PROJECT SUMMARY / ABSTRACT: The cell envelope of Mycobacterium tuberculosis (Mtb) serves an important role as protection against common antimicrobials and host cell defenses, but also a barrier through which Mtb must transport host derived nutrients. The MCE protein family (originally implicated in Mammalian Cell Entry) is nearly ubiquitous in double-membraned bacteria, and MCE proteins are now believed to act as transporters that facilitate lipid movement between the inner and outer membranes. While Mtb MCE proteins are critical for virulence, their mechanism in Mtb remains unclear. Due to Mtb’s distinct cell envelope Mtb MCE proteins likely adopt a unique architecture to mediate transport. In addition, throughout the course of Mtb infection MCE transporters are believed to be differentially expressed by MCE transcriptional regulators (MceRs). This suggests that environmental sensing in Mycobacteria in turn alters transcription of the mce operons. Using structural biology, biochemistry, and genetic complementation assays, I will provide the first glimpse into the architecture of Mtb Mce protein systems and elucidate how MceRs interact with their operator sequences at the molecular level to regulate mce operons. To do so I propose the following aims: (Aim 1) structure determination of the Mce4 lipid transport system from Mycobacterium smegmatis and (Aim 2) structural characterization and DNA-binding specificity of MceRs. My work will provide the first glimpse at the architecture of the M. smegmatis Mce4 transporter system, as well as the molecular mechanisms of MceR mediated mce operon regulation, which may shed light on the mechanisms of cell envelope maintenance in Mtb and provide a novel target for therapeutics.