RelA is a component of the nutritional stress activation pathway of the Bacillus subtilis transcription factor σB

The general stress regulon of Bacillus subtilis is induced by the activation of the sigma(B) transcription factor. Activation of sigma(B) occurs when one of two phosphatases (RsbU and RsbP), each responding to a unique type of stress, actuates a positive regulator of sigma(B) by dephosphorylation. Nutritional stress triggers the RsbP phosphatase. The mechanism by which RsbP becomes active is unknown; however, its activation coincides with culture conditions that are likely to reduce the cell's levels of high-energy nucleotides. We now present evidence that RelA, a (p)ppGpp synthetase and the key enzyme of the stringent response, plays a role in nutritional stress activation of sigma(B). An insertion mutation that disrupts relA blocks the activation of sigma(B) in response to PO4 or glucose limitation and inhibits the drop in ATP/GTP levels that normally accompanies sigma(B) induction under these conditions. In contrast, the activation of sigma(B) by physical stress (e.g., ethanol treatment) is not affected by the loss of RelA. RelA's role in sigma(B) activation appears to be distinct from its participation in the stringent response. Amino acid analogs which induce the stringent response and RelA-dependent (p)ppGpp synthesis do not trigger sigma(B) activity. In addition, neither a missense mutation in relA (relA240GE) nor a null mutation in rplK (rplK54), either of which is sufficient to inhibit the stringent response and RelA-dependent (p)ppGpp synthesis, fails to block sigma(B) activation by PO4 or glucose limitation.