Transcription of the Escherichia coli rrnB P1 promoter by the heat shock RNA polymerase (E sigma 32) in vitro

Threshold accumulation of a constitutive protein explains E. coli cell-division behavior in nutrient upshifts

Despite a boost of recent progress in dynamic single-cell measurements and analyses in Escherichia coli, we still lack a mechanistic understanding of the determinants of the decision to divide. Specifically, the debate is open regarding the processes linking growth and chromosome replication to division and on the molecular origin of the observed "adder correlations," whereby cells divide, adding roughly a constant volume independent of their initial volume. In order to gain insight into these questions, we interrogate dynamic size-growth behavior of single cells across nutrient upshifts with a high-precision microfluidic device. We find that the division rate changes quickly after nutrients change, much before growth rate goes to a steady state, and in a way that adder correlations are robustly conserved. Comparison of these data to simple mathematical models falsifies proposed mechanisms, where replication-segregation or septum completions are the limiting step for cell division. Instead, we show that the accumulation of a putative constitutively expressed "P-sector divisor" protein explains the behavior during the shift.

Regulation of heat-shock genes in bacteria: from signal sensing to gene expression output

The heat-shock response is a mechanism of cellular protection against sudden adverse environmental growth conditions and results in the prompt production of various heat-shock proteins. In bacteria, specific sensory biomolecules sense temperature fluctuations and transduce intercellular signals that coordinate gene expression outputs. Sensory biomolecules, also known as thermosensors, include nucleic acids (DNA or RNA) and proteins. Once a stress signal is perceived, it is transduced to invoke specific molecular mechanisms controlling transcription of genes coding for heat-shock proteins. Transcriptional regulation of heat-shock genes can be under either positive or negative control mediated by dedicated regulatory proteins. Positive regulation exploits specific alternative sigma factors to redirect the RNA polymerase enzyme to a subset of selected promoters, while negative regulation is mediated by transcriptional repressors. Interestingly, while various bacteria adopt either exclusively positive or negative mechanisms, in some microorganisms these two opposite strategies coexist, establishing complex networks regulating heat-shock genes. Here, we comprehensively summarize molecular mechanisms that microorganisms have adopted to finely control transcription of heat-shock genes.

Characterization of the transcriptional stimulatory properties of the Pseudomonas putida RapA protein

RNA polymerase-associated factors can significantly affect its performance at specific promoters. Here we identified a Pseudomonas putida RNA polymerases-associated protein as a homolog of Escherichia coli RapA. We found that P. putida RapA stimulates the transcription from promoters dependent on a variety of σ-factors (σ(70), σ(S), σ(54), σ(32), σ(E)) in vitro. The level of stimulation varied from 2- to 10-fold, with the maximal effect observed with the σ(E)-dependent PhtrA promoter. Stimulation by RapA was apparent in the multi-round reactions and was modulated by salt concentration in vitro. However, in contrast to findings with E. coli RapA, P. putida RapA-mediated stimulation of transcription was also evident using linear templates. These properties of P. putida RapA were apparent using either E. coli- or P. putida-derived RNA polymerases. Analysis of individual steps of transcription revealed that P. putida RapA enhances the stability of competitor-resistant open-complexes formed by RNA polymerase at promoters. In vivo, P. putida RapA can complement the inhibitory effect of high salt on growth of an E. coli RapA null strain. However, a P. putida RapA null mutant was not sensitive to high salt. The in vivo effects of lack of RapA were only detectable for the σ(E)-PhtrA promoter where the RapA-deficiency resulted in lower activity. The presented characteristics of P. putida RapA indicate that its functions may extend beyond a role in facilitating RNA polymerase recycling to include a role in transcription initiation efficiency.

Cooperative regulation of the Vibrio vulnificus nan gene cluster by NanR protein, cAMP receptor protein, and N-acetylmannosamine 6-phosphate

The nan cluster of Vibrio vulnificus, a food-borne pathogen, consists of two divergently transcribed operons, nanT(PSL)AR and nanEK nagA, required for transport and catabolism of N-acetylneuraminic acid (Neu5Ac). A mutation of nanR abolished the extensive lag phase observed for the bacteria growing on Neu5Ac and increased transcription of nanT(P) and nanE, suggesting that NanR is a transcriptional repressor of both nan operons. Intracellular accumulation of Neu5Ac was dependent on the carbon source, implying that the nan operons are also subject to catabolite repression. Hence, cAMP receptor protein (CRP) appeared to activate and repress transcription of nanT(PSL)AR and nanEK nagA, respectively. Direct bindings of NanR and CRP to the nanT(P)-nanE intergenic DNA were demonstrated by EMSA. Two adjacent NanR-binding sites centered at +44.5 and -10 and a CRP-binding site centered at -60.5 from the transcription start site of nanT(P) were identified by DNase I protection assays. Mutagenesis approaches, in vitro transcription, and isothermal titration calorimetry experiments demonstrated that N-acetylmannosamine 6-phosphate specifically binds to NanR and functions as the inducer of the nan operons. The combined results propose a model in which NanR, CRP, and N-acetylmannosamine 6-phosphate cooperate for precise adjustment of the expression level of the V. vulnificus nan cluster.

