Analysis of sequence elements important for the synthesis and control of ribosomal RNA in E coli

Biosynthesis and Regulation of the Branched-Chain Amino Acids†

This review focuses on more recent studies concerning the systems biology of branched-chain amino acid biosynthesis, that is, the pathway-specific and global metabolic and genetic regulatory networks that enable the cell to adjust branched-chain amino acid synthesis rates to changing nutritional and environmental conditions. It begins with an overview of the enzymatic steps and metabolic regulatory mechanisms of the pathways and descriptions of the genetic regulatory mechanisms of the individual operons of the isoleucine-leucine-valine (ilv) regulon. This is followed by more-detailed discussions of recent evidence that global control mechanisms that coordinate the expression of the operons of this regulon with one another and the growth conditions of the cell are mediated by changes in DNA supercoiling that occur in response to changes in cellular energy charge levels that, in turn, are modulated by nutrient and environmental signals. Since the parallel pathways for isoleucine and valine biosynthesis are catalyzed by a single set of enzymes, and because the AHAS-catalyzed reaction is the first step specific for valine biosynthesis but the second step of isoleucine biosynthesis, valine inhibition of a single enzyme for this enzymatic step might compromise the cell for isoleucine or result in the accumulation of toxic intermediates. The operon-specific regulatory mechanisms of the operons of the ilv regulon are discussed in the review followed by a consideration and brief review of global regulatory proteins such as integration host factor (IHF), Lrp, and CAP (CRP) that affect the expression of these operons.

A proximal promoter element required for positive transcriptional control by guanosine tetraphosphate and DksA protein during the stringent response

The alarmone guanosine tetraphosphate (ppGpp) acts as both a positive and a negative regulator of gene expression in the presence of DksA, but the underlying mechanisms of this differential control are unclear. Here, using uspA hybrid promoters, we show that an AT-rich discriminator region is crucial for positive control by ppGpp/DksA. The AT-rich discriminator makes the RNA polymerase-promoter complex extremely stable and therefore easily saturated with RNA polymerase. A more efficient transcription is achieved when the RNA polymerase-promoter complex is destabilized with ppGpp/DksA. We found that exchanging the AT-rich discriminator of uspA with the GC-rich rrnB-P1 discriminator made the uspA promoter negatively regulated by ppGpp/DksA both in vivo and in vitro. In addition, the GC-rich discriminator destabilized the RNA polymerase-promoter complex, and the effect of ppGpp/DksA on the kinetic properties of the promoter was reversed. We propose that the transcription initiation rate from promoters with GC-rich discriminators, in contrast to the uspA-promoter, is not limited by the stability of the open complex. The findings are discussed in view of models for both direct and indirect effects of ppGpp/DksA on transcriptional trade-offs.

Growth rate regulation in Escherichia coli

Growth rate regulation in bacteria has been an important issue in bacterial physiology for the past 50 years. This review, using Escherichia coli as a paradigm, summarizes the mechanisms for the regulation of rRNA synthesis in the context of systems biology, particularly, in the context of genome-wide competition for limited RNA polymerase (RNAP) in the cell under different growth conditions including nutrient starvation. The specific location of the seven rrn operons in the chromosome and the unique properties of the rrn promoters contribute to growth rate regulation. The length of the rrn transcripts, coupled with gene dosage effects, influence the distribution of RNAP on the chromosome in response to growth rate. Regulation of rRNA synthesis depends on multiple factors that affect the structure of the nucleoid and the allocation of RNAP for global gene expression. The magic spot ppGpp, which acts with DksA synergistically, is a key effector in both the growth rate regulation and the stringent response induced by nutrient starvation, mainly because the ppGpp level changes in response to environmental cues. It regulates rRNA synthesis via a cascade of events including both transcription initiation and elongation, and can be explained by an RNAP redistribution (allocation) model.

