spxA2, encoding a regulator of stress resistance in Bacillus anthracis, is controlled by SaiR, a new member of the Rrf2 protein family.

Spx, a member of the ArsC (arsenate reductase) protein family, is conserved in Gram-positive bacteria, and interacts with RNA polymerase to activate transcription in response to toxic oxidants. In Bacillus anthracis str. Sterne, resistance to oxidative stress requires the activity of two paralogues, SpxA1 and SpxA2.