Signal Peptidase-Mediated Cleavage of the Anti-sigma-Factor RsiP at Site 1 Controls sigma(P) Activation and beta-Lactam Resistance in Bacillus thuringiensis

In Bacillus thuringiensis, beta-lactam antibiotic resistance is controlled by the extracytoplasmic function (ECF) sigma factor sigma(P) . sigma(P) activity is inhibited by the anti-sigma factor RsiP. In the presence of beta-lactam antibiotics, RsiP is degraded and sigma(P) is activated. Previous work found that RsiP degradation requires cleavage of RsiP at site 1 by an unknown protease, followed by cleavage at site 2 by the site 2 protease RasP. The penicillin-binding protein PbpP acts as a sensor for beta-lactams. PbpP initiates P activation and is required for site 1 cleavage of RsiP but is not the site 1 protease. Here, we describe the identification of a signal peptidase, SipP, which cleaves RsiP at a site 1 signal peptidase cleavage site and is required for sigma(P) activation. Finally, many B. anthracis strains are sensitive to beta-lactams yet encode the sigma(P)-RsiP signal transduction system. We identified a naturally occurring mutation in the signal peptidase cleavage site of B. anthracis RsiP that renders it resistant to SipP cleavage. We find that B. anthracis RsiP is not degraded in the presence of beta-lactams. Altering the B. anthracis RsiP site 1 cleavage site by a single residue to resemble B. thuringiensis RsiP results in beta-lactam-dependent degradation of RsiP. We show that mutation of the B. thuringiensis RsiP cleavage site to resemble the sequence of B. anthracis RsiP blocks degradation by SipP. The change in the cleavage site likely explains many reasons why B. anthracis strains are sensitive to beta-lactams. IMPORTANCE beta-Lactam antibiotics are important for the treatment of many bacterial infections. However, resistance mechanisms have become increasingly more prevalent. Understanding how beta-lactam resistance is conferred and how bacteria control expression of beta-lactam resistance is important for informing the future treatment of bacterial infections. sigma(P) is an alternative s factor that controls the transcription of genes that confer beta-lactam resistance in Bacillus thuringiensis, Bacillus cereus, and Bacillus anthracis. Here, we identify a signal peptidase as the protease required for initiating activation of sigma(P) by the degradation of the anti-sigma factor RsiP. The discovery that the signal peptidase SipP is required for sigma(P) activation highlights an increasing role for signal peptidases in signal transduction, as well as in antibiotic resistance.

Automated summary

This study identified the signaling pathway regulating beta-lactam antibiotic resistance in Bacillus thuringiensis, providing important insights for future treatment of bacterial infections. The discovery of the signal peptidase SipP required for sigma(P) activation highlights the increasing role of signal peptidases in signal transduction and antibiotic resistance.