Late steps of ribosome assembly in E. coli are sensitive to a severe heat stress but are assisted by the HSP70 chaperone machine

The late stages of 30S and 50S ribosomal subunits biogenesis have been studied in a wild-type (wt) strain of Escherichia coli (MC4100) subjected to a severe heat stress (45–46°C). The 32S and 45S ribosomal particles (precursors to 50S subunits) and 21S ribosomal particles (precursors to 30S subunits) accumulate under these conditions. They are authentic precursors, not degraded or dead-end particles. The 21S particles are shown, by way of a modified 3′5′ RACE procedure, to contain 16S rRNA unprocessed, or processed at its 5′ end, and not at the 3′ end. This implies that maturation of 16S rRNA is ordered and starts at its 5′-terminus, and that the 3′-terminus is trimmed at a later step. This observation is not limited to heat stress conditions, but it also can be verified in bacteria growing at a normal temperature (30°C), supporting the idea that this is the general pathway. Assembly defects at very high temperature are partially compensated by plasmid-driven overexpression of the DnaK/DnaJ chaperones. The ribosome assembly pattern in wt bacteria under a severe heat stress is therefore reminiscent of that observed at lower temperatures in E. coli mutants lacking the chaperones DnaK or DnaJ.

Development and application of versatile high density microarrays for genome-wide analysis of Streptomyces coelicolor: characterization of the HspR regulon

BACKGROUND

DNA microarrays are a key resource for global analysis of genome content, gene expression and the distribution of transcription factor binding sites. We describe the development and application of versatile high density ink-jet in situ-synthesized DNA arrays for the G+C rich bacterium Streptomyces coelicolor. High G+C content DNA probes often perform poorly on arrays, yielding either weak hybridization or non-specific signals. Thus, more than one million 60-mer oligonucleotide probes were experimentally tested for sensitivity and specificity to enable selection of optimal probe sets for the genome microarrays. The heat-shock HspR regulatory system of S. coelicolor, a well-characterized repressor with a small number of known targets, was exploited to test and validate the arrays for use in global chromatin immunoprecipitation-on-chip (ChIP-chip) and gene expression analysis.

RESULTS

In addition to confirming dnaK, clpB and lon as in vivo targets of HspR, it was revealed, using a novel ChIP-chip data clustering method, that HspR also apparently interacts with ribosomal RNA (rrnD operon) and specific transfer RNA genes (the tRNAGln/tRNAGlu cluster). It is suggested that enhanced synthesis of Glu-tRNAGlu may reflect increased demand for tetrapyrrole biosynthesis following heat-shock. Moreover, it was found that heat-shock-induced genes are significantly enriched for Gln/Glu codons relative to the whole genome, a finding that would be consistent with HspR-mediated control of the tRNA species.

CONCLUSIONS

This study suggests that HspR fulfils a broader, unprecedented role in adaptation to stresses than previously recognized -- influencing expression of key components of the translational apparatus in addition to molecular chaperone and protease-encoding genes. It is envisaged that these experimentally optimized arrays will provide a key resource for systems level studies of Streptomyces biology.

Vibrio vulnificus rpoS expression is repressed by direct binding of cAMP-cAMP receptor protein complex to its two promoter regions

Vibrio vulnificus, a septicemia-causing pathogenic bacterium, acquires resistance against various stresses and expresses virulence factors via an rpoS gene product. In this study, we investigated the transcriptional characteristics of this global regulator. Two distinct transcriptional initiation sites for the rpoS gene, the proximal promoter (P(p)) and the distal promoter (P(d)), were defined by primer extension experiments. Various rpoS::luxAB transcriptional fusions indicated that P(d) is a major promoter of rpoS expression. Western blot analysis showed that RpoS levels were inversely correlated with intracellular levels of 3',5'-cyclic AMP (cAMP). The expressions of both P(d) and P(p) were increased in cya and crp mutants. The exogenous addition of cAMP to the cya mutant resulted in repressed expression of rpoS. In addition, rpoS expression was significantly lowered in the cpdA mutant, in which the level of cAMP was elevated because of the absence of 3',5'-cAMP phosphodiesterase. In vitro transcription assays using the V. vulnificus RNA polymerase showed that the transcripts from both promoters were reduced by addition of the cAMP-cAMP receptor protein (CRP). The cAMP-CRP was shown to bind to two rpoS promoters by electrophoretic mobility shift assays. The alteration of the putative CRP-binding site on each rpoS promoter, via site-directed mutagenesis, abolished the binding of cAMP-CRP as well as regulation by cAMP-CRP. Therefore, this study shows a relationship between the level of intracellular cAMP and the degree of rpoS expression and further demonstrates, for the first time, the direct binding of the cAMP-CRP complex to rpoS upstream regions, which results in repression of rpoS gene expression.

Substitution of a highly conserved histidine in the Escherichia coli heat shock transcription factor, sigma32, affects promoter utilization in vitro and leads to overexpression of the biofilm-associated flu protein in vivo

The heat shock sigma factor (sigma(32) in Escherichia coli) directs the bacterial RNA polymerase to promoters of a specific sequence to form a stable complex, competent to initiate transcription of genes whose products mitigate the effects of exposure of the cell to high temperatures. The histidine at position 107 of sigma(32) is at the homologous position of a tryptophan residue at position 433 of the main sigma factor of E. coli, sigma(70). This tryptophan is essential for the strand separation step leading to the formation of the initiation-competent RNA polymerase-promoter complex. The heat shock sigma factors of all gammaproteobacteria sequenced have a histidine at this position, while in the alpha- and deltaproteobacteria, it is a tryptophan. In vitro the alanine-for-histidine substitution at position 107 (H107A) destabilizes complexes between the GroE promoter and RNA polymerase containing sigma(32), implying that H107 plays a role in formation or maintenance of the strand-separated complex. In vivo, the H107A substitution in sigma(32) impedes recovery from heat shock (exposure to 42 degrees C), and it also leads to overexpression at lower temperatures (30 degrees C) of the Flu protein, which is associated with biofilm formation.

