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

Coping with High Temperature: A Unique Regulation in A. tumefaciens

Elevation of temperature is a frequent and considerable stress for mesophilic bacteria. Therefore, several molecular mechanisms have evolved to cope with high temperature. We have been studying the response of Agrobacterium tumefaciens to temperature stress, focusing on two aspects: the heat-shock response and the temperature-dependent regulation of methionine biosynthesis. The results indicate that the molecular mechanisms involved in A. tumefaciens control of growth at high temperature are unique and we are still missing important information essential for understanding how these bacteria cope with temperature stress.

Differential control of Salmonella heat shock operons by structured mRNAs

DnaK-DnaJ-GrpE and GroES-GroEL are the major chaperone machineries in bacteria. In many species, dnaKJ and groESL are encoded in bicistronic operons. Quantitative proteomics revealed that DnaK and GroEL amounts in Salmonella dominate over DnaJ and GroES respectively. An imperfect transcriptional terminator in the intergenic region of dnaKJ is known to result in higher transcript levels of the first gene. Here, we examined the groESL operon and asked how the second gene in a heat shock operon can be preferentially expressed and found that an RNA structure in the 5'untranslated region of groES is responsible. The secondary structure masks the Shine-Dalgarno (SD) sequence and AUG start codon and thereby modulates translation of groES mRNA. Reporter gene assays combined with structure probing and toeprinting analysis revealed a dynamic temperature-sensitive RNA structure. Following an increase in temperature, only the second of two RNA hairpins melts and partially liberates the SD sequence, thus facilitating translation. Translation of groEL is not temperature-regulated leading to an excess of the chaperonin in the cell at low temperature. Discussion in a broader context shows how structured RNA segments can differentially control expression of temperature-affected operons in various ways.

Characterization and expression analysis of the groESL operon of Bartonella bacilliformis

The groESL operon of Bartonella bacilliformis, a facultative intracellular, Gram-negative bacterium and etiologic agent of Oroya Fever, was characterized. Sequence analysis revealed an operon containing two genes of 294 (groES) and 1632 nucleotides (groEL) separated by a 55-nt intergenic spacer. The operon is preceded by a 72-nt ORF (ORF1) that encodes a hypothetical protein with homology to a portion of the HrcA repressor for groESL. A divergent fumarate hydratase C (fumC) gene lies further upstream. Deduced amino acid sequences for B. bacilliformis GroEL and GroES revealed a high degree of identity with homologues from other Bartonella and alpha-Protebacteria. A single transcriptional start site (TSS) was mapped 79 nucleotides upstream of the groES start codon, regardless of incubation temperature. The TSS was located immediately 5' to a potential controlling inverted repeat of chaperonin expression (CIRCE) element and is preceded by a sigma70-like promoter. The operon is followed by a predicted rho-independent transcriptional terminator. Northern blot analysis indicated that groES and groEL are co-transcribed as a single mRNA of approximately 2.4 kb. A 6-h time course analysis by qRT-PCR showed that groEL expression increases 1.3-fold within 30 min of a temperature upshift from 30 to 37 degrees C, with maximum transcription reached after 60 min (approximately 4.3-fold), followed by a steady decrease to background (30 degrees C) transcription levels by 6 h. Western blot analysis revealed a 1.4- and 1.5-fold increase in GroEL synthesis following a temperature upshift or by inhibiting DNA supercoiling with coumermycin A1, respectively. Functional expression and complementation of temperature-sensitive Escherichia coli groES or groEL mutants with the cloned operon allowed them to grow at otherwise restrictive temperatures.

Two of the three groEL homologues in Rhizobium leguminosarum are dispensable for normal growth

Although many bacteria contain only a single groE operon encoding the essential chaperones GroES and GroEL, examples of bacteria containing more than one groE operon are common. The root-nodulating bacterium Rhizobium leguminosarum contains at least three operons encoding homologues to Escherichia coli GroEL, referred to as Cpn60.1, Cpn60.2 and Cpn60.3, respectively. We report here a detailed analysis of the requirement for and relative levels of these three proteins. Cpn60.1 is present at higher levels than Cpn60.2, and Cpn60.3 protein could not be detected under any conditions although the cpn60.3 gene is transcribed under anaerobic conditions. Insertion mutations could not be constructed in cpn60.1 unless a complementing copy was present, showing that this gene is essential for growth under the conditions used here. Both cpn60.2 and cpn60.3 could be inactivated with no loss of viability, and a double cpn60.2 cpn60.3 mutant was also constructed which was fully viable. Thus only Cpn60.1 is required for growth of this organism.

