The CpxR/CpxA two-component system up-regulates two tat-dependent peptidoglycan amidases to confer bacterial resistance to antimicrobial peptide

We demonstrate that the twin arginine translocation (Tat) system contributes to bacterial resistance to cationic antimicrobial peptides (CAMPs). Our results show that a deletion at the tatC gene, which encodes a subunit of the Tat complex, caused Salmonella and Escherichia coli to become susceptible to protamine. We screened chromosomal loci that encode known and predicted Tat-dependent proteins and found that two N-acetylmuramoyl-L-alanine amidases, encoded by amiA and amiC, elevated bacterial resistance to protamine and α-helical peptides magainin 2 and melittin but not to β-sheet defensin HNP-1 and lipopeptide polymyxin B. Genetic analysis suggests that transcription of both amiA and amiC loci in Salmonella is up-regulated by the CpxR/ CpxA two-component system when nlpE is overexpressed. A footprinting analysis reveals that CpxR protein can interact with amiA and amiC promoters at the CpxR box, which is localized between the predicted -10 and -35 regions but present on different strands in these two genes. In addition, our results show that activation of the CpxR/CpxA system can facilitate protamine resistance because nlpE overexpression elevates this resistance in the wild-type strain but not the cpxR deletion mutant. Thus, we uncover a new transcriptional regulation pathway in which the Cpx envelope stress response system modulates the integrity of the cell envelope in part by controlling peptidoglycan amidase activity, which confers bacterial resistance to protamine and α-helical CAMPs. Our studies have important implications for understanding transcriptional regulation of peptidoglycan metabolism and also provide new insights into the role of the bacterial envelope in CAMP resistance.