Promoter analysis of palindromic transcription units in the ribosomal DNA circle of Entamoeba histolytica

rRNA genes of Entamoeba histolytica are organized as palindromic ribosomal DNA (rDNA) units (I and II) in a 24.5-kb circle. Although the two rDNAs are identical in sequence, their upstream spacers are completely different. Since the intergenic sequences (IGS) of all rDNA copies in other organisms are conserved and contain transcription regulatory sequences, the lack of sequence conservation in the IGS prompted the question of whether both rDNAs are indeed transcriptionally active. We mapped the transcriptional start points (tsp's) and promoters of the two rDNAs. A 51-bp sequence immediately upstream of the tsp's was highly conserved in both units. In addition, both units had an A+T-rich stretch upstream of the 51-bp core. Analysis of reporter gene transcription showed promoter activity to reside in the regions from positions -86 to +123 (rDNA I) and positions -101 to +140 (rDNA II). The promoter-containing fragments from both units could bind and compete with each other for protein(s) from nuclear extracts. Protein binding was especially dependent on the A+T-rich region upstream of the 51-bp core (positions -53 to -68). The requirement of >80 bp downstream of the tsp was striking. Although this sequence was not conserved in the two units, it could potentially fold into very long stem-loops. Both rDNAs transcribed with comparable efficiency, as measured by nuclear runon. Thus, both rDNAs share very similar organization of promoter sequences, and in exponential culture both rDNAs are transcribed. It remains to be seen whether the different IGS affect the regulation of the two units under adverse conditions.

Promoter prediction in the rhizobia

The ability to recognize and predict non-σ54promoters in the alphaproteobacteria is not well developed. In this study, 25 experimentally verifiedSinorhizobium melilotipromoter sequences were compiled and used to predict the location of other related promoters in theS. melilotigenome. Fourteen candidate predictions were targeted for verification and of these at least 12 proved to be genuine promoters. As a result, the experimental identification of 12 novel promoters linked to genesrpoD,topA,rpmJ,trpS,ropB1,metC,rpsT,secE,trkHand three tRNA genes is reported. In all, 99 predicted and verified promoters are reported, including those linked with 13 tRNA genes, eight ribosomal protein genes and a number of other physiologically important or essential genes. On the basis of sequence conservation and a mutational analysis of promoter activity, the −35 and −10 consensus for these promoters is 5-CTTGAC-N17-CTATAT. This promoter structure, which seems to be widely conserved amongst several other genera in the alphaproteobacteria, shares significant similarity with, but is skewed by a 1 nt step from, the canonicalEscherichia coliσ70promoter. Perhaps this difference is responsible for the observation thatS. melilotipromoters are often poorly expressed inE. coli. In this regard, expression data from plasmid-bornegfp-reporter fusions to eight of theS. melilotipromoters verified in this work revealed that while these promoters were very active inS. melilotiandAgrobacterium tumefaciensonly very low, near-background activity was detected inE. coli.

The seven E. coli ribosomal RNA operon upstream regulatory regions differ in structure and transcription factor binding efficiencies

Ribosomal RNAs in E. coli are transcribed from seven operons, which are highly conserved in their organization and sequence. However, the upstream regulatory DNA regions differ considerably, suggesting differences in regulation. We have therefore analyzed the conformation of all seven DNA elements located upstream of the major E. coli rRNA P1 promoters. As judged by temperature-dependent gel electrophoresis with isolated DNA fragments comprising the individual P1 promoters and the complete upstream regulatory regions, all seven rRNA upstream sequences are intrinsically curved. The degree of intrinsic curvature was highest for the rrnB and rrnD fragments and less pronounced for the rrnA and rrnE operons. Comparison of the experimentally determined differences in curvature with programs for the prediction of DNA conformation revealed a generally high degree of conformity. Moreover, the analysis showed that the center of curvature is located at about the same position in all fragments. The different upstream regions were analyzed for their capacity to bind the transcription factors FIS and H-NS, which are known as antagonists in the regulation of rRNA synthesis. Gel retardation experiments revealed that both proteins interact with the upstream promoter regions of all seven rDNA fragments, with the affinities of the different DNA fragments for FIS and H-NS and the structure of the resulting complexes deviating considerably. FIS binding was non-cooperative, and at comparable protein concentrations the occupancy of the different DNA fragments varied between two and four binding sites. In contrast, H-NS was shown to bind cooperatively and intermediate states of occupancy could not be resolved for each fragment. The different gel electrophoretic mobilities of the individual DNA/protein complexes indicate variable structures and topologies of the upstream activating sequence regulatory complexes. Our results are highly suggestive of differential regulation of the individual rRNA operons.

