The Escherichia coli heat shock regulatory gene is immediately downstream of a cell division operon: the fam mutation is allelic with rpoH

Signal recognition particle-dependent protein targeting, universal to all kingdoms of life

The signal recognition particle (SRP) and its membrane-bound receptor represent a ubiquitous protein-targeting device utilized by organisms as different as bacteria and humans, archaea and plants. The unifying concept of SRP-dependent protein targeting is that SRP binds to signal sequences of newly synthesized proteins as they emerge from the ribosome. In eukaryotes this interaction arrests or retards translation elongation until SRP targets the ribosome-nascent chain complexes via the SRP receptor to the translocation channel. Such channels are present in the endoplasmic reticulum of eukaryotic cells, the thylakoids of chloroplasts, or the plasma membrane of prokaryotes. The minimal functional unit of SRP consists of a signal sequence-recognizing protein and a small RNA. The as yet most complex version is the mammalian SRP whose RNA, together with six proteinaceous subunits, undergo an intricate assembly process. The preferential substrates of SRP possess especially hydrophobic signal sequences. Interactions between SRP and its receptor, the ribosome, the signal sequence, and the target membrane are regulated by GTP hydrolysis. SRP-dependent protein targeting in bacteria and chloroplasts slightly deviate from the canonical mechanism found in eukaryotes. Pro- and eukaryotic cells harbour regulatory mechanisms to prevent a malfunction of the SRP pathway.

Stress proteins: nomenclature, division and functions

The heat shock response, characterized by increased expression of heat shock proteins (Hsps) is induced by exposure of cells and tissues to extreme conditions that cause acute or chronic stress. Hsps function as molecular chaperones in regulating cellular homeostasis and promoting survival. If the stress is too severe, a signal that leads to programmed cell death, apoptosis, is activated, thereby providing a finely tuned balance between survival and death. In addition to extracellular stimuli, several nonstressfull conditions induce Hsps during normal cellular growth and development. The enhanced heat shock gene expression in response to various stimuli is regulated by heat shock transcription factors.

Molecular characterization of Escherichia coli FtsE and FtsX

The genes ftsE and ftsX are organized in one operon together with ftsY. FtsY codes for the receptor of the signal recognition particle (SRP) that functions in targeting a subset of inner membrane proteins. We have found no indications for a structural relationship between FtsE/X and FtsY. Evidence is presented that FtsE and FtsX form a complex in the inner membrane that bears the characteristics of an ATP-binding cassette (ABC)-type transporter. FtsE is a hydrophilic nucleotide-binding protein that has a tendency to dimerize and associates with the inner membrane through an interaction with the integral membrane protein FtsX. An FtsE null mutant showed filamentous growth and appeared viable on high salt medium only, indicating a role for FtsE in cell division and/or salt transport.

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 cluster of cell division genes maps to the terC region of the chromosome of Escherichia coli K-12

Thirty-nine cell division mutants were isolated in Escherichia coli K-12 and were mapped in the terminus region of the chromosome, between 33.5 and 36 min. They were obtained by two different approaches involving specific mutagenesis of the terC region. The mutants could be divided into eight classes (I to VIII) based on their map position and phenotype at the restrictive temperature, and constitute a new cell division gene cluster.

Escherichia coli cells with mutations in the gene for adenylate cyclase (cya) exhibit a heat shock response

Adenylate cyclase mutants of Escherichia coli showed the heat-shock response. The heat-shock response was studied in two different mutants and in different growth media, including rich and minimal media. These results are in disagreement with the proposal that the cya gene regulates the expression of the heat-shock genes.

Regulation of the Escherichia coli heat-shock response

Steady-state- and stress-induced expression of Escherichia coli heat-shock genes is regulated at the transcriptional level through controls of concentration and activity of the positive regulator, the heat-shock promoter-specific subunit of RNA polymerase, sigma 32. Central to these controls are functions of the DnaK, DnaJ, GrpE heat-shock proteins as negative modulators that mediate degradation as well as repression of activity and, in some conditions, of synthesis of sigma 32. DnaJ has a key role in modulation since it binds sigma 32 and, jointly with DnaK and GrpE, represses its activity. Furthermore, DnaJ is capable of binding heat-damaged proteins, targeting DnaK and GrpE to these substrates, and thereby mediating DnaK-, DnaJ-, GrpE-dependent repair. It is proposed that one important signal transduction pathway that converts stress to a heat-shock response relies on the sequestering of DnaJ through binding to damaged proteins which derepresses and stabilizes sigma 32. Damage repair ameliorates the inducing signal and frees DnaJ, DnaK, GrpE to shut off the heat-shock response.