Crl facilitates RNA polymerase holoenzyme formation

The Escherichia coli Crl protein has been described as a transcriptional coactivator for the stationary-phase sigma factor sigma(S). In a transcription system with highly purified components, we demonstrate that Crl affects transcription not only by the Esigma(S) RNA polymerase holoenzyme but also by Esigma(70) and Esigma(32). Crl increased transcription dramatically but only when the sigma concentration was low and when Crl was added to sigma prior to assembly with the core enzyme. Our results suggest that Crl facilitates holoenzyme formation, the first positive regulator identified with this mechanism of action.

Regulon and promoter analysis of the E. coli heat-shock factor, sigma32, reveals a multifaceted cellular response to heat stress

The heat-shock response (HSR), a universal cellular response to heat, is crucial for cellular adaptation. In Escherichia coli, the HSR is mediated by the alternative sigma factor, sigma32. To determine its role, we used genome-wide expression analysis and promoter validation to identify genes directly regulated by sigma32 and screened ORF overexpression libraries to identify sigma32 inducers. We triple the number of genes validated to be transcribed by sigma32 and provide new insights into the cellular role of this response. Our work indicates that the response is propagated as the regulon encodes numerous global transcriptional regulators, reveals that sigma70 holoenzyme initiates from 12% of sigma32 promoters, which has important implications for global transcriptional wiring, and identifies a new role for the response in protein homeostasis, that of protecting complex proteins. Finally, this study suggests that the response protects the cell membrane and responds to its status: Fully 25% of sigma32 regulon members reside in the membrane and alter its functionality; moreover, a disproportionate fraction of overexpressed proteins that induce the response are membrane localized. The intimate connection of the response to the membrane rationalizes why a major regulator of the response resides in that cellular compartment.

Extensive functional overlap between sigma factors in Escherichia coli

Bacterial core RNA polymerase (RNAP) must associate with a sigma factor to recognize promoter sequences. Escherichia coli encodes seven sigma factors, each believed to be specific for a largely distinct subset of promoters. Using microarrays representing the entire E. coli genome, we identify 87 in vivo targets of sigma32, the heat-shock sigma factor, and estimate that there are 120-150 sigma32 promoters in total. Unexpectedly, 25% of these sigma32 targets are located within coding regions, suggesting novel regulatory roles for sigma32. The majority of sigma32 promoter targets overlap with those of sigma70, the housekeeping sigma factor. Furthermore, their DNA sequence motifs are often interdigitated, with RNAPsigma70 and RNAPsigma32 initiating transcription in vitro with similar efficiency and from identical positions. SigmaE-regulated promoters also overlap extensively with those for sigma70. These results suggest that extensive functional overlap between sigma factors is an important phenomenon.

Holoenzyme Switching and Stochastic Release of Sigma Factors from RNA Polymerase In Vivo

We investigated the binding of E. coli RNA polymerase holoenzymes bearing sigma70, sigma(S), sigma32, or sigma54 to the ribosomal RNA operons (rrn) in vivo. At the rrn promoter, we observed "holoenzyme switching" from Esigma70 to Esigma(S) or Esigma32 in response to environmental cues. We also examined if sigma factors are retained by core polymerase during transcript elongation. At the rrn operons, sigma70 translocates briefly with the elongating polymerase and is released stochastically from the core polymerase with an estimated half-life of approximately 4-7 s. Similarly, at gadA and htpG, operons that are targeted by Esigma(S) and Esigma32, respectively, we find that sigma(S) and sigma32 also dissociate stochastically, albeit more rapidly than sigma70, from the elongating core polymerase. Up to approximately 70% of Esigma70 (the major vegetative holoenzyme) in rapidly growing cells is engaged in transcribing the rrn operons. Thus, our results suggest that at least approximately 70% of cellular holoenzymes release sigma70 during transcript elongation. Release of sigma factors during each round of transcription provides a simple mechanism for rapidly reprogramming polymerase with the relevant sigma factor and is consistent with the occurrence of a "sigma cycle" in vivo.

rRNA antitermination functions with heat shock promoters

Transcription antitermination in the rRNA operons of Escherichia coli requires a unique nucleic acid sequence that serves as a signal for modification of the elongating RNA polymerase, making it resistant to Rho-dependent termination. We examined the antitermination ability of RNA polymerase elongation complexes that had initiated at three different heat shock promoters, dnaK, groE, and clpB, and then transcribed the antitermination sequence to read through a Rho-dependent terminator. Terminator bypass comparable to that seen with sigma(70) promoters was obtained. Lack of or inversion of the sequence abolished terminator readthrough. We conclude that RNA polymerase that uses sigma(32) to initiate transcription can adopt a conformation similar to that of sigma(70)-containing RNA polymerase, enabling it to interact with auxiliary modifying proteins and bypass Rho-dependent terminators.

