Functional cooperation of three non-essential proteins is required for Escherichia coli survival under cell envelope stress conditions

Gram-negative bacteria have a unique cell envelope consisting of a lipopolysaccharide-containing outer membrane (OM) that is covalently linked to the thin layer of peptidoglycan (PG). The OM serves as a barrier against toxic molecules including many antibiotics and allows the cells to survive in many environmental stress conditions; more recently it has been shown to contribute to the mechanical integrity of the bacterial cell, a function traditionally assigned to the PG. The growth of OM and PG layers needs to be tightly coordinated. Our laboratory recently found that when the OM biogenesis is compromised a PG remodeling program is required to avoid cell lysis. In Escherichia coli cells this modification program relies on the activity of LD-transpeptidase family proteins that introduce the non-canonical 3-3 cross-links in the PG layer to restore the mechanical strength and the overall stability of the bacterial cell envelope. Among the member of this family, DpaA (previously LdtF) is the enzyme that detaches Lpp from PG. Notably, Lpp is the abundant E. coli OM lipoprotein that covalently links the OM to PG. Previous works of our laboratory have shown that a mutant deleted for dpaA undergoes lysis when LPS transport to the OM is blocked. However, the lysis phenotype of lipopolysaccharide defective dpaA-deleted mutant is suppressed by the deletion of actS which codes for an activator of amidases, the enzymes that hydrolyze septal PG during cell separation. Our goal is to understand the interplay between dpaA and actS to better define the physiological role of dpaA under envelope stress conditions and the biological meaning of Lpp attachment from PG.