In vitro effect of the Escherichia coli heat shock regulatory protein on expression of heat shock genes

Resistance in antimicrobial photodynamic inactivation of bacteria

Antibiotics have increasingly lost their impact to kill bacteria efficiently during the last 10 years. The emergence and dissemination of superbugs with resistance to multiple antibiotic classes have occurred among Gram-positive and Gram-negative strains including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter strains. These six superbugs can "escape" more or less any single kind of antibiotic treatment. That means bacteria are very good at developing resistance against antibiotics in a short time. One new approach is called photodynamic antimicrobial chemotherapy (PACT) which already has demonstrated an efficient antimicrobial efficacy among multi-resistant bacteria. Until now it has been questionable if bacteria can develop resistance against PACT. This perspective summarises the current knowledge about the susceptibility of bacteria towards oxidative stress and sheds some light on possible strategies of the development of photodynamic inactivation of bacteria (PACT)-induced oxidative stress resistance by bacteria.

Efficient prediction of co-complexed proteins based on coevolution

The prediction of the network of protein-protein interactions (PPI) of an organism is crucial for the understanding of biological processes and for the development of new drugs. Machine learning methods have been successfully applied to the prediction of PPI in yeast by the integration of multiple direct and indirect biological data sources. However, experimental data are not available for most organisms. We propose here an ensemble machine learning approach for the prediction of PPI that depends solely on features independent from experimental data. We developed new estimators of the coevolution between proteins and combined them in an ensemble learning procedure.

We applied this method to a dataset of known co-complexed proteins in Escherichia coli and compared it to previously published methods. We show that our method allows prediction of PPI with an unprecedented precision of 95.5% for the first 200 sorted pairs of proteins compared to 28.5% on the same dataset with the previous best method.

A close inspection of the best predicted pairs allowed us to detect new or recently discovered interactions between chemotactic components, the flagellar apparatus and RNA polymerase complexes in E. coli.

Production of recombinant proteins in E. coli by the heat inducible expression system based on the phage lambda pL and/or pR promoters

The temperature inducible expression system, based on the pL and/or pR phage lambda promoters regulated by the thermolabile cI857 repressor has been widely use to produce recombinant proteins in prokaryotic cells. In this expression system, induction of heterologous protein is achieved by increasing the culture temperature, generally above 37 degrees C. Concomitant to the overexpression of heterologous protein, the increase in temperature also causes a variety of complex stress responses. Many studies have reported the use of such temperature inducible expression system, however only few discuss the simultaneous stress effects caused by recombinant protein production and the up-shift in temperature. Understanding the integral effect of such responses should be useful to develop improved strategies for high yield protein production and recovery. Here, we describe the current status of the heat inducible expression system based on the pL and/or pR lambda phage promoters, focusing on recent developments on expression vehicles, the stress responses at the molecular and physiological level that occur after heat induction, and bioprocessing factors that affect protein overexpression, including culture operation variables and induction strategies.

Heat shock activation of the groESL operon of Agrobacterium tumefaciens and the regulatory roles of the inverted repeat

Deletions were constructed in the conserved inverted repeat (IR) found in the groESL operon of Agrobacterium tumefaciens and in many other groE and dnaK operons and genes in eubacteria. These deletions affected the level of expression of the operon and the magnitude of its heat shock activation. The IR seems to operate at the DNA level, probably as an operator site that binds a repressor under non-heat shock conditions. The IR was also found to function at the mRNA level, since under non-heat shock conditions transcripts containing deletions of one side of the IR had longer half-lives than did transcripts containing the wild-type IR. Under heat shock conditions, the half-life of the mRNA was unaffected by this deletion because of heat shock-dependent cleavage. However, the groESL operon was found to be heat shock activated even after most of the IR was deleted. This observation, together with the fact that the groESL operon of A. tumefaciens was heat shock activated in Escherichia coli and vice versa, suggests that a heat shock promoter regulates the heat shock activation of this operon. The primary role of the IR appears to be in reducing the MRNA levels from this promoter under non-heat shock conditions.

