Synthetic Phenotypes Provide Insight into the Outer Membrane Biogenesis of Escherichia coli

Abstract

The outer membrane (OM) of Gram-negative bacteria such as <italic>Escherichia coli</italic> is essential for their resistance to a wide variety of toxins, small molecules, and other environmental insults. A number of the proteins involved in the biogenesis of the outer membrane are essential. However, the loss of others results in only mild defects in the assembly of the outer membrane. Combining mutations in these nonessential genes can exacerbate their phenotypes, lead to the discovery of genetic interactions, and provide insight into protein function.

In the experiments reported here, we have used synthetic phenotypes to better understand the relationship between the periplasmic chaperones that deliver outer membrane proteins (OMPs) to the OM. We have investigated the OM proteome in a strain lacking multiple periplasmic chaperones to determine the contributions of these chaperones to OMP assembly. The striking synthetic phenotypes associated with the loss of the Bam complex member <italic>bamE</italic> have led to a model describing the functions of two members of the Bam complex, BamD and BamE. The investigation of a temperature-sensitive strain lacking three periplasmic chaperones has led to the discovery of a novel function for the chaperone Skp in LptD assembly. Finally, the synthetic lethality of one pair of periplasmic OMP chaperones has led to the discovery of a novel allele of the extracytoplasmic stress sigma factor, RpoE. Clearly, the use of strains with mutations in multiple genes, and the exploration of their synthetic phenotypes, is an invaluable tool for studying the essential process of OM biogenesis.