Roles of the Two-MnSOD System of Stenotrophomonas maltophilia in the Alleviation of Superoxide Stress

Abstract

Manganese-dependent superoxide dismutase (MnSOD, SodA) and iron-dependent SOD (FeSOD, SodB) are critical cytosolic enzymes for alleviating superoxide stress. Distinct from the singular <i>sodA</i> gene in most bacteria, <i>Stenotrophomonas maltophilia</i> harbors two <i>sodA</i> genes, <i>sodA1</i> and <i>sodA2</i>. The roles of SodA1, SodA2, and SodB of <i>S. maltophilia</i> in alleviating superoxide stress were investigated. The expression of <i>sod</i> genes was determined by promoter-<i>xylE</i> transcriptional fusion assay and qRT-PCR. <i>SodA2</i> and <i>sodB</i> expressions were proportional to the bacterial logarithmic growth, but unaffected by menadione (MD), iron, or manganese challenges. SodA1 was intrinsically unexpressed and inducibly expressed by MD. Complementary expression of <i>sodA1</i> was observed when <i>sodA2</i> was inactivated. The individual or combined <i>sod</i> deletion mutants were constructed using the gene replacement strategy. The functions of SODs were assessed by evaluating cell viabilities of different <i>sod</i> mutants in MD, low iron-stressed, and/or low manganese-stressed conditions. Inactivation of SodA1 or SodA2 alone did not affect bacterial viability; however, simultaneously inactivating <i>sodA1</i> and <i>sodA2</i> significantly compromised bacterial viability in both aerobic growth and stressed conditions. SodA1 can either rescue or support SodA2 when SodA2 is defective or insufficiently potent. The presence of two MnSODs gives <i>S. maltophilia</i> an advantage against superoxide stress.