Stress-induced transcriptional activation

Living cells, both prokaryotic and eukaryotic, employ specific sensory and signalling systems to obtain and transmit information from their environment in order to adjust cellular metabolism, growth, and development to environmental alterations. Among external factors that trigger such molecular communications are nutrients, ions, drugs and other compounds, and physical parameters such as temperature and pressure. One could consider stress imposed on cells as any disturbance of the normal growth condition and even as any deviation from optimal growth circumstances. It may be worthwhile to distinguish specific and general stress circumstances. Reasoning from this angle, the extensively studied response to heat stress on the one hand is a specific response of cells challenged with supra-optimal temperatures. This response makes use of the sophisticated chaperoning mechanisms playing a role during normal protein folding and turnover. The response is aimed primarily at protection and repair of cellular components and partly at acquisition of heat tolerance. In addition, heat stress conditions induce a general response, in common with other metabolically adverse circumstances leading to physiological perturbations, such as oxidative stress or osmostress. Furthermore, it is obvious that limitation of essential nutrients, such as glucose or amino acids for yeasts, leads to such a metabolic response. The purpose of the general response may be to promote rapid recovery from the stressful condition and resumption of normal growth. This review focuses on the changes in gene expression that occur when cells are challenged by stress, with major emphasis on the transcription factors involved, their cognate promoter elements, and the modulation of their activity upon stress signal transduction. With respect to heat shock-induced changes, a wealth of information on both prokaryotic and eukaryotic organisms, including yeasts, is available. As far as the concept of the general (metabolic) stress response is concerned, major attention will be paid to Saccharomyces cerevisiae.

A mutant at position 87 of the GroEL chaperonin is affected in protein binding and ATP hydrolysis

The highly conserved aspartic acid residue at position 87 of the Escherichia coli chaperonin GroEL was mutated to glutamic acid. When expressed in an E. coli groEL mutant strain deficient for phage morphogenesis, plasmid-encoded GroEL mutant D87E restored T4 phage morphogenesis. It did not, however, reactivate the transcription of a recombinant luciferase operon from Vibrio fischeri. In vitro, GroEL mutant D87E was found to be impaired in the ability to bind nonnative proteins and to hydrolyze ATP, resulting in less efficient refolding of urea-denatured ribulose-1,5-bisphosphate carboxylase/oxygenase. Mutant oligomer D87E GroEL14 was able to bind GroES7 as efficiently as wild-type GroEL14. The conserved aspartic acid residue at position 87 located in the equatorial domain of GroEL (Braig, K., Otwinowski, Z., Hegde, R., Boisvert, D.C., Joachimiak, A., Horwich, A.L., and Sigler, P.B. (1994) Nature 371, 578-586) is thus inferred to have a dual effect on the binding of nonnative proteins to the GroEL14 core chaperonin and on ATP hydrolysis.

The groESL operon of Agrobacterium tumefaciens: evidence for heat shock-dependent mRNA cleavage

The heat shock response of the groESL operon of Agrobacterium tumefaciens was studied at the RNA level. The operon was found to be activated under heat shock conditions and transcribed as a polycistronic mRNA that contains the groES and groEL genes. After activation, the polycistronic mRNA appeared to be cleaved between the groES and groEL genes and formed two monocistronic mRNAs. The groES cleavage product appeared to be unstable and subjected to degradation, while the groEL cleavage product appeared to be stable and became the major mRNA representing the groESL operon after long periods of growth at a high temperature. The polycistronic mRNA containing the groES and groEL genes was the major mRNA representing the groESL operon at a low temperature, and it reappeared when the cells were returned to the lower growth temperature after heat shock induction. These findings indicate that the cleavage event is part of the heat shock regulation of the groESL operon in A. tumefaciens.

Identification of a conditionally essential heat shock protein in Escherichia coli

Protein D48.5 was recognized as a heat-inducible protein of Escherichia coli during the screening of a group of random, temperature-inducible Mud-Lac fusion mutants. Physiological and genetic analysis demonstrated that (i) the structural gene for this protein, designated htpI, is a member of the sigma 32-dependent heat shock regulon, (ii) at 37 degrees C the synthesis of protein D48.5 is nearly constitutive, increasing slightly with growth rate in media of different composition, and (iii) this protein is essential for growth at high temperature.

