The phage shock protein (Psp) response is found in many Gram-negative
enterobacteria, where it helps to maintain the proton motive force (PMF) when the
integrity of the inner membrane (IM) is impaired and promotes virulence of
pathogens such as Yersinia or Salmonella. In Escherichia coli, Psp comprises seven
genes (pspF pspABCDE and pspG) which are organised in a regulon under the
control of two Sigma54-dependent promoters. Despite considerable advances,
neither the mechanism of Psp induction nor the functioning of the Psp response is
fully understood. Recent findings comparing the roles of ArcB in Yersinia
enterocolitica and E. coli caused a dispute over the requirement of the twocomponent
system ArcAB in Psp signal-transduction. The present study now
establishes that ArcAB involvement is conditional and appears to be mediated via
protein-protein interactions between ArcB and PspB. The study further suggests
that the cellular ubiquinone pool, which acts upstream of ArcAB, may also play a
role in Psp signalling whereas dissipation of proton motive force (PMF), generally
inferred to be the inducing signal, is not sufficient to mount a Psp response. To gain
further insight into its functioning, PspA (a negative regulator and effector of Psp)
and PspG (an effector of Psp) were visualised in vivo using fusions to Green
fluorescent protein (GFP). To maintain PMF, PspA was proposed to uniformly cover
the cytoplasmic face of the IM. However, the present study demonstrates that PspA
(and PspG) is highly organised into distinct complexes at the cell pole and the lateral
cell membrane. Real-time observations revealed lateral PspA and PspG complexes
are highly mobile, but absent in cells lacking MreB. Without the MreB cytoskeleton, induction of the Psp response is still observed, yet these cells fail to maintain PMF
under stress conditions.