The Superoxide Dismutases of Bacillus anthracis Do Not Cooperatively Protect against Endogenous Superoxide Stress

Abstract

The Bacillus anthracis chromosome encodes four unique, putative superoxide dismutase ( sod ) genes. During exponential growth and sporulation, sodA1 , sodA2 , and sodC are transcribed constitutively throughout the growth cycle as individual genes. In contrast, the transcription of sod15 occurs mainly during late exponential and sporulation phases as part of a four-gene operon that may be involved in spore formation. Vegetative cell and spore lysates of wild-type Sterne and superoxide dismutase deletion ( Δsod ) mutants show detectable SOD activity for SODA1 and SODA2, and protein analysis suggests that these two proteins form active homodimers and heterodimers. A comparison of the growth of parental versus Δ sod mutants under various chemical oxidative stresses indicates that Δ sodA1 mutants are particularly sensitive to endogenously produced superoxide, whereas Δ sodA2 , Δ sod15 , and Δ sodC mutants remain as resistant to this stress as the parental strain. In addition, in mouse survival assays, Δ sod15 and Δ sodA1 were responsible for less end-point death, but the level of decreased virulence does not fall within a statistically significant range. Collectively, these data show that sodA1 acts as a major protectant from intracellular superoxide stress, that sod15 is transcribed as part of an operon that may play a role in cell morphology, and that sodA2 and sodC may have minor roles that are not apparent in the conditions tested here.