Expression analysis of cloned chromosomal segments of Escherichia coli

The novel transcription system of bacteriophage T7 was used to express Escherichia coli genes preferentially with a new low-copy-number plasmid vector, pFN476, to minimize toxic gene effects. Selected E. coli chromosomal fragments from an ordered genomic library (Y. Kohara, K. Ikiyama, and K. Isono, Cell 50:495-508, 1987) were recloned into this vector, and their genes were preferentially expressed in vivo utilizing its T7 promoter. The protein products were analyzed by two-dimensional gel electrophoresis. By using DNA sequence information, the gel migration was predicted for the protein products of open reading frames from these segments, and this information was used to identify gene products visualized as spots on two-dimensional gels. Even in the absence of DNA sequence information, this approach offers the opportunity to identify all gene products of E. coli and map their genes to within 10 kb on the E. coli genome; with sequence information, this approach can produce a definitive expression map of the E. coli genome.

Localised mutagenesis of the fts YEX operon: conditionally lethal missense substitutions in the FtsE cell division protein of Escherichia coli are similar to those found in the cystic fibrosis transmembrane conductance regulator protein (CFTR) of human patients

After localised mutagenesis of the 76 min region of the Escherichia coli chromosome, we isolated a number of conditionally lethal mutants. Some of these mutants had a filamentation temperature sensitive (fts) phenotype and were assigned to the cell division genes ftsE of ftsX, whereas others were defective in the heat shock regulator gene rpoH. Both missense and amber mutant alleles of these genes were produced. The missense mutant ftsE alleles were cloned and sequenced to determine whether or not the respective mutations mapped to the region of the gene encoding the putative nucleotide binding site. Surprisingly, most of these mutant FtsE proteins had missense substitutions in a different domain of the protein. This region of the FtsE protein is highly conserved in a large family of proteins involved in diverse transport processes in all living cells, from bacteria to man. One of the proteins in this large family of homologues is the human cystic fibrosis transmembrane conductance regulator (CFTR), and the FtsE substitutions were found to be in very closely linked, or identical, amino acid residues to those which are frequently altered in the CFTR of human patients. These results confirm the structural importance of this highly conserved region of FtsE and CFTR and add weight to the current structural model for the human protein.

Transcriptional regulation of the heat shock regulatory gene rpoH in Escherichia coli: involvement of a novel catabolite-sensitive promoter

A catabolite-sensitive promoter was found to be involved in transcription of the heat shock regulatory gene rpoH encoding the sigma 32 protein. Expression of lacZ from the operon fusion, rpoHp-lacZ, was partially inhibited by glucose added to the broth medium. Dissection of the rpoH promoter region allowed us to localize the glucose-sensitive promoter to the 110-base-pair (bp) segment directly upstream of the rpoH coding region. Experiments on lacZ expression from the set of fusions in cya (adenylate cyclase) and crp (cyclic AMP [cAMP] receptor protein) mutants also supported the involvement of a catabolite-sensitive promoter. Analysis of rpoH mRNAs by S1 nuclease protection experiments led us to identify a novel promoter, designated P5, that is regulated by cAMP and the cAMP receptor protein. Studies of rpoH transcription in vitro demonstrated that RNA polymerase-sigma 70 can transcribe from the P5 promoter only in the presence of cAMP and its receptor protein. The 5' ends of P5 transcripts obtained in vivo and in vitro were found to be at 61 to 62 bp upstream of the initiation codon, and a putative binding sequence for the cAMP receptor protein was found at 38 to 39 bp further upstream. Transcription from the P5 promoter is increased by the addition of ethanol to the growth medium; however, the increase is greater in the presence of glucose than in its absence. These results add a new dimension to the transcriptional control of rpoH and to the regulation of the heat shock response in Escherichia coli.

The identification of the Escherichia coli ftsY gene product: an unusual protein

The ftsYEX operon in Escherichia coli encodes three proteins, two of which (FtsE and FtsX) are known to be required for cell division. Although FtsE and FtsX have been identified using SDS-PAGE, the FtsY protein has not. We have used in vitro insertion mutagenesis to identify FtsY as a 92 kD polypeptide in maxicell experiments, although predictions from the DNA sequence estimated FtsY to be 54 kD. Results suggest that this disparity could be due to the unusually high percentage of acidic residues within the protein. Complementation tests indicated the presence of a promoter within ftsY required for expression of ftsE and ftsX. The FtsY protein exhibits sequence homology with the SR alpha protein of eukaryotes which is involved in protein secretion. The essential nature of the ftsY gene was also demonstrated.

