SpxA1 and SpxA2 Act Coordinately To Fine-Tune Stress Responses and Virulence in .

SpxA is a unique transcriptional regulator highly conserved among members of the phylum that binds RNA polymerase and can act as an antiactivator. Why some members have two highly similar SpxA paralogs is not understood. Here, we show that the SpxA paralogs of the pathogen , SpxA1 and SpxA2, act coordinately to regulate virulence by fine-tuning toxin expression and stress resistance. Construction and analysis of mutants revealed that SpxA1 mutants were defective for growth under aerobic conditions, while SpxA2 mutants had severely attenuated responses to multiple stresses, including thermal and oxidative stresses. SpxA1 mutants had enhanced resistance to the cationic antimicrobial molecule polymyxin B, while SpxA2 mutants were more sensitive. In a murine model of soft tissue infection, a SpxA1 mutant was highly attenuated. In contrast, the highly stress-sensitive SpxA2 mutant was hypervirulent, exhibiting more extensive tissue damage and a greater bacterial burden than the wild-type strain. SpxA1 attenuation was associated with reduced expression of several toxins, including the SpeB cysteine protease. In contrast, SpxA2 hypervirulence correlated with toxin overexpression and could be suppressed to wild-type levels by deletion of These data show that SpxA1 and SpxA2 have opposing roles in virulence and stress resistance, suggesting that they act coordinately to fine-tune toxin expression in response to stress. SpxA2 hypervirulence also shows that stress resistance is not always essential for pathogenesis in soft tissue. For many pathogens, it is generally assumed that stress resistance is essential for pathogenesis. For , environmental stress is also used as a signal to alter toxin expression. The amount of stress likely informs the bacterium of the strength of the host's defense response, allowing it to adjust its toxin expression to produce the ideal amount of tissue damage, balancing between too little damage, which will result in its elimination, and too much damage, which will debilitate the host. Here we identify components of a genetic circuit involved in stress resistance and toxin expression that has a fine-tuning function in tissue damage. The circuit consists of two versions of the protein SpxA that regulate transcription and are highly similar but have opposing effects on the severity of soft tissue damage. These results will help us understand how virulence is fine-tuned in other pathogens that have two SpxA proteins.