B-281. Branched-Chain Keto Acid Dehydrogenase Is Required for Optimal Bacillus anthracis Virulence

W. A. Day, Jr.1, S. L. Rasmussen1, T. E. Blank1, S. N. Peterson2, A. M. Friedlander1;
1USAMRIID, Frederick, MD, 2The Inst. for Genomic Res., Rockville, MD.

A previous screen for B. anthracis virulence factors identified a putative protein utilization pathway that converts exogenous and endogenous proteins to amino acids and energy. The pathway includes the branched-chain keto acid dehydrogenase (BKD) multi-enzyme complex, encoded by the bkd locus, which converts branched-chain amino acids (BCAA) to coenzyme A thioesters that, in turn, may be converted to fatty acids or energy. We confirmed that bkdB, which encodes the BKD E2 subunit, is required for optimal B. anthracis virulence in a guinea pig model. The bkdB strain grew normally in casamino acids (CA) medium in air but poorly in CA medium with bicarbonate. Transcriptome analyses indicated that genes for fatty acid synthesis were induced in the mutant during growth in CA bicarbonate. Comparison of wild type and bkdB fatty acid profiles indicated that in air, branched-chain fatty acid (BCFA) synthesis in the bkdB strain was altered and likely occurred through the BKD paralogous pyruvate dehydrogenase (PDH) complex; in bicarbonate, BCFA synthesis was more dramatically altered in the bkdB mutant, likely due to reduced PDH activity. These results suggest that BKD is essential for B. anthracis BCFA synthesis in environments with bicarbonate, including host tissues, and is active in environments in which PDH is less active. Strikingly, we also found that B. anthracis is a leucine/valine auxotroph and must acquire BCAA from the environment. Collectively, these studies suggest that a protein utilization pathway required for B. anthracis virulence supplies the cell with BCAA essential for normal lipid synthesis and growth in host tissues.