Cloning, molecular characterization and expression analysis of two small heat shock genes isolated from wine Lactobacillus plantarum

AIM

Understanding the molecular response to stress tolerance of wine Lactobacillus plantarum.

METHODS AND RESULTS

Two genes codifying for heat shock proteins were cloned from wine L. plantarum. The coding regions of the two heat shock genes are 420 and 444 nucleotides long, and started with an ATG codon suggesting that they were translated. The protein sequences deduced from the isolated genes have a molecular mass of 18.483 and 19.282 kDa, respectively, and were therefore named hsp18.5 and hsp19.3. The expression of small heat shock genes was analysed by RT-PCR analysis. Moreover, the 5' and 3' noncoding regions were cloned and sequenced.

CONCLUSIONS

The expression of the heat shock genes was strongly induced by heat, cold and ethanol stress. Analysis of the 5' and 3' flanking regions of hsp18.5 and hsp19.3 genes, revealed the presence of an inverted repeat sequence (TTAGCACTC-N(9)-GAGTGCTAA) homologue to the CIRCE elements found to the upstream regulatory region of heat shock operons, and an inverted sequence that could form a stem and loop structure that it is likely to function as a transcriptional terminator. Based on their structures, the genes were classified as belonging to Class I of heat shock genes according to the B. subtilis nomenclature of heat response genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Small heat shock genes isolated from wine L. plantarum might have a role in preventing damage by cold stress.

Molecular and functional analysis of the Rickettsia typhi groESL operon

The groESL operon from an obligate, intracellular, Gram-negative bacterium Rickettsia typhi, the etiologic agent of murine typhus, was cloned and sequenced. The sequence analysis of 2229 bp of the groESL operon reveals two open reading frames of 288 nucleotides (groES) and 1653 nucleotides (groEL) separated by 20 nucleotides. The deduced amino acid sequence of R. typhi GroES and GroEL shows a high degree of identity with other bacterial GroES and GroEL. Reverse transcriptase-polymerase chain reaction and Northern blot analysis indicated that both groES and groEL are transcribed as a single mRNA. The transcriptional start point at 81 nucleotides upstream of the groES start codon was determined by primer extension. The promoter analysis shows no regulatory CIRCE element as it is known for many Gram-positive and Gram-negative bacteria. However, it contains the sequence similar to the putative sigma(70)-dependent promoter and lacks the -35 sequence of the putative sigma(32)-dependent promoter. Complementation assay by R. typhi groESL in a temperature sensitive Escherichia coli groEL mutant restored significant growth ability at non-permissive temperature.

Differential expression of virulence-related genes in Enterococcus faecalis in response to biological cues in serum and urine

Enterococci rank among leading causes of nosocomial bacteremia and urinary tract infection and are also a leading cause of community acquired subacute endocarditis. Limited evidence suggests that biological cues in serum and urine may play an important role in modulating enterococcal virulence at sites of infection. To determine the extent to which biological cues affect enterococcal virulence-associated gene expression, we used quantitative real-time PCR to compare mRNA levels in Enterococcus faecalis cultures grown in serum or urine to that achieved in laboratory medium. Both environment- and growth phase-specific variations were observed, demonstrating the occurrence of as-yet-uncharacterized mechanisms for control of gene expression in E. faecalis that may play an important role in vivo.

