The conserved σDstress response monitors multiple aspects of envelope integrity in corynebacteria

Abstract

ABSTRACTThe cell envelope fortifies bacterial cells against antibiotics and other insults. Species in theMycobacterialesorder have a complex envelope that includes an outer layer of mycolic acids called the mycomembrane (MM) and a cell wall composed of peptidoglycan and arabinogalactan. This envelope architecture is unique among bacteria and contributes significantly to the virulence of pathogenicMycobacterialeslikeMycobacterium tuberculosis. Characterization of pathways that govern envelope biogenesis in these organisms is therefore critical in understanding their biology and for identifying new antibiotic targets. To better understand MM biogenesis, we developed a cell sorting-based screen for mutants defective in the surface exposure of a porin normally embedded in the MM of the model organismCorynebacterium glutamicum.The results revealed a requirement for the conserved σDenvelope stress response in porin export and identified MarPas the site-1 protease, respectively, that activate the response by cleaving the membrane-embedded anti-sigma factor. A reporter system revealed that the σDpathway responds to defects in mycolic acid and arabinogalactan biosynthesis, suggesting that the stress response has the unusual property of being induced by activating signals that arise from defects in the assembly of two distinct envelope layers. Our results thus provide new insights into howC. glutamicumand related bacteria monitor envelope integrity and suggest a potential role for members of the σDregulon in protein export to the MM.AUTHOR SUMMARYBacteria within theMycobacterialesorder, which includes the pathogenMycobacterium tuberculosis,have a unique multilayered cell surface architecture. How they sense and respond to defects in the construction of this distinct envelope to maintain homeostasis remains poorly understood. Here, we used the model organismCorynebacterium glutamicumto reveal that the conserved σDenvelope stress response pathway responds to two distinct signals resulting from defects in different envelope layers. The results provide new insight into how members of theMycobacterialesmonitor the integrity of their surface and pave the way for the identification of novel vulnerabilities in envelope biogenesis that may be useful for targeting with antibiotics.