Project Summary Branched amino acids (valine, leucine, and isoleucine) are essential for life. Some organisms, like humans, acquire these amino acids from their diet and thus lack the ability to make them. Others, like plants, harbor the full biosynthetic machinery for their production and do so when nutrients are scarce. Some pathogenic bacteria seemingly do both; they have production capacity but also possess transporters that mediate their uptake from the environment. However, it is unclear which of these two arms, acquisition versus synthesis, is most important during infection of their vertebrate hosts. In this project, we employ the use of B. anthracis, the causative agent of anthrax disease whose prolific replication in blood and tissues makes it ideal for studying nutrient uptake, to determine the importance of branched amino acid metabolism to the multi-stage infectious process of this pathogen. Working under the premise that bacilli needs to liberate branched amino acids from the breakdown of blood proteins to sustain high levels of growth, we hypothesize that the transport of freed branched amino acids in serum is necessary for anthrax disease. In Aim 1, we use isogenic mutant strains deficient in each arm (transport versus synthesis) of branched amino acid metabolism to determine their overall contribution to pulmonary anthrax, the most lethal type. In Aim 2, we explore the exact role of these two arms to each stage in the infectious cycle of bacilli, including outgrowth inside infected macrophages and rapid expansion of vegetative bacilli in blood and blood-like environments. This work takes the first step towards knowing if branched amino acid production and/or transport represents a viable entry point for new antimicrobial strategies.