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.

Lysis of Escherichia coli by the bacteriophage phi X174 E protein: inhibition of lysis by heat shock proteins

Lysis of Escherichia coli by the cloned E protein of bacteriophage phi X174 was more rapid than expected when bacteria were shifted from 30 to 42 degrees C at the time of E induction. Since such treatment also induces the heat shock response, we investigated the effect of heat shock proteins on lysis. An rpoH mutant was more sensitive to lysis by E, but a secondary suppressor mutation restored lysis resistance to parental levels, which suggests that the sigma 32 subunit itself did not directly increase lysis resistance. At 30 degrees C, mutants in five heat shock genes (dnaK, dnaJ, groEL, groES, and grpE) were more sensitive to lysis than were their wild-type parents. The magnitude of lysis sensitivity varied with mutation and strain background, with dnaK, dnaJ, and groES mutants consistently exhibiting the greatest sensitivities. Extended protection against lysis occurred when overproduction of heat shock proteins was induced artificially in cells that contained a plasmid with the rpoH+ gene under control of the tac promoter. This protective effect was completely abolished by mutations in dnaK, dnaJ, or groES but not by grpE or groEL mutations. Altered membrane behavior probably explains the contradiction whereby an actual temperature shift sensitized cells to lysis, but production of heat shock proteins exhibited protective effects. The results demonstrate that E-induced lysis can be divided into two distinct operations which may now be studied separately. They also emphasize a role for heat shock proteins under non-heat-shock conditions and suggest cautious interpretation of lysis phenomena in systems where E protein production is under control of a temperature-sensitive repressor.

DNA supercoiling and temperature shift affect the promoter activity of the Escherichia coli rpoH gene encoding the heat-shock sigma subunit of RNA polymerase

The effects of DNA supercoiling and temperature shift on the activity of two major promoters of the Escherichia coli rpoH gene, which codes for the heat-shock sigma subunit of RNA polymerase, were examined using an in vitro transcription system. Upstream promoter P1 is not affected by DNA supercoiling nor temperature upshift but downstream promoter P2 is enhanced by both of these factors. Both factors facilitate the formation of an open complex on rpoH P2, and these two effects are not additive. Hence these two factors act at the same step of transcription, probably the local unwinding of the DNA double helix at the promoter region. The different sensitivities of the rpoH major promoters to DNA supercoiling and temperature shift indicate that these promoters are under differential control. Taken together with the in vivo enhancement by heat shock of rpoH P2 transcription, we propose that at least one of the mechanism(s) of induction of sigma 32 synthesis by heat shock is the activation of the rpoH P2 promoter by temperature up-shift.

Division behavior and shape changes in isogenic ftsZ, ftsQ, ftsA, pbpB, and ftsE cell division mutants of Escherichia coli during temperature shift experiments

Isogenic ftsZ, ftsQ, ftsA, pbpB, and ftsE cell division mutants of Escherichia coli were compared with their parent strain in temperature shift experiments. To improve detection of phenotypic differences in division behavior and cell shape, the strains were grown in glucose-minimal medium with a decreased osmolality (about 100 mosM). Already at the premissive temperature, all mutants, particularly the pbpB and ftsQ mutants, showed an increased average cell length and cell mass. The pbpB and ftsQ mutants also exhibited a prolonged duration of the constriction period. All strains, except ftsZ, continued to initiate new constrictions at 42 degrees C, suggesting the involvement of FtsZ in an early step of the constriction process. The new constrictions were blunt in ftsQ and more pronounced in ftsA and pbpB filaments, which also had elongated median constrictions. Whereas the latter strains showed a slow recovery of cell division after a shift back to the permissive temperature, ftsZ and ftsQ filaments recovered quickly. Recovery of filaments occurred in all strains by the separation of newborn cells with an average length of two times LO, the length of newborn cells at the permissive temperature. The increased size of the newborn cells could indicate that the cell division machinery recovers too slowly to create normal-sized cells. Our results indicate a phenotypic resemblance between ftsA and pbpB mutants and suggest that the cell division gene products function in the order FtsZ-FtsQ-FtsA, PBP3. The ftsE mutant continued to constrict and divide at 42 degrees C, forming short filaments, which recovered quickly after a shift back to the permissive temperature. After prolonged growth at 42 degree C, chains of cells, which eventually swelled up, were formed. Although the ftsE mutant produced filaments in broth medium at the restrictive temperature, it cannot be considered a cell division mutant under the presently applied conditions.

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.