The Global Transcriptional Responses of Bacillus anthracis Sterne (34F ₂ ) and a Δ sodA1 Mutant to Paraquat Reveal Metal Ion Homeostasis Imbalances during Endogenous Superoxide Stress

Microarray analyses were conducted to evaluate the paraquat-induced global transcriptional response of Bacillus anthracis Sterne (34F 2 ) to varying levels of endogenous superoxide stress. Data revealed that the transcription of genes putatively involved in metal/ion transport, bacillibactin siderophore biosynthesis, the glyoxalase pathway, and oxidoreductase activity was perturbed most significantly. A B. anthracis mutant lacking the superoxide dismutase gene sodA1 (Δ sodA1 ) had transcriptional responses to paraquat similar to, but notably larger than, those of the isogenic parental strain. A small, unique set of genes was found to be differentially expressed in the Δ sodA1 mutant relative to the parental strain during growth in rich broth independently of induced oxidative stress. The bacillibactin siderophore biosynthetic genes were notably overexpressed in Sterne and Δ sodA1 cells after treatment with paraquat. The bacillibactin siderophore itself was isolated from the supernatants and lysates of cells grown in iron-depleted medium and was detected at lower levels after treatment with paraquat. This suggests that, while transcriptional regulation of these genes is sensitive to changes in the redox environment, additional levels of posttranscriptional control may exist for bacillibactin biosynthesis, or the enzymatic siderophore pipeline may be compromised by intracellular superoxide stress or damage. The Δ sodA1 mutant showed slower growth in a chelated iron-limiting medium but not in a metal-depleted medium, suggesting a connection between the intracellular redox state and iron/metal ion acquisition in B. anthracis . A double mutant lacking both the sodA1 and sodA2 genes (Δ sodA1 Δ sodA2 ) was attenuated for growth in manganese-depleted medium, suggesting a slight level of redundancy between sodA1 and sodA2 , and a role for the sod genes in manganese homeostasis.