UP element-dependent transcription at the Escherichia coli rrnB P1 promoter: positional requirements and role of the RNA polymerase alpha subunit linker

The UP element stimulates transcription from the rrnB P1 promoter through a direct interaction with the C-terminal domain of the RNA polymerase alpha subunit (alphaCTD). We investigated the effect on transcription from rrnB P1 of varying both the location of the UP element and the length of the alpha subunit interdomain linker, separately and in combination. Displacement of the UP element by a single turn of the DNA helix resulted in a large decrease in transcription from rrnB P1, while displacement by half a turn or two turns totally abolished UP element-dependent transcription. Deletions of six or more amino acids from within the alpha subunit linker resulted in a decrease in UP element-dependent stimulation, which correlated with decreased binding of alphaCTD to the UP element. Increasing the alpha linker length was less deleterious to RNA polymerase function at rrnB P1 but did not compensate for the decrease in activation that resulted from displacing the UP element. Our results suggest that the location of the UP element at rrnB P1 is crucial to its function and that the natural length of the alpha subunit linker is optimal for utilisation of the UP element at this promoter.

Heat shock RNA polymerase (E sigma(32)) is involved in the transcription of mlc and crucial for induction of the Mlc regulon by glucose in Escherichia coli

Mlc is a global regulator of carbohydrate metabolism. Recent studies have revealed that Mlc is depressed by protein-protein interaction with enzyme IICB(Glc), a glucose-specific permease, which is encoded by ptsG. The mlc gene has been previously known to be transcribed by two promoters, P1(+1) and P2(+13), and have a binding site of its own gene product at +16. However, the mechanism of transcriptional regulation of the gene has not yet been established. In vitro transcription assays of the mlc gene showed that P2 promoter could be recognized by RNA polymerase containing the heat shock sigma factor final sigma(32) (E sigma(32)) as well as E sigma(70), while P1 promoter is only recognized by E sigma(70). The cyclic AMP receptor protein and cyclic AMP complex (CRP.cAMP) increased expression from P2 but showed negative effect on transcription from P1 by E sigma(70), although it had little effect on transcription from P2 by E sigma(32) in vitro. Purified Mlc repressed transcription from both promoters, but with different degrees of inhibition. In vivo transcription assays using wild type and mlc strains indicated that the level of mlc expression was modulated less than 2-fold by glucose in the medium with concerted action of CRP.cAMP and Mlc. A dramatic increase in mlc expression was observed upon heat shock or in cells overexpressing final sigma(32), confirming that E sigma(32) is involved in the expression of mlc. Induction of ptsG P1 and pts P0 transcription by glucose was also dependent on E sigma(32). These results indicate that E sigma(32) plays an important role in balancing the relative concentration of Mlc and EIICB(Glc) in response to availability of glucose in order to maintain inducibility of the Mlc regulon at high growth temperature.

UPs and downs in bacterial transcription initiation: the role of the alpha subunit of RNA polymerase in promoter recognition

In recent years, it has become clear that promoter recognition by bacterial RNA polymerase involves interactions not only between core promoter elements and the sigma subunit, but also between a DNA element upstream of the core promoter and the alpha subunit. DNA binding by alpha can increase transcription dramatically. Here we review the current state of our understanding of the alpha interaction with DNA during basal transcription initiation (i.e. in the absence of proteins other than RNA polymerase) and activated transcription initiation (i.e. when stimulated by transcription factors).

Bacterial promoter architecture: subsite structure of UP elements and interactions with the carboxy-terminal domain of the RNA polymerase alpha subunit

We demonstrate here that the previously described bacterial promoter upstream element (UP element) consists of two distinct subsites, each of which, by itself, can bind the RNA polymerase holoenzyme alpha subunit carboxy-terminal domain (RNAP alphaCTD) and stimulate transcription. Using binding-site-selection experiments, we identify the consensus sequence for each subsite. The selected proximal subsites (positions -46 to -38; consensus 5'-AAAAAARNR-3') stimulate transcription up to 170-fold, and the selected distal subsites (positions -57 to -47; consensus 5'-AWWWWWTTTTT-3') stimulate transcription up to 16-fold. RNAP has subunit composition alpha(2)betabeta'sigma and thus contains two copies of alphaCTD. Experiments with RNAP derivatives containing only one copy of alphaCTD indicate, in contrast to a previous report, that the two alphaCTDs function interchangeably with respect to UP element recognition. Furthermore, function of the consensus proximal subsite requires only one copy of alphaCTD, whereas function of the consensus distal subsite requires both copies of alphaCTD. We propose that each subsite constitutes a binding site for a copy of alphaCTD, and that binding of an alphaCTD to the proximal subsite region (through specific interactions with a consensus proximal subsite or through nonspecific interactions with a nonconsensus proximal subsite) is a prerequisite for binding of the other alphaCTD to the distal subsite.

