Characterization of Six Lipoproteins in the σ E Regulon

Abstract

ABSTRACT In Escherichia coli , σ E regulon functions are required for envelope homeostasis during stress and are essential for viability under all growth conditions. The E. coli genome encodes approximately 100 lipoproteins, and 6 of these are regulated by σ E . Phenotypes associated with deletion of each of these lipoproteins are the subject of this report. One lipoprotein, YfiO, is essential for cellular viability. However, overexpression of this protein is not sufficient to alleviate the requirement of σ E for viability, suggesting that the σ E regulon provides more than one essential function. The remaining five lipoproteins in the σ E regulon are nonessential; cells are viable even when all five are removed simultaneously. Deletion of three nonessential lipoprotein genes ( nlpB , yraP , ygfL ) results in the exhibition of phenotypes that suggest they are important for maintenance of the integrity of the cell envelope. Δ nlpB cells are selectively sensitive to rifampin; Δ yraP cells are selectively sensitive to sodium dodecyl sulfate. Such selective sensitivity has not been previously reported. Both Δ yraP and Δ nlpB are synthetically lethal with surA :: Cm , which encodes a periplasmic chaperone and PPIase, suggesting that NlpB and YraP play roles in a periplasmic folding pathway that functions in parallel with that of SurA. Finally, the Δ yfgL mutant exhibits a broad range of envelope defects, including sensitivity to several membrane-impermeable agents, an altered outer membrane protein profile, synthetic lethality with both surA :: Cm and Δ fkpA :: Cm strains, and sensitivity to a bactericidal permeability-increasing peptide. We suggest that this lipoprotein performs a very important but as-yet-unknown function in maintaining the integrity of the cell envelope.