Control of rRNA synthesis in Escherichia coli: a systems biology approach

The first part of this review contains an overview of the various contributions and models relating to the control of rRNA synthesis reported over the last 45 years. The second part describes a systems biology approach to identify the factors and effectors that control the interactions between RNA polymerase and rRNA (rrn) promoters of Escherichia coli bacteria during exponential growth in different media. This analysis is based on measurements of absolute rrn promoter activities as transcripts per minute per promoter in bacterial strains either deficient or proficient in the synthesis of the factor Fis and/or the effector ppGpp. These absolute promoter activities are evaluated in terms of rrn promoter strength (V(max)/K(m)) and free RNA polymerase concentrations. Three major conclusions emerge from this evaluation. First, the rrn promoters are not saturated with RNA polymerase. As a consequence, changes in the concentration of free RNA polymerase contribute to changes in rrn promoter activities. Second, rrn P2 promoter strength is not specifically regulated during exponential growth at different rates; its activity changes only when the concentration of free RNA polymerase changes. Third, the effector ppGpp reduces the strength of the rrn P1 promoter both directly and indirectly by reducing synthesis of the stimulating factor Fis. This control of rrn P1 promoter strength forms part of a larger feedback loop that adjusts the synthesis of ribosomes to the availability of amino acids via amino acid-dependent control of ppGpp accumulation.

Activation of transcription initiation from a stable RNA promoter by a Fis protein-mediated DNA structural transmission mechanism

The leuV operon of Escherichia coli encodes three of the four genes for the tRNA1Leu isoacceptors. Transcription from this and other stable RNA promoters is known to be affected by a cis-acting UP element and by Fis protein interactions with the carboxyl-terminal domain of the alpha-subunits of RNA polymerase. In this report, we suggest that transcription from the leuV promoter also is activated by a Fis-mediated, DNA supercoiling-dependent mechanism similar to the IHF-mediated mechanism described previously for the ilvP(G) promoter (S. D. Sheridan et al., 1998, J Biol Chem 273: 21298-21308). We present evidence that Fis binding results in the translocation of superhelical energy from the promoter-distal portion of a supercoiling-induced DNA duplex destabilized (SIDD) region to the promoter-proximal portion of the leuV promoter that is unwound within the open complex. A mutant Fis protein, which is defective in contacting the carboxyl-terminal domain of the alpha-subunits of RNA polymerase, remains competent for stimulating open complex formation, suggesting that this DNA supercoiling-dependent component of Fis-mediated activation occurs in the absence of specific protein interactions between Fis and RNA polymerase. Fis-mediated translocation of superhelical energy from upstream binding sites to the promoter region may be a general feature of Fis-mediated activation of transcription at stable RNA promoters, which often contain A+T-rich upstream sequences.

A motif co-occurrence approach for genome-wide prediction of transcription-factor-binding sites in Escherichia coli

Various computational approaches have been developed for predicting cis-regulatory DNA elements in prokaryotic genomes. We describe a novel method for predicting transcription-factor-binding sites in Escherichia coli. Our method takes advantage of the principle that transcription factors frequently coregulate gene expression, but without requiring prior knowledge of which groups of genes are coregulated. Using position weight matrices for 49 known transcription factors, we examined spacings between pairs of matrix hits. These pairs were assigned probabilities according to the overrepresentation of their separation distance. The functions of many open reading frames (ORFs) downstream from predicted binding sites are unknown, and may correspond to novel regulon members. For five predictions, knockouts with mutated replacements of the predicted binding sites were created in E. coli MG1655. Quantitative real-time PCR (RT-PCR) indicates that for each of the knockouts, at least one gene immediately downstream exhibits a statistically significant change in mRNA expression. This approach may be useful in analyzing binding sites in a variety of organisms.

The Effects of Glucose Concentration on the Reciprocal Regulation of rRNA Promoters in Plasmodium falciparum

The developmental progression of Plasmodium falciparum is remarkably sensitive to glucose concentration. We have investigated the effects of glucose concentration on the parasite development cycle as reflected by changes of ribosomal RNA (rRNA) transcription. We showed that glucose starvation differentially affects transcriptional control of the rRNA genes by sharply repressing transcription from those loci involved with asexual development of the parasite while up-regulating transcription at those loci involved with sexual development of the parasite. Temperature change also effects regulation of transcription. We found that the effects of temperature and glucose were synergistic. We identified and compared the upstream region of the transcription start sites of each gene. These putative promoter structures are considerably different from one another and contain structures remarkably similar to rRNA control elements in other organisms.

