S3-4 Cell death caused by a defect in lipoprotein maturation in E. coli

Escherichia coli entericidin locus is a chromosomally encoded toxin/antitoxin module and implicated in programmed death of a subpopulation of stationary phase cells under hiypertonic conditions (Bishop et al., 1998, J. Mol. Biol. 280, 583-596). Its products EcnA and B are both predicted to be outer membrane lipoproteins. Maturation of lipoproteins begins with modification of the prospective N-terminal Cys residue with diacylglycerol. This modification is a prerequisite for the following processing and N-terminal acylation and thus for translocation to the outer membrane. A principal diacylglycerol donor in the modification reaction is phosphatidylglycerol, whose committed biosynthetic step is catalyzed by phosphatidylglycerophosphate synthase encoded by the pgsA gene. Overproduction of the toxin protein EcnB in a pgsA null mutant which completely lacks phosphatidylglycerol led to cell lysis during exponential growth phase, whereas its overproduction in the wild type caused lysis only after the entry into stationary phase. We conclude that it is the unmodified precursor form of EcnB accumulated in the cytoplasmic membrane that causes cell death.