Proteome analysis in the study of the bacterial heat-shock response

In recent years, it has become clear that, in addition to the regulation of the expression of specific genes, there are global regulatory systems that control the simultaneous expression of a large number of genes in response to a variety of environmental stresses. The first of these global control systems, and of substantial importance, is the heat-shock response. The heat-shock response is characterized by the induction of a large set of proteins (heat-shock proteins-HSPs) upon shifts to higher temperature and upon exposure to conditions in which proteins are denatured (i.e., alcohols, heavy metals). The heat-shock response is universal and many of the heat-shock proteins are highly conserved among species. In bacteria, the heat-shock response has been studied extensively in several Gram-positive bacteria (Bacillus subtilis) and in the Gram-negative bacteria (i.e., Escherichia coli, Agrobacterium tumefaciens). The first recognition of the molecular abundance of the bacterial heat-shock proteins took place with the introduction of high-resolution two-dimensional polyacrylamide gels (2D gels) to analyze complex mixtures of cellular proteins. Two-dimensional gels, followed by mass spectrometry, were used to define the heat-shock stimulons in several bacteria, and to study the regulatory elements that control the heat-shock response. Here, we review the heat-shock response and its regulation in bacteria. The review will emphasize the use of proteome analysis in the study of this response, and will point out those open questions that can be investigated with proteomics, including mass spectrometry techniques.

Heat shock proteome of Agrobacterium tumefaciens: evidence for new control systems

The regulation of Agrobacterium tumefaciens heat shock genes involves a transcriptional activator (RpoH) and repressor elements (HrcA-CIRCE). Using proteome analysis and mutants in these control elements, we show that the heat shock induction of 32 (out of 56) heat shock proteins is independent of RpoH and HrcA. These results indicate the existence of additional regulatory factors in the A. tumefaciens heat shock response.

Stress-induced proteins of Agrobacterium tumefaciens

The pattern of proteins produced by bacteria represents the physiological state of the organism as well as the environmental conditions encountered. Environmental stress induces the expression of several regulons encoding stress proteins. Extensive information about the proteins which constitute these regulons (or stimulons) and their control is available for very few bacteria, such as the Gram-positive Bacillus subtilis and the Gram-negative Escherichia coli (gamma-proteobacteria) and is minimal for all other bacteria. Agrobacterium tumefaciens is a Gram-negative plant pathogen of the alpha-proteobacteria, which constitutes the main tool for plant recombinant genetics. Our previous studies on the control of chaperone-coding operons indicated that A. tumefaciens has unique features and combines regulatory elements from both B. subtilis and E. coli. Therefore, we examined the patterns of proteins induced in A. tumefaciens by environmental changes using two-dimensional gel electrophoresis and dual-channel image analysis. Shifts to high temperature, oxidative and mild acid stresses stimulated the expression of 97 proteins. The results indicate that most of these stress-induced proteins (80/97) were specific to one stress stimulon. Only 10 proteins appear to belong to a general stress regulon.

Transcription of the nfrA-ywcH operon from Bacillus subtilis is specifically induced in response to heat

The NfrA protein, an oxidoreductase from the soil bacterium Bacillus subtilis, is synthesized during the stationary phase and in response to heat. Analysis of promoter mutants revealed that the nfrA gene belongs to the class III heat shock genes in B. subtilis. An approximate 10-fold induction at both the transcriptional and the translational levels was found after thermal upshock. This induction resulted from enhanced synthesis of mRNA. Genetic and Northern blot analyses revealed that nfrA and the gene downstream of nfrA are transcribed as a bicistronic transcriptional unit. The unstable full-length transcript is processed into two short transcripts encoding nfrA and ywcH. The nfrA-ywcH operon is not induced by salt stress or by ethanol. According to previously published data, the transcription of class III genes in general is activated in response to the addition of these stressors. However, this conclusion is based on experiments which lacked a valid control. Therefore, it seems possible that the transcription of all class III genes is specifically induced by heat shock.

Messenger RNA stability and its role in control of gene expression in bacteria and phages

The stability of mRNA in prokaryotes depends on multiple factors and it has not yet been possible to describe the process of mRNA degradation in terms of a unique pathway. However, important advances have been made in the past 10 years with the characterization of the cis-acting RNA elements and the trans-acting cellular proteins that control mRNA decay. The trans-acting proteins are mainly four nucleases, two endo- (RNase E and RNase III) and two exonucleases (PNPase and RNase II), and poly(A) polymerase. RNase E and PNPase are found in a multienzyme complex called the degradosome. In addition to the host nucleases, phage T4 encodes a specific endonuclease called RegB. The cis-acting elements that protect mRNA from degradation are stable stem-loops at the 5' end of the transcript and terminators or REP sequences at their 3' end. The rate-limiting step in mRNA decay is usually an initial endonucleolytic cleavage that often occurs at the 5' extremity. This initial step is followed by directional 3' to 5' degradation by the two exonucleases. Several examples, reviewed here, indicate that mRNA degradation is an important step at which gene expression can be controlled. This regulation can be either global, as in the case of growth rate-dependent control, or specific, in response to changes in the environmental conditions.

