Bacillus anthracis, a Gram-positive spore-forming soil bacterium and member of the Bacillus cereus group species, is distinguished by its ability to cause lethal anthrax disease in mammals, including humans. Well-established virulence factors unique to this organism are the anthrax toxin proteins and a poly-D-glutamic acid capsule. Findings of numerous investigators have established the structure and function of the anthrax toxins and capsule. Work in our laboratory has focused on the genetic basis for expression of the structural genes for the toxin proteins, pagA, lef, and cya, and more recently, the capsule biosynthesis operon, capBCAD. Our model for virulence gene regulation is of increasing complexity and includes numerous trans-acting regulators. The most critical and far-reaching regulator is atxA, a gene that appears to be unique to B. anthracis, atxA is essential for expression of all three toxin genes and contributes to control of the capsule operon. In experiments proposed here, we will continue our investigations of virulence gene expression by testing our current model for regulation of virulence. Our overall approach will be to determine the function of virulence gene regulators in B. anthracis cultured in vitro and to test for significance in a mouse inhalation model for anthrax. We will also assess the physiological roles of newly identified targets of established regulators. Finally, we will probe the molecular basis for differences in beta-lactamase gene expression between prototypical penicillin-susceptible and less common penicillin-resistant B. anthracis stains. Bacillus anthracis is the lead bacterium on the Select Agent List. The intentional release of spores in the fall of 2001 in the U.S. that resulted in eleven confirmed cases of anthrax and five deaths dramatically illustrated the public health threat this organism can pose when as a bioweapon. As the recent U.S. cases showed, inhalation of B. anthracis spores can result in a fatal clinical outcome in humans and only timely post-exposure intervention can limit the extent of the disease. Our overall objective is to identify and characterize B. anthracis determinants that impact B. anthracis infection in a mouse model for inhalation anthrax. Such determinants are potential targets for therapeutic intervention and/or possible components for new subunit vaccines.