The rpoE operon regulates heat stress response in Burkholderia pseudomallei

Burkholderia pseudomallei is a gram-negative bacterium and the causative agent of melioidosis, one of the important lethal diseases in tropical regions. In this article, we demonstrate the crucial role of the B. pseudomallei rpoE locus in the response to heat stress. The rpoE operon knockout mutant exhibited growth retardation and reduced survival when exposed to a high temperature. Expression analysis using rpoH promoter-lacZ fusion revealed that heat stress induction of rpoH, which encodes heat shock sigma factor (sigma(H)), was abolished in the B. pseudomallei rpoE mutant. Analysis of the rpoH promoter region revealed sequences sharing high homology to the consensus sequence of sigma(E)-dependent promoters. Moreover, the putative heat-induced sigma(H)-regulated heat shock proteins (i.e. GroEL and HtpG) were also absent in the rpoE operon mutant. Altogether, our data suggest that the rpoE operon regulates B. pseudomallei heat stress response through the function of rpoH.     

Function of the sigma(E) regulon in dead-cell lysis in stationary-phase Escherichia coli

Elevation of active sigma(E) levels in Escherichia coli by either repressing the expression of rseA encoding an anti-sigma(E) factor or cloning rpoE in a multicopy plasmid, led to a large decrease in the number of dead cells and the accumulation of cellular proteins in the medium in the stationary phase. The numbers of CFU, however, were nearly the same as those of the wild type or cells devoid of the cloned gene. In the wild-type cells, rpoE expression was increased in the stationary phase and a low-level release of intracellular proteins was observed. These results suggest that dead cell lysis in stationary-phase E. coli occurs in a sigma(E)-dependent fashion. We propose there is a novel physiological function of the sigma(E) regulon that may guarantee cell survival in prolonged stationary phase by providing nutrients from dead cells for the next generation.     

A two-plasmid system for identification of promoters recognized by RNA polymerase containing extracytoplasmic stress response sigma(E) in Escherichia coli

We have previously established a two-plasmid system in Escherichia coli for identification of promoters recognized by RNA polymerase containing a heterologous sigma factor. Attempts to optimize this system for identification of promoters recognized by RNA polymerase containing E. coli extracytoplasmic stress response sigma(E) failed owing to high toxicity of the expressed rpoE. A new system for identification of sigma(E)-cognate promoters was established, and verified using the two known sigma(E)-dependent promoters, rpoEp2 and degPp. Expression of the sigma(E)-encoding rpoE gene was under the control of the AraC-dependent P(BAD) promoter. A low level of arabinose induced a non-toxic, however, sufficient level of sigma(E) to interact with the core enzyme of RNA polymerase. Such an RNA polymerase holoenzyme recognized both known sigma(E)-dependent promoters, rpoEp2 and degPp, which were cloned in the compatible promoter probe plasmid, upstream of a promoterless lacZ alpha reporter gene. This new system has proved to be useful for identification of E. coli sigma(E)-cognate promoters. Moreover, the system could be used for identification of ECF sigma-cognate promoters from other bacteria.     

Reversal of the ΔdegP phenotypes by a novel rpoE allele of Escherichia coli

RseA sequesters RpoE (σ(E)) to the inner membrane of Escherichia coli when envelope stress is low. Elevated envelope stress triggers RseA cleavage by the sequential action of two membrane proteases, DegS and RseP, releasing σ(E) to activate an envelope stress reducing pathway. Revertants of a ΔdegP ΔbamB strain, which fails to grow at 37°C due to high envelope stress, harbored mutations in the rseA and rpoE genes. Null and missense rseA mutations constitutively hyper-activated the σ(E) regulon and significantly reduced the major outer membrane protein (OMP) levels. In contrast, a novel rpoE allele, rpoE3, resulting from the partial duplication of the rpoE gene, increased σ(E) levels greater than that seen in the rseA mutant background but did not reduce OMP levels. A σ(E)-dependent RybB::LacZ construct showed only a weak activation of the σ(E) pathway by rpoE3. Despite this, rpoE3 fully reversed the growth and envelope vesiculation phenotypes of ΔdegP. Interestingly, rpoE3 also brought down the modestly activated Cpx envelope stress pathway in the ΔdegP strain to the wild type level, showing the complementary nature of the σ(E) and Cpx pathways. Through employing a labile mutant periplasmic protein, AcrA(L222Q), it was determined that the rpoE3 mutation overcomes the ΔdegP phenotypes, in part, by activating a σ(E)-dependent proteolytic pathway. Our data suggest that a reduction in the OMP levels is not intrinsic to the σ(E)-mediated mechanism of lowering envelope stress. They also suggest that under extreme envelope stress, a tight homeostasis loop between RseA and σ(E) may partly be responsible for cell death, and this loop can be broken by mutations that either lower RseA activity or increase σ(E) levels.