Differential and independent roles of a sigma(32) homolog (RpoH) and an HrcA repressor in the heat shock response of Agrobacterium tumefaciens

The heat shock response in alpha proteobacteria is unique in that a combination of two regulators is involved: a positive regulator, RpoH (sigma(32) homolog), found in the alpha, beta, and gamma proteobacteria, and a negative regulator, HrcA, widely distributed in eubacteria but not in the gamma proteobacteria. To assess the differential roles of the two regulators in these bacteria, we cloned the hrcA-grpE operon of Agrobacterium tumefaciens, analyzed its transcription, and constructed deletion mutants lacking RpoH and/or HrcA. The DeltarpoH mutant and DeltarpoH DeltahrcA double mutant were unable to grow above 30 degrees C. Whereas the synthesis of heat shock proteins (e.g., DnaK, GroEL, and ClpB) was transiently induced upon temperature upshift from 25 to 37 degrees C in the wild type, such induction was not observed in the DeltarpoH mutant, except that GroEL synthesis was still partially induced. By contrast, the DeltahrcA mutant grew normally and exhibited essentially normal heat induction except for a higher level of GroEL expression, especially before heat shock. The DeltarpoH DeltahrcA double mutant showed the combined phenotypes of each of the single mutants. The amounts of dnaK and groE transcripts before and after heat shock, as determined by primer extension, were consistent with those of the proteins synthesized. The cellular level of RpoH but not HrcA increased significantly upon heat shock. We conclude that RpoH plays a major and global role in the induction of most heat shock proteins, whereas HrcA plays a restricted role in repressing groE expression under nonstress conditions.

Molecular characterisation of the dnaK operon of Lactobacillus sakei LTH681

The use of lactobacilli as starter organisms in food fermentation processes requires thorough knowledge of their reaction to the multitude of ecological factors including their response to stress. We have characterised the dnaK gene region of Lactobacillus sakei LTH681. Two chromosomal EcoRI fragments of 2.5 and 4.0 kb were identified using a homologous dnaK probe generated by PCR. The sequence analysis of the cloned fragments showed that the dnaK gene region consists of four heat shock genes with the organisation hrcA-grpE-dnaK-dnaJ. Comparison of the deduced amino acid sequences revealed high similarity to the corresponding heat shock proteins of Gram-positive bacteria. An upstream located orfY was found which exhibited substantial similarity (41.5%) to the chloramphenicol acetyltransferase of Enterobacter aerogenes. Northern hybridisation analysis revealed that the transcription of the genes is induced by heat shock (42 degrees C) as well as salt (6%) or ethanol (10%) stress. Several transcripts were detected including a polycistronic mRNA of 4.9 kb which represents the transcript of the complete dnaK gene region indicating a tetracistronic organisation of the dnaK operon. The other RNA fragments were identified as shorter transcripts (3.7 and 1.3 kb) or cleavage products of the polycistronic mRNAs. The transcription start sites of the dnaK operon were determined under inducing and non-inducing conditions. The site varied with the applied stress condition. A regulatory CIRCE element was identified located between the transcription and translation start site. The promoter region including CIRCE was transcriptionally fused to the beta-glucuronidase reporter gene gusA and expressed in L. sakei LTH681. The kinetics of transcriptional induction of gusA by heat shocking were identical to those of the dnaK operon confirming the involvement of the CIRCE element in regulation of gene expression.

Regulation of heat-shock response in bacteria

Stress response in bacteria is essential for effective adaptation to changes in the environment, as well as to the changes in the physiological state of the bacterial culture itself. This response is mediated by global regulatory mechanisms affecting several pathways. It now appears that these regulatory mechanisms operate by transcriptional control, translational control, and proteolysis. One example to be discussed extensively is the heat-shock response. In Escherichia coli, where it has been studied initially and most extensively, the expression of the heat-shock operon is transcriptionally controlled by the employment of the heat-shock transcription factor sigma 32, that recognizes specific heat-shock promoters. Later studies indicated that in most bacteria the control of the major heat-shock genes is much more complicated, and involves additional--or alternative--control channels. These regulatory elements will be reviewed looking at the groE and dnaK operons. These operons, coding for the bacterial equivalent of Hsp10+60 and Hsp70, respectively, contain in many bacteria a conserved regulatory inverted repeat (IR = CIRCE), and are transcribed either by the vegetative sigma factor--sigma 70--or by a sigma 32-like factor. The IR functions at the DNA level as a repressor binding site and also controls the half life of the transcript. In addition, in Agrobacterium tumefaciens there also exists a system for mRNA processing that involves a temperature-controlled cleavage of the groE transcript.

