The Extracytoplasmic Stress Factor, σ(E), Is Required to Maintain Cell Envelope Integrity in Escherichia coli

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 σ(E) protein. σ(E) has been well studied for its role in combating extracytoplasmic stress, and the members of its regulon have been largely defined. σ(E) is required for viability of E. coli, yet none of the studies to date explain why σ(E) is essential in seemingly unstressed cells. In this work we investigate the essential role of σ(E) in E. coli by analyzing the phenotypes associated with loss of σ(E) activity and isolating suppressors that allow cells to live in the absence of σ(E). We demonstrate that when σ(E) 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 σ(E) and cell envelope integrity, we identified two multicopy suppressors of the essentiality of σ(E), ptsN and yhbW. yhbW is a gene of unknown function, while ptsN is a member of the σ(E) 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 σ(E).