Sequence-based analysis of metabolic demands for protein synthesis in prokaryotes

Constraints-based models for microbial metabolism can currently be constructed on a genome-scale. These models do not account for RNA and protein synthesis. A scalable formalism to describe translation and transcription that can be integrated with the existing metabolic models is thus needed. Here, we developed such a formalism. The fundamental protein synthesis network described by this formalism was analysed via extreme pathway and flux balance analyses. The protein synthesis network exhibited one extreme pathway per messenger RNA synthesized and one extreme pathway per protein synthesized. The key parameters in this network included promoter strengths, messenger RNA half-lives, and the availability of nucleotide triphosphates, amino acids, RNA polymerase, and active ribosomes. Given these parameters, we were able to calculate a cell's material and energy expenditures for protein synthesis using a flux balance approach. The framework provided herein can subsequently be integrated with genome-scale metabolic models, providing a sequence-based accounting of the metabolic demands resulting from RNA and protein polymerization.

Kinetic properties of rrn promoters in Escherichia coli

How do bacteria adapt and optimize their growth in response to different environments? The answer to this question is intimately related to the control of ribosome bio-synthesis. During the last decades numerous proposals have been made to explain this control but none has been definitive. To readdress the problem, we have used measurements of rRNA synthesis rates and rrn gene dosages in E. coli to find the absolute transcription rates of the average rrn operon (transcripts per min per operon) at different growth rates. By combining these rates with lacZ expression data from rRNA promoter-lacZ fusions, the abolute activities of the isolated rrnB P1 and P2 promoters were determined as functions of the growth rate in the presence and absence of Fis and of the effector ppGpp. The promoter activity data were analyzed to obtain the relative concentrations of free RNA polymerase, [R(f)], and the ratio of the Michaelis-Menten parameters, V(max)/K(m) (promoter strength), that characterize the promoter-RNA polymerase interaction. The results indicate that changes in the basal concentration of ppGpp can account for all growth-medium dependent regulation of the rrn P1 promoter strength. The P1 promoter strength was maximal when Fis was present and the level of ppGpp was undetectable during growth in rich media or in ppGpp-deficient strains; this maximal strength was 3-fold reduced when Fis was removed and the level of ppGpp remained undetectable. At ppGpp levels above 55 pmol per cell mass unit (OD(460)) during growth in poor media, the P1 promoter strength was minimal and not affected by the presence or absence of fis. The half-maximal value occurred at 20 pmol ppGpp/OD(460) and corresponds to an intracellular concentration of about 50 microM. In connection with previously published data, the results suggest that ppGpp reduces the P1 promoter strength directly, by binding RNA polymerase, and indirectly, by inhibiting the synthesis of Fis.

The bacterial DNA-binding protein H-NS represses ribosomal RNA transcription by trapping RNA polymerase in the initiation complex

The interaction of the bacterial regulatory protein H-NS with RNA polymerase and the ribosomal RNA P1 promoter was analyzed to better understand the mechanism of H-NS-dependent transcriptional repression. We could show that initial binding of RNA polymerase to the promoter was not inhibited by the simultaneous interaction of H-NS, although H-NS binding sites extend into the core promoter region. Binding of sigma(70)-saturated RNA polymerase and H-NS to the promoter DNA occurs cooperatively and results in a stable complex of slower gel electrophoretic mobility as compared to complexes formed with the single proteins. The presence of the upstream curved H-NS binding site contributes strongly to the cooperative RNA polymerase-promoter interaction. By KMnO(4) modification of single-stranded template nucleotides we could show that open complex formation at the rrnB P1 promoter was not inhibited by H-NS binding. An increased KMnO(4) reactivity of several positions within the open complex rather supports the view that open complex formation is stimulated in presence of H-NS. Moreover, subtle changes in the modification pattern indicate that the open complex formed in the presence of H-NS are structurally distinct from the H-NS-free complex. In vitro transcriptional analysis of the abortive and productive yields revealed that the formation of transcription products longer than three nucleotides is dramatically reduced in the presence of H-NS, while the amount of shorter abortive products remained unaffected. Together the results demonstrate that H-NS inhibits transcription at the rrnB P1 promoter not by interfering with initial RNA polymerase binding but by blocking chain elongation steps subsequent to the first (two) phosphodiester bond formations. The mechanism of H-NS dependent repression at rRNA promoters can thus be explained as a trap which inhibits substrate NTP incorporation beyond template position +3 into the initial transcribing complex.