Potato leafroll virus binds to the equatorial domain of the aphid endosymbiotic GroEL homolog

A GroEL homolog with a molecular mass of 60 kDa, produced by the primary endosymbiotic bacterium (a Buchnera sp.) of Myzus persicae and released into the hemolymph, has previously been shown to be a key protein in the transmission of potato leafroll virus (PLRV). Like other luteoviruses and pea enation mosaic virus, PLRV readily binds to extracellular Buchnera GroEL, and in vivo interference in this interaction coincides with reduced capsid integrity and loss of infectivity. To gain more knowledge of the nature of the association between PLRV and Buchnera GroEL, the groE operon of the primary endosymbiont of M. persicae (MpB groE) and its flanking sequences were characterized and the PLRV-binding domain of Buchnera GroEL was identified by deletion mutant analysis. MpB GroEL has extensive sequence similarity (92%) with Escherichia coli GroEL and other members of the chaperonin-60 family. The genomic organization of the Buchnera groE operon is similar to that of the groE operon of E. coli except that a constitutive promoter sequence could not be identified; only the heat shock promoter was present. By a virus overlay assay of protein blots, it was shown that purified PLRV bound as efficiently to recombinant MpB GroEL (expressed in E. coli) as it did to wild-type MpB GroEL. Mutational analysis of the gene encoding MpB GroEL revealed that the PLRV-binding site was located in the so-called equatorial domain and not in the apical domain which is generally involved in polypeptide binding and folding. Buchnera GroEL mutants lacking the entire equatorial domain or parts of it lost the ability to bind PLRV. The equatorial domain is made up of two regions at the N and C termini that are not contiguous in the amino acid sequence but are in spatial proximity after folding of the GroEL polypeptide. Both the N- and C-terminal regions of the equatorial domain were implicated in virus binding.

Cloning, characterization and functional analysis of groESL operon from thermophilic cyanobacterium Synechococcus vulcanus

Genes encoding 10914 Da and 58267 Da polypeptides homologous to groES and groEL of Escherichia coli were cloned and sequenced from a thermophilic cyanobacterium, Synechococcus vulcanus. The deduced amino acid sequence of the GroEL protein was much more homologous to GroELs of other cyanobacteria which accompany GroES than another GroEL homolog of S. vulcanus (GroEL2) reported previously (M. Furuki, N. Tanaka, T. Hiyama, and H. Nakamoto, Biochim. Biophys. Acta 1294 (1996) 106-110). We designate the gene as groEL1 to distinguish it from the non-operon forming groEL2 gene. A 9-base pair inverted repeat sequence (TTAGCACTC-N9-GAGTGCTAA) was located upstream of the promoter region of groEL1, which was absent in groEL2. Southern blot analysis indicated that only one groESL1 operon was present in the genomic DNA of S. vulcanus. The amount of the bicistronic, 2.3 kb transcript of groESL1 operon increased 30-fold within 30 min upon heat shock. The increase was completely inhibited by chloramphenicol, suggesting the involvement of heat-induced production of a polypeptide. Introduction of the cloned groEL1 gene into a groEL defective mutant of E. coli resulted in the complementation of heat sensitivity, which contrasted with the previous result with groEL2.

Transcriptional analysis of the groESL operon from Porphyromonas gingivalis

Transcriptional analysis of the groESL operon from Porphyromonas gingivalis, one of the obligative anaerobic oral microorganisms implicated in adult periodontitis, was performed. P. gingivalis 381 cultured at 37 degrees C was shifted to 42 degrees C, 45 degrees C or 48 degrees C for 10 mins. Northern hybridization analysis revealed that a band with 2.1-kb (kilo base pair) was observed, and the transcripts increased greatly by heat shock. Primer extension and S1 mapping detected four different 5'-ending sites of the mRNAs at the upstream region of the groES. Three sites out of the four were heat-inducible. There were inverted repeats and a Escherichia coli sigma 32-recognizing consensus sequence in the promoter region of the groESL, which may be relevant to the regulation of transcription of groESL operon in P. gingivalis. Both a heat shock promoter and inverted repeats may be relevant to the transcriptional regulation of the groESL operon in P. gingivalis.