DnaK mutants defective in ATPase activity are defective in negative regulation of the heat shock response: expression of mutant DnaK proteins results in filamentation

Site-directed mutagenesis has previously been used to construct Escherichia coli dnaK mutants encoding proteins that are altered at the site of in vitro phosphorylation (J. S. McCarty and G. C. Walker, Proc. Natl. Acad. Sci. USA 88:9513-9517, 1991). These mutants are unable to autophosphorylate and are severely defective in ATP hydrolysis. These mutant dnaK genes were placed under the control of the lac promoter and were found not to complement the deficiencies of a delta dnaK mutant in negative regulation of the heat shock response. A decrease in the expression of DnaK and DnaJ below their normal levels at 30 degrees C was found to result in increased expression of GroEL. The implications of these results for DnaK's role in the negative regulation of the heat shock response are discussed. Evidence is also presented indicating the existence of a 70-kDa protein present in a delta dnaK52 mutant that cross-reacts with antibodies raised against DnaK. Derivatives of the dnaK+ E. coli strain MC4100 expressing the mutant DnaK proteins filamented severely at temperatures equal to or greater than 34 degrees C. In the dnaK+ E. coli strain W3110, expression of these mutant proteins caused extreme filamentation even at 30 degrees C. Together with other observations, these results suggest that DnaK may play a direct role in the septation pathway, perhaps via an interaction with FtsZ. Although delta dnaK52 derivatives of strain MC4100 filament extensively, a level of underexpression of DnaK and DnaJ that results in increased expression of the other heat shock proteins did not result in filamentation. The delta dnaK52 allele could be transduced successfully, at temperatures of up to 45 degrees C, into strains carrying a plasmid expressing dnaK+ dnaJ+, although the yield of transductants decreased above 37 degrees C. In contrast, with a strain that did not carry a plasmid expressing dnaK+ dnaJ+, the yield of delta dnaK52 transductants decreased extremely sharply between 39 and 40 degrees C, suggesting that DnaK and DnaJ play one or more roles critical for growth at temperatures of 40 degrees C or greater.

Cloning, sequencing, and transcriptional analysis of the gene coding for the vegetative sigma factor of Agrobacterium tumefaciens

The sigA gene of Agrobacterium tumefaciens was cloned and sequenced. Comparison with previously analyzed sigA genes revealed a high degree of similarity in nucleotide and amino acid sequences of regions two, three, and four of vegetative sigma factors. However, the upstream regulatory region shows no sequence homology with the Escherichia coli heat shock (sigma 32) promoters. It also does not contain the hairpin-loop structure (inverted repeat sequence) that was found in the upstream region of the groE operon in A. tumefaciens. The transcription initiation site of the gene was determined and found to be at the same position during normal growth and under heat shock conditions. Furthermore, no heat shock activation was observed at the transcriptional level.

Heat shock transcription of the groESL operon of Agrobacterium tumefaciens may involve a hairpin-loop structure

The groESL operon of Agrobacterium tumefaciens was cloned and sequenced and found to be highly homologous to previously analyzed groE operons in nucleotides of the coding region and in amino acid sequence. Transcription of this operon in A. tumefaciens was considerably stimulated by heat shock. Primer extension analysis revealed that the groE transcripts from cells under heat shock were initiated from the same promoter (a sigma-70-like promoter) as transcripts from untreated cells, and no sequence homology with the Escherichia coli heat shock promoters was observed. The DNA sequence downstream of the transcription start site contains an inverted repeat that has a strong similarity to other groESL operons of both gram-positive and gram-negative bacteria (such as cyanobacteria and chlamydiae). This conserved region is thought to form a hairpin-loop structure and may play a role in gene regulation during heat shock.

DnaK, DnaJ, and GrpE are required for flagellum synthesis in Escherichia coli

The DnaK, DnaJ, and GrpE heat shock proteins are required for motility of Escherichia coli. Cells deleted for dnaK or dnaJ, or with some mutations in the dnaK or grpE gene, are nonmotile, lack flagella, exhibit a 10- to 20-fold decrease in the rate of synthesis of flagellin, and show reduced rates of transcription of both the flhD master operon (encoding FlhD and FlhC) and the fliA operon (encoding sigma F). Genetic studies suggest that DnaK and DnaJ define a regulatory pathway affecting flhD and fliA synthesis that is independent of cyclic AMP-catabolite gene activator protein or the chemotaxis system.

