Activation of the Extracytoplasmic Function σ Factor σP by β-Lactams in Bacillus thuringiensis Requires the Site-2 Protease RasP

Bacteria can utilize alternative sigma factors to regulate sets of genes in response to changes in the environment. The largest and most diverse group of alternative sigma factors are the extracytoplasmic function(ECF) u factors. sigma(P) is an ECF sigma factor found in Bacillus anthrocis, Bacillus cereus, and Bacillus thuringiensis. Previous work showed that sigma(P) is induced by ampicillin, a beta-lactam antibiotic, and required for resistance to ampicillin. However, it was not known how activation of e is controlled or what other antibiotics may activate sigma(P). Here, we report that activation of sigma(P) is specific to a subset of beta-lactams and that sigma(P) is required for resistance to these beta-lactams. We demonstrate that activation of sigma(P) is controlled by the proteolytic destruction of the anti-sigma factor RsiP and that degradation of sigma(P) requires multiple pro teases. Upon exposure to beta-lactams, the extracellular domain of RsiP is cleaved by an unknown protease, which we predict cleaves at site-1. Following cleavage by the unknown protease, the N terminus of RsiP is further degraded by the site-2 intramembrane protease RasP. Our data indicate that RasP cleavage of RsiP is not the rate-limiting step in sigma(P) activation. This proteolytic cascade leads to activation of sigma(P), which induces resistance to beta-lactams likely via increased expression of beta-lactamases. IMPORTANCE The discovery of antibiotics to treat bacterial infections has had a dramatic and positive impact on human health. However, shortly after the introduction of a new antibiotic, bacteria often develop resistance. The bacterial cell envelope is essential for cell viability and is the target of many of the most commonly used antibiotics, including beta-lactam antibiotics. Resistance to beta-lactams is often dependent upon beta-lactamases. In B. cereus, B. thuringiensis, and some B. anthracis strains, the expression of some beta-lactamases is inducible. This inducible beta-lactamase expression is controlled by activation of an alternative sigma factor called sigma(P). Here, we show that 0-lactam antibiotics induce sigma(P) activation by degradation of the anti-sigma factor RsiP.