Dual Molecular Signals Mediate the Bacterial Response to Outer-Membrane Stress

Abstract

In Gram-negative bacteria, outer-membrane integrity is essential for survival and is monitored by the σE stress-response system, which initiates damage-repair pathways. One activating signal is unassembled outer-membrane proteins. Using biochemical and genetic experiments in Escherichia coli, we found that off-pathway intermediates in lipopolysaccharide transport and assembly provided an additional required signal. These distinct signals, arising from disruptions in the transport and assembly of the major outer-membrane components, jointly determined the rate of proteolytic destruction of a negative regulator of the σE transcription factor, thereby modulating expression of stress-response genes. This dual-signal system permits a rapid response to dysfunction in outer-membrane biogenesis, while buffering responses to transient fluctuations in individual components, and may represent a broad strategy for bacteria to monitor their interface with the environment.

Multiple Transcriptional Factors Regulate Transcription of the rpoE Gene in Escherichia coli under Different Growth Conditions and When the Lipopolysaccharide Biosynthesis Is Defective

“…The molecular basis of RpoE induction due to specific LPS defects and participation of any specific signal relay system, like Rcs or other two-component systems, has not been addressed. Two models have been proposed, suggesting sensing of acylation defects in lipid A and a trade-off between the lipopolysaccharide transporter LptA and RpoE negative regulator RseB (38,57). However, as shown here, the Rcs system can independently sense LPS defects.…”

Regulation of bacterial virulence gene expression by cell envelope stress responses

“…Indeed, mutational disruption of LPS biosynthesis also generates the OMP inducing signal for DegS. 6 Furthermore, the OMP LptD is part of the machinery that assembles LPS in the OM, and LptD requires the same assembly machinery as other OMPs. 42 Thus, defects in OMP biogenesis will disrupt LptD function, and consequently LPS transport and assembly.…”

“…1). 6 Both of these RpoE-inducing signals are likely to be generated simultaneously if defects in LPS biogenesis cause problems in OMP biogenesis and vice versa. Indeed, mutational disruption of LPS biosynthesis also generates the OMP inducing signal for DegS.…”

“…A proteolytic cascade degrades an inhibitory antisigma factor to control RpoE activity and this cascade is triggered by misfolded/mislocalized OM proteins and by so-called "off-pathway" LPS molecules. 5,6 RpoE is essential in some species, with E. coli being a notable example. 7 This indicates that it also plays a critical role during normal growth conditions.…”

“…51 A proteolytic cascade analogous to that in E. coli is triggered to degrade the antisigma factor RseA (called MucA in P. aeruginosa) when misfolded OMPs interact with its DegS homolog AlgW, and LPS interacts with it RseB homolog, MucB. 6,52 However, the mucoid isolates from CF lungs almost always have mutations that inactivate MucA/RseA. 52,53 The great majority of these mutations are predicted to cause an aberrant form of MucA (MucA22) with the C-terminal periplasmic domain truncated, which renders it susceptible to proteolysis.…”