Genetic mapping and biochemical characterization of suppressor mutations sukA and sukB for a dnaK7(Ts) mutation of Escherichia coli K-12

Temperature-resistant pseudorevertants were isolated from a dnaK7(Ts) mutant of Escherichia coli K-12. Two of these pseudorevertants were shown to carry suppressor mutations, sukA and sukB, respectively. Genetic mapping by conjugation and P1-transduction revealed that these suppressor mutations were located at two distinct sites between 76 and 77 min close to the suhA and rpoH genes. Labeled cellular proteins were extracted from suppressor mutants grown at various temperatures and subjected to SDS-gel electrophoresis. Autoradiograms of the gels indicated that these suppressor mutations each resulted in increased synthesis of the heat shock protein Lon (an ATP-dependent protease, La) at both permissive and nonpermissive temperatures.

Involvement of GroEL in nif gene regulation and nitrogenase assembly

Several approaches were used to study the role of GroEL, the prototype chaperonin, in the nitrogen fixation (nif) system. An Escherichia coli groEL mutant transformed with the Klebsiella pneumoniae nif gene cluster accumulated very low to nondetectable levels of nitrogenase components compared with the isogenic wild-type strain or the mutant cotransformed with the wild-type groE operon. In K. pneumoniae, overexpression of the E. coli groE operon markedly accelerated the rate of appearance of the MoFe protein and its constituent polypeptides after the start of derepression. The groEL mutation in E. coli decreased NifA-dependent beta-galactosidase expression from the nifH promoter but did not affect the constitutive expression of nifA from the tet promoter of ntr-controlled expression from the nifLA promoter. The possibility that GroEL is required for the correct folding of NifA was supported by coimmunoprecipitation of NifA with anti-GroEL antibodies. Kinetic analyses of nitrogenase assembly in 35S pulse-chased K. pneumoniae pointed to the existence of high-molecular-weight intermediates in MoFe protein assembly and demonstrated the transient binding of newly synthesized NifH and NifDK to GroEL. Overall, these results indicate that GroEL fulfills both regulatory and structural functions in the nif system.

Interaction of DnaK with ATP: binding, hydrolysis and Ca+2-stimulated autophosphorylation

The autophosphorylation of DnaK from Escherichia coli using ATP as phosphate donor is markedly stimulated by Ca+2 and to a lesser degree by Mn+2. Mg+2 and other divalent ions are without effect in this reaction. Lanthanum, an agonist/antagonist of Ca+2, is also effective in stimulating the autophosphorylation. In contrast, Mg+2 but not Ca+2, markedly stimulates the hydrolysis of ATP catalyzed by DnaK. Also at 0 degrees, ATP forms a stable complex with DnaK without hydrolysis that is independent of cations. About 15% of the DnaK in E. coli is associated with membrane vesicles where it also can be phosphorylated in the presence of Ca+2.

A cya deletion mutant of Escherichia coli develops thermotolerance but does not exhibit a heat-shock response

An adenyl cyclase deletion mutant (cya) of E. coli failed to exhibit a heat-shock response even after 30 min at 42 degrees C. Under these conditions, heat-shock protein synthesis was induced by 10 min in the wild-type strain. These results suggest that synthesis of heat-shock proteins in E. coli requires the cya gene. This hypothesis is supported by the finding that a presumptive cyclic AMP receptor protein (CRP) binding site exists within the promoter region of the E. coli htpR gene. In spite of the absence of heat-shock protein synthesis, when treated at 50 degrees C, the cya mutant is relatively more heat resistant than wild type. Furthermore, when heat shocked at 42 degrees C prior to exposure at 50 degrees C, the cya mutant developed thermotolerance. These results suggest that heat-shock protein synthesis is not essential for development of thermotolerance in E. coli.

Identification of the sigma E subunit of Escherichia coli RNA polymerase: a second alternate sigma factor involved in high-temperature gene expression

The rpoH gene of Escherichia coli encodes sigma 32, the 32-kD sigma-factor responsible for the heat-inducible transcription of the heat shock genes. rpoH is transcribed from at least three promoters. Two of these promoters are recognized by RNA polymerase containing sigma 70, the predominant sigma-factor. We purified the factor responsible for recognizing the third rpoH promoter (rpoH P3) and identified it as RNA polymerase containing a novel sigma-factor with an apparent Mr of 24,000. This new sigma, which we call sigma E, is distinct from the known sigma factors in molecular weight and promoter specificity. sigma E holoenzyme will not recognize the sigma 70- or sigma 32-controlled promoters we tested, but it does transcribe the htrA gene, which is required for viability at temperatures greater than 42 degrees C. The in vivo role of sigma E is not known. The transcripts from the sigma E-controlled rpoH P3 and htrA promoters are most abundant at very high temperature, suggesting the sigma E holoenzyme may transcribe a second set of heat-inducible genes that are involved in growth at high temperature or in thermotolerance.

