Autoregulation and Virulence Control by the Toxin-Antitoxin System SavRS in Staphylococcus aureus

Toxin-antitoxin (TA) systems play diverse physiological roles, such as plasmid maintenance, growth control, and persister cell formation, but their involvement in bacterial pathogenicity remains largely unknown. Here, we have identified a novel type II toxin-antitoxin system, SavRS, and revealed the molecular mechanisms of its autoregulation and virulence control in Staphylococcus aureus. Electrophoretic mobility shift assay and isothermal titration calorimetry data indicated that the antitoxin SavR acted as the primary repressor bound to its own promoter, while the toxin SavS formed a complex with SavR to enhance the ability to bind to the operator site. DNase I footprinting assay identified the SavRS-binding site containing a short and long palindrome in the promoter region. Further, mutation and DNase I footprinting assay demonstrated that the two palindromes were crucial for DNA binding and transcriptional repression. More interestingly, genetic deletion of the savRS system led to the increased hemolytic activity and pathogenicity in a mouse subcutaneous abscess model. We further identified two virulence genes, hla and efb, by real-time quantitative reverse transcription-PCR and demonstrated that SavR and SavRS could directly bind to their promoter regions to repress virulence gene expression.