Effect of the pufQ-pufB intercistronic region on puf mRNA stability in Rhodobacter capsulatus

Differential expression of genes localized within the polycistronic puf operon of Rhodobacter capsulatus is partly due to altered stabilities of individual mRNA segments. We show that the 5' untranslated region (UTR) of pufB contributes to the unusual longevity of the 0.5 kb light-harvesting (LH) I specific pufBA mRNA and of the 2.7 kb pufBALMX mRNA. Three stem-loop structures have been identified within the pufQ-pufB intercistronic region by means of RNA secondary-structure analysis in vitro and in vivo. Deletion analysis of the pufB 5' UTR indicates that the complete set of secondary structures is required to maintain wild-type levels of pufBA mRNA stability. A phylogenetic comparison of pufB 5' UTRs of other photosynthetic bacteria reveals an evolutionary conservation of the base-pairing potential despite sequence divergence. Comparison of puf mRNA decay in Escherichia coli strains with or without endoribonuclease E (RNase E) activity suggests that the pufB 5' secondary structures protect the downstream mRNA segment against degradation by RNase E. Removal of the 117-nucleotide pufQ-pufB intercistronic region results in loss of stability for the pufBA and pufBALMX mRNAs with concomitant stabilization of the full-length puf primary transcript (QBALMX). We therefore conclude that the deleted sequence functions both as a stabilizing element for pufBALMX and pufBA segments and as a target site for initial rate-limiting decay of the unstable pufQBALMX mRNA.

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.

Regulation of groE expression in Bacillus subtilis: the involvement of the sigma A-like promoter and the roles of the inverted repeat sequence (CIRCE)

To study the regulatory mechanism controlling the heat-inducible expression of Bacillus subtilis groE, two regulatory elements, the sigma A-like promoter and the inverted repeat (IR [CIRCE]) in the control region, were characterized. The groE promoter was shown to be transcribed by the major RNA polymerase under both heat shock and non-heat shock conditions. The IR was found to have two functions. (i) It ensures the fast turnover of the groE transcript, and (ii) it serves as an operator. This IR acts as a negative heat shock regulatory element, since deletion of this sequence resulted in high-level expression of groE even at 37 degrees C. Although this IR is present in the 5' untranslated region of the groE transcript, groE transcripts under heat shock and non-heat shock conditions showed similar in vivo half-lives of 5 min. This rapid turnover at 37 degrees C requires the presence of the IR. Without the IR, the groE transcript showed a longer half-life of 17 min. Increasing the distance between the groE transcription start site and the IR systematically by inserting nucleotide sequences from 5 to 21 bp in length resulted in a gradual abolition of the negative regulatory effect mediated by the IR. This effect was not due to a significant change in transcript stability or the transcription start site and is consistent with the model that this IR serves as an operator.

The dnaKJ operon of Agrobacterium tumefaciens: transcriptional analysis and evidence for a new heat shock promoter

The dnaKJ operon of Agrobacterium tumefaciens was cloned and sequenced and was found to be highly homologous to previously analyzed dnaKJ operons. Transcription of this operon in A. tumefaciens was stimulated by heat shock as well as by exposure to ethanol and hydrogen peroxide. There were two transcripts representing the dnaKJ operon: one containing the dnaK and dnaJ genes and the second containing only the dnaK gene. Primer extension analysis indicated that transcription started from the same site in heat-shocked cells and in untreated cells. The upstream regulatory region of the dnaKJ operon of A. tumefaciens does not contain the highly conserved inverted repeat sequence previously found in the groESL operon of this bacterium, as well as in many other groE and dnaK operons. Sequence analysis of the promoter region of several groESL and dnaK operons from alpha-purple proteobacteria indicates the existence of a putative promoter sequence different from the known consensus promoter sequences recognized by the Escherichia coli vegetative or heat shock sigma factor. This promoter may constitute the heat shock promoter of these alpha-purple proteobacteria.