The Extracytoplasmic Stress Factor, σE, Is Required to Maintain Cell Envelope Integrity in Escherichia coli

Jennifer D Hayden,Sarah E Ades,Steve Sandler

doi:10.1371/journal.pone.0001573

Abstract

Extracytoplasmic function or ECF sigma factors are the most abundant class of alternative sigma factors in bacteria. Members of the rpoE subclass of ECF sigma factors are implicated in sensing stress in the cell envelope of Gram-negative bacteria and are required for virulence in many pathogens. The best-studied member of this family is rpoE from Escherichia coli, encoding the σE protein. σE has been well studied for its role in combating extracytoplasmic stress, and the members of its regulon have been largely defined. σE is required for viability of E. coli, yet none of the studies to date explain why σE is essential in seemingly unstressed cells. In this work we investigate the essential role of σE in E. coli by analyzing the phenotypes associated with loss of σE activity and isolating suppressors that allow cells to live in the absence of σE. We demonstrate that when σE is inhibited, cell envelope stress increases and envelope integrity is lost. Many cells lyse and some develop blebs containing cytoplasmic material along their sides. To better understand the connection between transcription by σE and cell envelope integrity, we identified two multicopy suppressors of the essentiality of σE, ptsN and yhbW. yhbW is a gene of unknown function, while ptsN is a member of the σE regulon. Overexpression of ptsN lowers the basal level of multiple envelope stress responses, but not that of a cytoplasmic stress response. Our results are consistent with a model in which overexpression of ptsN reduces stress in the cell envelope, thereby promoting survival in the absence of σE.

Highlights

All organisms have stress responses that allow them to sense and respond to damaging conditions by altering gene expression
PtsN and yhbW, were isolated that suppress both the lethality due to inhibition of sE activity and the essentiality of the rpoE gene. ptsN is a member of the sE regulon, while yhbW is not, and neither gene has previously been associated with cell envelope functions
Inhibition of sE activity is toxic and increases envelope stress To gain a better understanding of the role of a particular gene in cellular physiology, it is often informative to examine the phenotypes of a deletion mutant lacking the gene of interest

Summary

Introduction

All organisms have stress responses that allow them to sense and respond to damaging conditions by altering gene expression. The cell envelope is a complex, dynamic compartment that is crucial for the survival of the cell It is not a static structure and can be remodeled in response to environmental conditions. Gram-negative bacteria possess stress responses that are uniquely targeted to the cell envelope. These stress responses include the CpxAR (Cpx), BaeRS (Bae), and Rcs phosphorelays, the response governed by the alternative sigma factor sE, and the phage shock (PSP) response [3–6]. Each of these responses is activated following perturbation of particular components of the envelope

Methods

Results

Conclusion