Identification of an UP element consensus sequence for bacterial promoters

The UP element, a component of bacterial promoters located upstream of the -35 hexamer, increases transcription by interacting with the RNA polymerase alpha-subunit. By using a modification of the SELEX procedure for identification of protein-binding sites, we selected in vitro and subsequently screened in vivo for sequences that greatly increased promoter activity when situated upstream of the Escherichia coli rrnB P1 core promoter. A set of 31 of these upstream sequences increased transcription from 136- to 326-fold in vivo, considerably more than the natural rrnB P1 UP element, and was used to derive a consensus sequence: -59 nnAAA(A/T)(A/T)T(A/T)TTTTnnAAAAnnn -38. The most active selected sequence contained the derived consensus, displayed all of the properties of an UP element, and the interaction of this sequence with the alpha C-terminal domain was similar to that of previously characterized UP elements. The identification of the UP element consensus should facilitate a detailed understanding of the alpha-DNA interaction. Based on the evolutionary conservation of the residues in alpha responsible for interaction with UP elements, we suggest that the UP element consensus sequence should be applicable throughout eubacteria, should generally facilitate promoter prediction, and may be of use for biotechnological applications.

Promoter selectivity of the Bradyrhizobium japonicum RpoH transcription factors in vivo and in vitro

Expression of the dnaKJ and groESL1 heat shock operons of Bradyrhizobium japonicum depends on a sigma32-like transcription factor. Three such factors (RpoH1, RpoH2, and RpoH3) have previously been identified in this organism. We report here that they direct transcription from some but not all sigma32-type promoters when the respective rpoH genes are expressed in Escherichia coli. All three RpoH factors were purified as soluble C-terminally histidine-tagged proteins, although the bulk of overproduced RpoH3 was insoluble. The purified proteins were recognized by an anti-E. coli sigma32 serum. While RpoH1 and RpoH2 productively interacted with E. coli core RNA polymerase and produced E. coli groE transcript in vitro, RpoH3 was unable to do so. B. japonicum core RNA polymerase was prepared and reconstituted with the RpoH proteins. Again, RpoH1 and RpoH2 were active, and they initiated transcription at the B. japonicum groESL1 and dnaKJ promoters. In all cases, the in vitro start site was shown to be identical to the start site determined in vivo. Promoter competition experiments revealed that the B. japonicum dnaKJ and groESL1 promoters were suboptimal for transcription by RpoH1- or RpoH2-containing RNA polymerase from B. japonicum. In a mixture of different templates, the E. coli groESL promoter was preferred over any other promoter. Differences were observed in the specificities of both sigma factors toward B. japonicum rpoH-dependent promoters. We conclude that the primary function of RpoH2 is to supply the cell with DnaKJ under normal growth conditions whereas RpoH1 is responsible mainly for increasing the level of GroESL1 after a heat shock.

CRP interacts with promoter-bound sigma54 RNA polymerase and blocks transcriptional activation of the dctA promoter

The cAMP receptor protein (CRP) is an activator of sigma70-dependent transcription. Analysis of the sigma54-dependent dctA promoter reveals a novel negative regulatory function for CRP. CRP can bind to two distant sites of the dctA promoter, sites which overlap the upstream activator sequences for the DctD activator. CRP interacts with Esigma54 bound at the dctA promoter via DNA loop formation. When the CRP-binding sites are deleted, CRP still interacts in a cAMP-dependent manner with the stable Esigma54 closed complex via protein-protein contacts. CRP is able to repress activation of the dctA promoter, even in the absence of specific CRP-binding sites. CRP affects both the final level and the kinetics of activation. The establishment of the repression and its release by the NtrC activator proceed via slow processes. The kinetics suggest that CRP favours a new form of closed complex which interconverts slowly with the classical closed intermediate. Only the latter is capable of interacting with an activator to form an open promoter complex. Thus, Esigma54 promoters are responsive to CRP, a protein unrelated to sigma54 activators, and the repression exerted is the direct result of an interaction between Esigma54 and the CRP-cAMP complex.

Metabolic roles of a Rhodobacter sphaeroides member of the sigma32 family

We report the role of a gene (rpoH) from the facultative phototroph Rhodobacter sphaeroides that encodes a protein (sigma37) similar to Escherichia coli sigma32 and other members of the heat shock family of eubacterial sigma factors. R. sphaeroides sigma37 controls genes that function during environmental stress, since an R. sphaeroides deltaRpoH mutant is approximately 30-fold more sensitive to the toxic oxyanion tellurite than wild-type cells. However, the deltaRpoH mutant lacks several phenotypes characteristic of E. coli cells lacking sigma32. For example, an R. sphaeroides deltaRpoH mutant is not generally defective in phage morphogenesis, since it plates the lytic virus RS1, as well as its wild-type parent. In characterizing the response of R. sphaeroides to heat, we found that its growth temperature profile is different when cells generate energy by aerobic respiration, anaerobic respiration, or photosynthesis. However, growth of the deltaRpoH mutant is comparable to that of a wild-type strain under each of these conditions. The deltaRpoH mutant mounted a heat shock response when aerobically grown cells were shifted from 30 to 42 degrees C, but it exhibited altered induction kinetics of approximately 120-, 85-, 75-, and 65-kDa proteins. There was also reduced accumulation of several presumed heat shock transcripts (rpoD P(HS), groESL1, etc.) when aerobically grown deltaRpoH cells were placed at 42 degrees C. Under aerobic conditions, it appears that another sigma factor enables the deltaRpoH mutant to mount a heat shock response, since either RNA polymerase preparations from an deltaRpoH mutant, reconstituted Esigma37, or a holoenzyme containing a 38-kDa protein (sigma38) each transcribed E. coli Esigma32-dependent promoters. The lower growth temperature profile of photosynthetic cells is correlated with a difference in heat-inducible gene expression, since neither wild-type cells or the deltaRpoH mutant mount a typical heat shock response after such cultures were shifted from 30 to 37 degrees C.