Activities of constitutive promoters in Escherichia coli

The in vivo activities of seven constitutive promoters in Escherichia coli have been determined as functions of growth rate in wild-type relA+ spoT+ strains with normal levels of guanosine tetraphosphate (ppGpp) and in ppGpp-deficient DeltarelADeltaspoT derivatives. The promoters include (i) the spc ribosomal protein operon promotor Pspc; (ii) the beta-lactamase gene promotor Pblaof plasmid pBR322; (iii) the PLpromoter of phage lambda; (iv) and (v) the replication control promoters PRNAIand PRNAIIof plasmid pBR322; and (vi) and (vii) the P1 and P2 promoters of the rrnB ribosomal RNA operon. Each strain carried an operon fusion consisting of one of the respective promoter regions linked to lacZ and recombined into the chromosome at the mal locus of a lac deletion strain. The amount of 5'-terminal lacZ mRNA and of beta-galactosidase activity expressed from these promoters were determined by standard hybridization or enzyme activity assays, respectively. In addition, DNA, RNA and protein measurements were used to obtain information about gene dosage, rRNA synthesis and translation rates. By combining lacZ mRNA hybridization data with gene dosage and rRNA synthesis data, the absolute activity of the different promoters, in transcripts/minute per promoter, was determined. In ppGpp-proficient (relA+ spoT+) strains, the respective activities of rrnB P1 and P2 increased 40 and fivefold with increasing growth rate between 0.7 and 3.0 doublings/hour. The activities of Pspc, PL, Pbla, and PRNAIincreased two- to threefold and reached a maximum at growth rates above 2.0 doublings/hour. In contrast, PRNAIIactivity decreased threefold over this range of growth rates. In ppGpp-deficient (DeltarelA DeltaspoT) bacterial strains, the activities of rrnB P1 and P2 promoters both increased about twofold between 1.6 and 3.0 doublings/hour, whereas the activities of Pspc, PL, Pbla, and PRNAI, and PRNAIIwere about constant. To explain these observations, we suggest that the cellular concentration of free RNA polymerase increases with increasing growth rate; for saturation the P1 and P2 rRNA promoters require a high RNA polymerase concentration that is approached only at the highest growth rates, whereas the other promoters are saturated at lower polymerase concentrations achieved at intermediate growth rates. In addition, the data indicate that the respective rrnB P1 and PRNAIIpromoters were under negative and positive control by ppGpp. This caused a reduced activity of rrnB P1 and an increased activity of PRNAIIduring slow growth in wild-type (relA+ spoT+) relative to ppGpp-deficient (DeltarelA DeltaspoT) bacterial strains.

Effects of the Escherichia coli DNA-binding protein H-NS on rRNA synthesis in vivo

The Escherichia coli DNA-binding protein H-NS is known to interact specifically with the upstream region of ribosomal RNA transcription units, where it causes transcriptional repression in vitro. Here, we present results demonstrating the effect of H-NS on rRNA transcription in vivo. rRNA synthesis rates were compared in cells that differ in the expression of functional H-NS or FIS molecules. We could show that in the absence of H-NS derepression of rRNA synthesis occurs at low growth rates. During the cell cycle H-NS is responsible for the rapid shut-off of rRNA synthesis at the end of the exponential phase. As it is known for FIS-dependent activation, the inhibitory function of H-NS is specific for P1, the first of the tandem rRNA promoters. The effect of H-NS on rRNA synthesis was further assessed under stress conditions. While under osmotic upshift the reduction in rRNA synthesis is clearly H-NS-dependent, no such influence could be detected at cold shock. Determination of the cellular ppGpp concentrations revealed that H-NS does not mediate its function via alterations in the synthesis of the global effector ppGpp.