A novel sigma factor is involved in expression of the rpoH gene of Escherichia coli

The Escherichia coli rpoH gene encoding sigma 32, which is involved in the heat shock response, is transcribed from as many as four promoters. We have isolated a novel sigma factor of about 24 kilodaltons that allows core RNA polymerase to transcribe preferentially from one of these promoters, rpoH3p. This promoter is known to be regulated by DnaA protein. The sigma 24 factor was isolated from a preparation of RNA polymerase by electroelution from sodium dodecyl sulfate-polyacrylamide gels followed by renaturation. Expression of heat shock proteins is induced by treatments which include those that induce the stringent response. Under such conditions, decreased transcription from rpoH3p and no increase in transcription from other rpoH promoters were observed. This result suggests that induction of heat shock proteins by the stringent response is not mediated by increased transcription of the rpoH gene.

Identification, characterization, and mapping of the Escherichia coli htrA gene, whose product is essential for bacterial growth only at elevated temperatures

We identified and cloned an Escherichia coli gene called htrA (high temperature requirement). The htrA gene was originally discovered because mini-Tn10 transposon insertions in it allowed E. coli growth at 30 degrees C but prevented growth at elevated temperatures (above 42 degrees C). The htrA insertion mutants underwent a block in macromolecular synthesis and eventually lysed at the nonpermissive temperature. The htrA gene was located at approximately 3.7 min (between the fhuA and dapD loci) on the genetic map of E. coli and between 180 and 187.5 kilobases on the physical map. It coded for an unstable, 51-kilodalton protein which was processed by removal of an amino-terminal fragment, resulting in a stable, 48-kilodalton protein.

Modulation of stability of the Escherichia coli heat shock regulatory factor sigma

The heat shock response of Escherichia coli is under the positive control of the sigma 32 protein (the product of the rpoH gene). We found that overproduction of the sigma 32 protein led to concomitant overproduction of the heat shock proteins, suggesting that the intracellular sigma 32 levels limit heat shock gene expression. In support of this idea, the intracellular half-life of the sigma 32 protein synthesized from a multicopy plasmid was found to be extremely short, e.g., less than 1 min at 37 and 42 degrees C. The half-life increased progressively with a decrease in temperature, reaching 15 min at 22 degrees C. Finally, conditions known previously to increase the rate of synthesis of the heat shock proteins, i.e., a mutation in the dnaK gene or expression of phage lambda early proteins, were shown to simultaneously result in a three- to fivefold increase in the half-life of sigma 32.

In vitro reconstitution of osmoregulated expression of proU of Escherichia coli

Osmoregulated expression of proU has been reconstituted in a cell-free system. proU encodes an osmotically inducible, high-affinity transport system for the osmoprotectant glycine betaine in Escherichia coli. Previously, a proU-lacZ fusion gene had been cloned, resulting in plasmid pOS3. In vivo osmoregulation of this extrachromosomal proU-lacZ fusion gene at low copy number showed that the plasmid-encoded fusion contained all the necessary sequences in cis for correctly receiving osmoregulatory signals during induction by osmotic stress and repression by glycine betaine. Using a cell-free (S-30) extract, plasmid pOS3 was then used to program protein synthesis in vitro. The ionic compound potassium glutamate specifically stimulated proU-lacZ expression in a concentration-dependent manner. Potassium acetate also induced some proU expression, but other salts were ineffective, thereby ruling out ionic strength as the stimulatory signal. High concentrations of sucrose, trehalose, or glycine betaine did not induce proU expression in vitro either, eliminating osmolarity per se as the stimulus. Reconstitution in a cell-free system rules out osmoregulatory mechanisms that depend on turgor, trans-membrane signaling, or trans-acting regulators synthesized after osmotic upshock.

