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

Bacteria can utilize alternative σ factors to regulate sets of genes in response to changes in the environment. The largest and most diverse group of alternative σ factors are the extracytoplasmic function (ECF) σ factors. σ P is an ECF σ factor found in Bacillus anthracis , Bacillus cereus , and Bacillus thuringiensis Previous work showed that σ P is induced by ampicillin, a β-lactam antibiotic, and required for resistance to ampicillin. However, it was not known how activation of σ P is controlled or what other antibiotics may activate σ P Here, we report that activation of σ P is specific to a subset of β-lactams and that σ P is required for resistance to these β-lactams. We demonstrate that activation of σ P is controlled by the proteolytic destruction of the anti-σ factor RsiP and that degradation of RsiP requires multiple proteases. Upon exposure to β-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 σ P activation. This proteolytic cascade leads to activation of σ P , which induces resistance to β-lactams likely via increased expression of β-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 β-lactam antibiotics. Resistance to β-lactams is often dependent upon β-lactamases. In B. cereus , B. thuringiensis , and some B. anthracis strains, the expression of some β-lactamases is inducible. This inducible β-lactamase expression is controlled by activation of an alternative σ factor called σ P Here, we show that β-lactam antibiotics induce σ P activation by degradation of the anti-σ factor RsiP.