Ribosomal RNA synthesis in Streptomyces lividans under heat shock conditions

Clones containing rRNA genes were isolated from a gene library of Streptomyces lividans when RNA produced under heat shock conditions was used as a probe. Two of the clones carried entire rRNA operons rrnA and rrnF, respectively, the expression of both operons being under the control of four different promoters. At least two of the promoters were fully functional when the temperature increased from 30 to 45 degrees C, ensuring transcription of the rRNA genes under the heat shock. A third clone carried a partial rRNA operon in which expression was controlled by a main promoter that was functional at both 30 and 45 degrees C.

Comparative analysis of the interactions of Escherichia coli sigma S and sigma 70 RNA polymerase holoenzyme with the stationary-phase-specific bolAp1 promoter

We have investigated the interactions of Escherichia coli sigma 70 and sigma S holoenzyme RNA polymerases (E sigma S and E sigma 70) with the stationary-phase-specific bolAp1 promoter by various footprinting methods in vitro. E sigma S and E sigma 70 have been shown to transcribe the bolApl promoter in vitro. We have determined the effects of salt and holoenzyme concentrations on E sigma S and E sigma 70 open complex formation at the bolAp1 promoter in vitro. We have obtained a high-resolution hydroxyl radical (OH.) footprint of E sigma S and E sigma 70 on the bolApl promoter. The OH. footprinting data show remarkable similarities between the footprints of the heparin-resistant transcription complexes of the two holoenzymes which have the same +1 transcription start site. However, there are distinctive differences in the protection patterns in the region between -20 and -10 of the bolAp1 promoter. KMnO4 reactivity assays reveal that, at 37 degrees C, both holoenzymes produced similar but not identical patterns of reactivities.

Transcription of the mutL repair, miaA tRNA modification, hfq pleiotropic regulator, and hflA region protease genes of Escherichia coli K-12 from clustered Esigma32-specific promoters during heat shock

The amiB-mutL-miaA-hfq-hflX-hflK-hflC superoperon of Escherichia coli contains genes that are important for diverse cellular functions, including DNA mismatch repair (mutL), tRNA modification (miaA), pleiotropic regulation (hfq), and proteolysis (hflX-hflK-hflC). We show that this superoperon contains three E simga(32)-dependent heat shock promoters, P(mutL)HS,P(miaA)HS, and P1(hfq)HS, in addition to four E sigma(70)-dependent promoters, P(mutL), P(miaA), P2(hfq), and P3(hfq). Transcripts from P(mutL)HS and P(miaA)HS were most prominent in vivo during extreme heat shock (50 degrees C), whereas P1(hfq)HS transcripts were detectable under nonshock conditions and increased significantly after heat shock at 50 degrees C. The P(mutL)HS, P(miaA)HS, and P1(hfq)HS transcripts were not detected in an rpoH null mutant. All three promoters were transcribed by E sigma (32) in vitro at 37 degrees C and contain -35 and -10 regions that resemble the E sigma(32) consensus. In experiments to assess the possible physiological relevance of the P(mutL)HS and P(miaA)HS promoters, we found that E. coli prototrophic strain MG 1655 increased in cell mass and remained nearly 100% viable for several hours at 50 degrees C in enriched media. In these cells, a significant fraction of mutL and hfq-hflA region transcripts were from P(mutL)HS and P1(hfq)HS, respectively, and the amounts of the miaA, hfq, hflX, hflK, and hflC transcripts increased in comparison with those in nonstressed cells. The cellular amounts of MutL and the hfq gene product (HF-I protein) were maintained during heat shock at 44 or 50 degrees C. Consistent with their expression patterns, miaA and hfq were essential for growth and viability, respectively, at temperatures of 45 degrees C and above. Together, these results suggest that there is a class of E sigma(32) promoters that functions mainly at high temperatures to ensure E. coli function and survival.

rRNA transcription and growth rate-dependent regulation of ribosome synthesis in Escherichia coli

The synthesis of ribosomal RNA is the rate-limiting step in ribosome synthesis in bacteria. There are multiple mechanisms that determine the rate of rRNA synthesis. Ribosomal RNA promoter sequences have evolved for exceptional strength and for regulation in response to nutritional conditions and amino acid availability. Strength derives in part from an extended RNA polymerase (RNAP) recognition region involving at least two RNAP subunits, in part from activation by a transcription factor and in part from modification of the transcript by a system that prevents premature termination. Regulation derives from at least two mechanistically distinct systems, growth rate-dependent control and stringent control. The mechanisms contributing to rRNA transcription work together and compensate for one another when individual systems are rendered inoperative.