Sequence analysis and regulation of the htrA gene of Escherichia coli: a sigma 32-independent mechanism of heat-inducible transcription

Previous work has established that the E. coli htrA gene product is essential for bacterial survival at temperatures above 42 degrees. We have sequenced the htrA gene region and found an open reading frame (ORF) coding for a protein of 491 amino acids with a calculated molecular weight of 51,163 daltons. This molecular weight corresponds well with that seen following electrophoresis on SDS-polyacrylamide gels. This protein has an amino-terminal sequence typical for a leader peptide and undergoes post-translational modification by cleavage of an amino-terminal portion. The insertional mutations which affect the function of the htrA gene map inside this ORF. The levels of htrA mRNA increase rapidly and transiently upon heat shock in a manner independent of the rpoH gene, which encodes the sigma 32 RNA polymerase subunit and is known to regulate transcription of typical heat shock genes. Using S1 mapping and RNA primer extension, we have identified the htrA promoter and found that it is similar to the P3 promoter of the rpoH gene. The P3 promoter is especially active at high temperatures and is recognized by a recently identified transcriptional factor, sigma E.

Antibody to sigma 32 cross-reacts with DnaK: association of DnaK protein with Escherichia coli RNA polymerase

A polyclonal antibody to sigma 32, the heat shock sigma factor, has been used to show the presence of low levels of sigma 32 in Escherichia coli RNA polymerase preparations (E sigma 70), which explains the observed in vitro activity of E sigma 70 towards heat shock genes. The sigma 32 antibody cross-reacts with DnaK, and DnaK has been found associated with purified preparations of both E sigma 70 and the heat shock RNA polymerase, E sigma 32.

Heat shock response of Pseudomonas aeruginosa

The general properties of the heat shock response in Pseudomonas aeruginosa were characterized. The transfer of cells from 30 to 45 degrees C repressed the synthesis of many cellular proteins and led to the enhanced production of 17 proteins. With antibodies raised against the Escherichia coli proteins, two polypeptides of P. aeruginosa with apparent molecular weights of 76,000 and 61,000 (76K and 61K proteins) were shown to be analogous to the DnaK and GroEL heat shock proteins of E. coli due to their immunologic cross-reactivity. The major sigma factor (sigma 87) of P. aeruginosa was shown to be a heat shock protein that was immunologically related to the sigma 70 of E. coli by using polyclonal antisera. A hybridoma was produced, and the monoclonal antibody MP-S-1 was specific for the sigma 87 and did not cross-react with sigma 70 of E. coli. A smaller 40K protein was immunoprecipitated with RNA polymerase antisera from cells that had been heat shocked. The 40K protein was also associated with RNA polymerase which had been purified from heat-shocked cells and may be the heat shock sigma factor of P. aeruginosa. Exposure to ethanol resulted in the production of seven new proteins, three of which appeared to be heat shock proteins.

Correlation between the 32-kDa sigma factor levels and in vitro expression of Escherichia coli heat shock genes

S-30 extracts from Escherichia coli cells were used to express heat shock (HS) and non-HS genes in vitro in a DNA-directed protein synthesis system. The S-30 extracts prepared from cells that have been shifted to 45 degrees C express HS genes in vitro approximately 8 times better than extracts from cells at 33 degrees C. In contrast, the expression of non-HS genes in extracts from heat-induced cells is only 40% of that seen in extracts from cells at 33 degrees C. These results correlate well with the levels of HS sigma factor and normal sigma factor bound to RNA polymerase. Thus, there was an 8-fold increase in the HS sigma factor and a 60% decrease in the normal sigma factor associated with RNA polymerase at the higher temperature. Part of the increase in the level of the HS sigma factor could be accounted for by a 3-fold increase in the level of HS sigma factor mRNA during heat induction.

Heat-shock induction of RNA polymerase sigma-32 synthesis in Escherichia coli: transcriptional control and a multiple promoter system

Transcriptional start sites of the rpoH gene which codes for a minor sigma factor (sigma 32) of Escherichia coli RNA polymerase were determined. The rpoH gene is transcribed, both in vivo and in vitro, from two major (P1 and P2) and one minor (P2*) promoters. In vitro synthesis of the rpoH mRNAs is dependent on the major species of RNA polymerase holoenzyme (E sigma 70) but not on the minor one (E sigma 32). S1 nuclease analysis of the in vivo RNA showed that the level of rpoH transcript from the downstream P2 promoter increases rapidly when E. coli cells are transferred from 30 degrees C to 42 degrees C, while the transcript from the upstream P1 promoter remains at a constant level. Under these conditions, the metabolic stabilities of rpoH mRNAs are virtually unaffected, suggesting that the synthesis of rpoH mRNA from the P2 promoter is specifically enhanced upon heat-shock.

