We will combine advanced cryo-electron tomography (cryo-ET) with genetic and biochemical approaches to determine the pathway of Dot/Icm machine assembly (Aim 1) and to determine the mechanism by which cytosolic ATPases recruit effectors and mediate changes in the Dot/Icm structure (Aim 2). Furthermore, we aim to characterize the translocation pore in the host cell membrane that serves as the protein-conducting channel for Dot/Icm effectors (Aim 3).