Mutations that render the promoter of the histidine operon of Salmonella typhimurium insensitive to nutrient-rich medium repression and amino acid downshift

The effects of mutations in the promoter of the histidine operon of Salmonella typhimurium were examined in vivo. The wild-type chromosomal copy of the his promoter was replaced with mutations in the -10 hexamer sequence and in the region between the -10 hexamer and the transcriptional start point-termed the discriminator sequence. The substitutions were performed with a phage M13 allele replacement system. Expression of the his operon is known to correlate with levels of guanosine 5',3'-bispyrophosphate (ppGpp) in vivo. Strains containing either the wild-type his promoter or his promoter mutations were grown in both nutrient-rich and minimal media under steady-state conditions known to alter intracellular levels of ppGpp in a predictable way. The effect of the presence or absence of the his attenuator was assessed under these conditions as well. Expression of the his operon was studied by measuring the differential rate of beta-galactosidase synthesis with a his-lac transcriptional fusion. Regulation of the his operon in the promoter mutants was also studied under conditions of a transient amino acid downshift induced by the addition of serine hydroxamate to cultures growing in nutrient-rich medium. These growth conditions cause elevated levels of ppGpp. The results provide physiological confirmation of previous evidence obtained with a coupled transcription-translation system in vitro which indicated that ppGpp regulates interaction of RNA polymerase at the his promoter. More specifically, the in vivo evidence shows that the region of the his promoter that includes the -10 hexamer and discriminator sequences is the target at which ppGpp stimulates transcription.

Effects of different growth conditions on the in vivo activity of the tandem Escherichia coli ribosomal RNA promoters P1 and P2

We have analyzed the relative activities of the Escherichia coli ribosomal RNA promoters P1 and P2 in vivo under different physiological conditions. Promoter efficiencies were determined by quantitative comparison of the transcript-specific primer extension products obtained from total RNA preparations. Cells were analyzed at different stages of the growth cycle, at different growth rates, and under conditions of stringent control. In addition, the rRNA gene dosage was altered by transformation with plasmids containing additional rrnD or rrnB transcription units, or rRNA operons in which one of the tandem promoters (P1) had been deleted. Under conditions of amino acid starvation (stringent control) we observed the expected strong reduction in P1-directed transcription. In contrast to the previous assumption that the P2 promoter is not regulated, we simultaneously noticed a smaller but significant repression of P2-directed transcription. In strains in which the rRNA gene dosage was increased by transformation with plasmids bearing rRNA transcription units, a similar degree of repression was observed. Repression of the P1 promoter activity was increased, however, when cells contained extra rRNA operons with P2 promoters only. As demonstrated under stringent control conditions, changes in the growth cycle also affected the activity of promoters P1 and P2. A greater proportion of P2-derived transcripts was observed when cells changed from exponential to stationary growth or if cultures were grown in minimal medium. Under steady-state, slow growth conditions (minimal medium) we obtained evidence showing that the ratio of P1/P2 transcription products is much lower for cells with extra rrnB as compared to extra rrnD operons or cells lacking extra rRNA operons, implying an operon-specific regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence analysis of the Escherichia coli transcription regulator H-NS reveals two distinguishable complexes dependent on binding to specific or nonspecific DNA sites

Here we report a structural investigation of the transcription factor H-NS and its DNA interaction. H-NS has a general effect on transcription by compacting DNA; but for a number of specific genes, it is known to act directly as repressor or activator. The homodimeric protein binds to the major groove of DNA in a sequence-nonspecific manner, recognizing a curved conformation of the target DNA. H-NS consists of 136 amino acids with a single tryptophanyl residue at position 108. To overcome the apparent lack of any other structural details, we took advantage of the intrinsic fluorescence of Trp-108. Static and dynamic quenching constants obtained with the neutral quencher molecule acrylamide are consistent with a hydrophilic environment and high degree of solvent exposure for Trp-108. In addition, quenching studies in the presence of the anionic quencher iodide indicate a positively charged microenvironment for the same amino acid residue. Specific and nonspecific H-NS.DNA complexes were studied by gel retardation and fluorescence analysis. While specific H-NS.DNA complex formation is accompanied by a clear enhancement of the tryptophanyl fluorescence intensity, interaction in the presence of the nonspecific competitor DNA poly(dI-dC) decreases the fluorescence quantum yield.