The heat shock response of E. coli is regulated by changes in the concentration of sigma 32

Cells subjected to a heat shock, or a variety of other stresses increase the synthesis of a set of proteins, known as heat shock proteins. This response is apparently universal, occurring in the entire range from bacterial to mammalian cells. In Escherichia coli heat shock protein synthesis transiently increases following a shift from 30 degrees C to 42 degrees C as a result of changes in transcription initiation at heat shock promoters. Heat shock promoters are recognized by RNA polymerase containing a sigma factor of relative molecular mass (Mr) 32,000 (32K) E sigma 32 and not E sigma 70, the major form of RNA polymerase holoenzyme. To determine whether changes in the concentration of sigma 32 regulate this response, we measured the amount of sigma 32 before and after shift to high temperature and found that it increased transiently during heat shock as a result of changes in sigma 32 synthesis and stability. Our results indicate that sigma 32 is directly responsible for regulation of the heat shock response.

Regulation of the promoters and transcripts of rpoH, the Escherichia coli heat shock regulatory gene

In Escherichia coli the product of the rpoH (htpR) gene, sigma 32, directs RNA polymerase to initiate transcription from heat shock promoters at all temperatures. Transcription of the heat shock genes is increased when cells are exposed to high temperatures because of increased transcription initiation by sigma 32-RNA polymerase. As a step toward understanding the regulation of the heat shock response we have examined the transcription of the rpoH gene. Using S1 mapping, promoter cloning, and in vitro transcription, we have identified the promoters and the terminator for the rpoH transcription unit. The rpoH transcripts are monocistronic and originate from at least three promoters. None of the promoters is recognized by sigma 32-RNA polymerase. Two are recognized by sigma 70-RNA polymerase and are active at both low and high growth temperatures. We do not know what form of RNA polymerase recognizes the third promoter. Transcripts from this promoter are abundant only at high temperature and are present after shift to the lethal temperature of 50 degrees C, even at times when there are no detectable transcripts from the other rpoH promoters. The amount of rpoH mRNA increases fivefold by 8 min after shift from 30 to 43.5 degrees C but rpoH mRNA synthesis increases by less than twofold, indicating that there is post-transcriptional control of the level of rpoH mRNA and presumably of sigma 32.

Sigma 32 synthesis can regulate the synthesis of heat shock proteins in Escherichia coli

The Escherichia coli rpoH (htpR) gene product, sigma 32, is required for the normal expression of heat shock genes and for the heat shock response. We present experiments indicating a direct role for sigma 32 in controlling the heat shock response. Both the induction and decline in the synthesis of heat shock proteins can be controlled by changes in the rate of synthesis of sigma 32. Specifically, we show that: (1) sigma 32 is an unstable protein, degraded with a half-life of approximately 4 min; (2) increasing the rate of synthesis of sigma 32, by inducing expression from a Plac or Ptac-rpoH fusion, is sufficient to increase the rate of synthesis of heat shock proteins; (3) during the shut-off phase of the heat shock response synthesis of sigma 32 is repressed post-transcriptionally, and the dnaK756 mutation, which causes a defect in the shut-off phase, prevents the post-transcriptional repression of synthesis of sigma 32. These results serve as a basis for understanding the role of DnaK in the heat shock response, the regulation of sigma 32 synthesis, and the role of sigma 32 in controlling transcription of heat shock genes.

Heat shock regulatory gene rpoH mRNA level increases after heat shock in Escherichia coli

The Escherichia coli rpoH gene product sigma 32 is essential for the increase in heat shock gene transcription found after exposure of the bacteria to a sudden temperature increase. It is not known how the concentration of active sigma 32 is modulated. We showed that rpoH transcript levels increased after heat shock and that the magnitude of the increase in the level of mRNA was correlated with the magnitude of the temperature shift. The increase in the level of rpoH mRNA was still found in rpoH mutants so the mechanism of induction differed from that of the set of previously identified heat shock genes. The increased concentration of rpoH mRNA should result in a higher level of sigma 32, which is likely to be important for increasing heat shock gene transcription.