Control of rRNA transcription in Escherichia coli

The control of rRNA synthesis in response to both extra- and intracellular signals has been a subject of interest to microbial physiologists for nearly four decades, beginning with the observations that Salmonella typhimurium cells grown on rich medium are larger and contain more RNA than those grown on poor medium. This was followed shortly by the discovery of the stringent response in Escherichia coli, which has continued to be the organism of choice for the study of rRNA synthesis. In this review, we summarize four general areas of E. coli rRNA transcription control: stringent control, growth rate regulation, upstream activation, and anti-termination. We also cite similar mechanisms in other bacteria and eukaryotes. The separation of growth rate-dependent control of rRNA synthesis from stringent control continues to be a subject of controversy. One model holds that the nucleotide ppGpp is the key effector for both mechanisms, while another school holds that it is unlikely that ppGpp or any other single effector is solely responsible for growth rate-dependent control. Recent studies on activation of rRNA synthesis by cis-acting upstream sequences has led to the discovery of a new class of promoters that make contact with RNA polymerase at a third position, called the UP element, in addition to the well-known -10 and -35 regions. Lastly, clues as to the role of antitermination in rRNA operons have begun to appear. Transcription complexes modified at the antiterminator site appear to elongate faster and are resistant to the inhibitory effects of ppGpp during the stringent response.

rRNA operon multiplicity in Escherichia coli and the physiological implications of rrn inactivation

Here we present evidence that only five of the seven rRNA operons present in Escherichia coli are necessary to support near-optimal growth on complex media. Seven rrn operons are necessary, however, for rapid adaptation to nutrient and temperature changes, suggesting it is the ability to adapt quickly to changing environmental conditions that has provided the selective pressure for the persistence of seven rrn operons in E. coli. We have also found that one consequence of rrn operon inactivation is a miscoordination of the concentrations of initiation factor IF3 and ribosomes.

In vitro transcription of pathogenesis-related genes by purified RNA polymerase from Staphylococcus aureus

The RNA polymerase (RNAP) holoenzyme of Staphylococcus aureus was purified by DNA affinity, gel filtration, and ion-exchange chromatography. This RNAP contained four major subunits with apparent molecular masses of 165, 130, 60, and 47 kDa. All four subunits of the RNAP were serologically related to the subunits of Escherichia coli E sigma 70 holoenzyme by Western immunoblot analysis. The 60-kDa subunit was subsequently isolated and found to react with a monoclonal antibody specific to the E. coli sigma 70 subunit. This sigma 70-related protein allowed E. coli core RNAP promoter-specific initiation and increased transcription by S. aureus RNAP that is unsaturated with sigma. We therefore suggest that this 60-kDa protein is a sigma factor. Purified S. aureus RNAP transcribed from the promoters of several important S. aureus virulence genes (sea, sec, hla, and agr P2) in vitro. The in vitro transcription start sites of the sea, sec, and agr P2 promoters, mapped by primer extension, were similar to those identified in vivo. The putative promoter hexamers of these three genes showed strong sequence similarity to the E. coli sigma 70 consensus promoter, and transcription by E sigma 70 from some of these promoters has been observed. Conversely, S. aureus RNAP does not transcribe from all E. coli sigma 70-dependent promoters. Taken together, our results indicate that the promoter sequences recognized by purified S. aureus RNAP are similar but not identical to those recognized by E. coli E sigma 70.

Overlapping promoters for two different RNA polymerase holoenzymes control Bradyrhizobium japonicum nifA expression

The Bradyrhizobium japonicum NifA protein, the central regulator for nitrogen fixation gene expression, is encoded in the fixRnifA operon. This operon is activated during free-living anaerobic growth and in the symbiotic root nodule bacteroid state. In addition, it is expressed in aerobic conditions, albeit at a low level. Here, we report that this pattern of expression is due to the presence of two overlapping promoters: fixRp1, which is of the -24/-12 class recognized by the RNA polymerase sigma 54, and fixRp2, which shares homology with the -35 and -10 regions found in other putative B. japonicum housekeeping promoters. Primer extension analyses showed that fixRp1 directed the synthesis of a transcript, P1, that starts 12 nucleotides downstream of the -12 region. In addition to sigma 54, P1 was dependent on NifA and low oxygen tension. Transcripts originating from fixRp2 started at two sites: one coincided with P1, while the most abundant, P2 initiated just two nucleotides further downstream of P1. Expression from fixRp2 was dependent on the upstream -68 promoter region, a region known to bind a putative activator protein, but it was independent of sigma 54 and NifA. This promoter was expressed in aerobic and anaerobic conditions but was not expressed in 30-day-old bacteroids. Mutations in the conserved 12 region for the sigma 54 promoter did not show any transcript, because these mutations also disrupted the overlapping -10 region of the fixRp2 promoter. Conversely, mutations at the -24 region only affected the sigma 54-dependent P1 transcript, having no effect on the expression of P2. In the absence of omega(54), anaerobic expression from the fixRp(2) promoter was enhanced threefold, suggesting that in the wild-type strain, the two RNA polymerase holoenzymes must compete for binding to the same promoter region.

Selectivity of the Escherichia coli RNA polymerase E sigma 38 for overlapping promoters and ability to support CRP activation

A series of gal promoter mutants has been used to compare the in vitro selectivities of the two forms of Escherichia coli RNA polymerase, E sigma 38 and E sigma 70. In the absence of the CRP-cAMP complex, E sigma 38 shows a strong preference for the ga/P1 promoter, whereas E sigma 70 preferentially initiates transcription from the ga/P2 promoter. E sigma 38 selectivity is not affected by the nature and position of the upstream sequences or by the phasing between synthetic upstream curved sequences and the -10 regions. In fact, all effects of mutations in the extended -10 region can be accounted for without evoking strong new sequence preferences for E sigma 38. Finally, both E sigma 38 and E sigma 70 initiate transcription from the ga/P1 promoter in the presence of CRP-cAMP complex and support direct cAMP-CRP activation at several CRP-dependent promoters.