The Escherichia coli ribosomal RNA leader nut region interacts specifically with mature 16S RNA

All ribosomal RNAs are preceded by leader sequences not present in the final ribosome particles. The highly conserved leader sequences of bacterial rRNAs are known to be important for the folding and assembly of functional ribosomes. Very likely transient binding of the leader to mature parts of the 16S RNA occurs during transcription. To better understand the mechanistic details of these functions we have performed a secondary structural analysis of E. coli ribosomal RNA leader transcripts by chemical modification and enzymatic hydrolysis studies. The data were combined with results from thermodynamic stability calculations to yield a generalized structural model. The same secondary structure of the leader core, comprising the nut-like sequences up to the mature 5' end of the 16S RNA, was deduced, irrespective if transcripts started at promoter P1 or 120 nucleotides downstream at P2. Employing gelshift and cross-linking studies we were able to demonstrate that a part of the leader core, namely the nut-like sequence elements bind directly to specific regions within the mature 16S RNA. The sites of RNA-RNA cross-linking could be localized by sequencing. They map in the 16S RNA 5' domain at nucleotide positions G27 to G42, C48, G68, G117 and G126. The results may explain the recently observed scaffolding function of the leader RNA during ribosome biogenesis.

Degradation of rRNA in Salmonella strains: a novel mechanism to regulate the concentrations of rRNA and ribosomes

We have previously shown that the 23S rRNA of Salmonella strains is highly fragmented by specific enzyme cleavages. In this article, we report that 23S rRNA of Salmonella strains is rapidly degraded as the cells enter the stationary phase. More than 90% of the 23S rRNA is degraded when the cells reach the stationary phase. The rate of degradation of 23S rRNA correlated with its degree of fragmentation. This degradation is probably mediated by newly synthesized protein factor(s), since treatment with chloramphenicol or rifampin inhibits the rRNA degradation. We propose that degradation of 23S rRNA is a novel mechanism in the regulation of the bacterial 23S rRNA and ribosome concentration and that this additional regulatory mechanism provides some selective advantage to cells.

Promoters responsive to DNA bending: a common theme in prokaryotic gene expression

The early notion of DNA as a passive target for regulatory proteins has given way to the realization that higher-order DNA structures and DNA-protein complexes are at the basis of many molecular processes, including control of promoter activity. Protein binding may direct the bending of an otherwise linear DNA, exacerbate the angle of an intrinsic bend, or assist the directional flexibility of certain sequences within prokaryotic promoters. The important, sometimes essential role of intrinsic or protein-induced DNA bending in transcriptional regulation has become evident in virtually every system examined. As discussed throughout this article, not every function of DNA bends is understood, but their presence has been detected in a wide variety of bacterial promoters subjected to positive or negative control. Nonlinear DNA structures facilitate and even determine proximal and distal DNA-protein and protein-protein contacts involved in the various steps leading to transcription initiation.

Evidence for a regulatory function of the histone-like Escherichia coli protein H-NS in ribosomal RNA synthesis

We have isolated a small Escherichia coli protein which stably interacts with ribosomal RNA P1 promoter DNA. We present evidence showing that the protein is identical to the histone-like E. coli protein, H-NS (H1). Binding of H-NS to the P1 promoter region is dependent on the DNA curvature. Mapping the H-NS-DNA contact sites by nuclease protection and high-resolution footprinting techniques reveal three H-NS-binding domains, and contacts of the protein in the major groove of the bent DNA. The binding region extends from position -18 to -89, relative to the P1 transcription start site, and shows an overlap with the known binding sites for Fis, another E. coli protein, which acts as transcriptional activator of P1. The binding of H-NS does not displace Fis; instead, heterologous complexes are formed. Apparently, H-NS and Fis bind to separated curved DNA segments, with the planes of the curves pointing into different directions. In vitro transcriptional analyses demonstrate that H-NS represses rRNA P1 promoter-directed transcription. Repression is most pronounced in the presence of Fis. Thus, H-NS seems specifically to antagonize Fis-dependent activation. No comparable inactivation is observed for the second rRNA promoter P2.