Heat shock-dependent transcriptional activation of the metA gene of Escherichia coli

In Escherichia coli, the growth rate at elevated temperatures is controlled by the availability of endogenous methionine, which is limited because of the temperature sensitivity of the metA gene product, homoserine transsuccinylase (HTS). In order to determine the relationship between this control mechanism and the heat shock response, we estimated the cellular levels of HTS during heat shock by Western (immunoblot) analysis and found an increase following induction by temperature shift and by addition of ethanol or cadmium ions. The elevated level of HTS was a result of transcriptional activation of the metA gene. This activation was heat shock dependent, as it did not take place in rpoH mutants, and probably specific to the metA gene, as another gene of the methionine regulon (metE) was not activated. These results suggest a metabolic link between the two systems that control the response of E. coli to elevated temperatures: the metA gene, which codes for the enzyme responsible for regulating cell growth as a function of temperature elevation (HTS), is transcriptionally activated by the heat shock response.

Identification of genes negatively regulated by Fis: Fis and RpoS comodulate growth-phase-dependent gene expression in Escherichia coli

Fis is a nucleoid-associated protein in Escherichia coli that has been shown to regulate recombination, replication, and transcription reactions. It is expressed in a transient manner under batch culturing conditions such that high levels are present during early exponential phase and low levels are present during late exponential phase and stationary phase. We have screened a random collection of transposon-induced lac fusions for those which give decreased expression in the presence of Fis. Thirteen different Fis-repressed genes were identified, including glnQ (glutamine high-affinity transport), mglA (methyl-galactoside transport), xylF (D-xylose-binding protein), sdhA (succinate dehydrogenase flavoprotein subunit), and a newly identified aldehyde dehydrogenase, aldB. The LacZ expression patterns revealed that many of the fusions were maximally expressed at different stages of growth, including early log phase, mid- to late log phase, and stationary phase. The expression of some of the late-exponential- and stationary-phase genes was dependent on the RpoS sigma factor, whereas that of others was affected negatively by RpoS. We conclude that Fis negatively regulates a diverse set of genes and that RpoS can function to both activate and inhibit the expression of specific genes.

Nucleotide sequence of the Escherichia coli groE promoter

The promoter region of the Escherichia coli groE operon was cloned using the polymerase chain reaction (PCR). The 118-bp nucleotide sequence of the cloned groE promoter was determined on both strands of DNA. Two bp were found between the -10 and -35 regions of this promoter that have not been reported in previous publications of this sequence.

The Escherichia coli gapA gene is transcribed by the vegetative RNA polymerase holoenzyme E sigma 70 and by the heat shock RNA polymerase E sigma 32

Escherichia coli D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is produced by the gapA gene and is structurally related to eukaryotic GAPDHs. These facts led to the proposal that the gapA gene originated by a horizontal transfer of genetic information. The yields and start sites of gapA mRNAs produced in various fermentation conditions and genetic contexts were analyzed by primer extension. The transcriptional regulatory region of the gapA gene was found to contain four promoter sequences, three recognized by the vegetative RNA polymerase E sigma 70 and one recognized by the heat shock RNA polymerase E sigma 32. Transcription of gapA by E sigma 32 is activated in the logarithmic phase under conditions of starvation and of heat shock. Using a GAPDH- strain, we found that GAPDH production has a positive effect on cell growth at 43 degrees C. Thus, E. coli GAPDH displays some features of heat shock proteins. One of the gapA promoter sequences transcribed by E sigma 70 is subject to catabolic repression. Another one has growth phase-dependent efficiency. This complex area of differentially regulated promoters allows the production of large amounts of gapA transcripts in a wide variety of environmental conditions. On the basis of these data, the present view of E sigma 32 RNA polymerase function has to be enlarged, and the various hypotheses on E. coli gapA gene origin have to be reexamined.

A third recognition element in bacterial promoters: DNA binding by the alpha subunit of RNA polymerase

A DNA sequence rich in (A+T), located upstream of the -10, -35 region of the Escherichia coli ribosomal RNA promoter rrnB P1 and called the UP element, stimulates transcription by a factor of 30 in vivo, as well as in vitro in the absence of protein factors other than RNA polymerase (RNAP). When fused to other promoters, such as lacUV5, the UP element also stimulates transcription, indicating that it is a separate promoter module. Mutations in the carboxyl-terminal region of the alpha subunit of RNAP prevent stimulation of these promoters by the UP element although the mutant enzymes are effective in transcribing the "core" promoters (those lacking the UP element). Protection of UP element DNA by the mutant RNAPs is severely reduced in footprinting experiments, suggesting that the selective decrease in transcription might result from defective interactions between alpha and the UP element. Purified alpha binds specifically to the UP element, confirming that alpha acts directly in promoter recognition. Transcription of three other promoters was also reduced by the COOH-terminal alpha mutations. These results suggest that UP elements comprise a third promoter recognition region (in addition to the -10, -35 recognition hexamers, which interact with the sigma subunit) and may account for the presence of (A+T)-rich DNA upstream of many prokaryotic promoters. Since the same alpha mutations also block activation by some transcription factors, mechanisms of promoter stimulation by upstream DNA elements and positive control by certain transcription factors may be related.