Escherichia coli Fis and DnaA proteins bind specifically to the nrd promoter region and affect expression of an nrd-lac fusion

The global population stru cture of Lacticaseibacillus rhamnosus and its application to an investigation of a rare case of infective endocarditis

BACKGROUND

Lacticaseibacillus (formerly Lactobacillus) rhamnosus is widely used in probiotics or food supplements to promote microbiome health and may also be part of the normal microbiota of the human gastrointestinal tract. However, it rarely also causes invasive or severe infections in patients. It has been postulated that these infections may originate from probiotics or from endogenous commensal reservoirs. In this report, we examine the population structure of Lacticaseibacillus rhamnosus and investigate the utility of using bacterial genomics to identify the source of invasive Lacticaseibacillus infections.

METHODS

Core genome phylogenetic analysis was performed on 602 L. rhamnosus genome sequences from the National Center for Biotechnology public database. This information was then used along with newly generated sequences of L. rhamnosus isolates from yogurt to investigate a fatal case of L. rhamnosus endocarditis.

RESULTS

Phylogenetic analysis demonstrated substantial genetic overlap of L. rhamnosus isolates cultured from food, probiotics, infected patients, and colonized individuals. This was applied to a patient who had both consumed yogurt and developed L. rhamnosus endocarditis to attempt to identify the source of his infection. The sequence of the isolate from the patient's bloodstream differed at only one nucleotide position from one of the yogurt isolates. Both isolates belonged to a clade, identified here as clade YC, composed of mostly gastrointestinal isolates from healthy individuals, some of which also differed by only a single nucleotide change from the patient's isolate.

CONCLUSIONS

As illustrated by this case, whole genome sequencing may be insufficient to reliably determine the source of invasive infections caused by L. rhamnosus.

Mapping the Escherichia coli DnaA-binding landscape reveals a preference for binding pairs of closely spaced DNA sites

DnaA is a widely conserved DNA-binding protein that is essential for the initiation of DNA replication in many bacterial species, including Escherichia coli. Cooperative binding of ATP-bound DnaA to multiple 9mer sites ('DnaA boxes') at the origin of replication results in local unwinding of the DNA and recruitment of the replication machinery. DnaA also functions as a transcription regulator by binding to DNA sites upstream of target genes. Previous studies have identified many sites of direct positive and negative regulation by E. coli DnaA. Here, we use a ChIP-seq to map the E. coli DnaA-binding landscape. Our data reveal a compact regulon for DnaA that coordinates the initiation of DNA replication with expression of genes associated with nucleotide synthesis, replication, DNA repair and RNA metabolism. We also show that DnaA binds preferentially to pairs of DnaA boxes spaced 2 or 3 bp apart. Mutation of either the upstream or downstream site in a pair disrupts DnaA binding, as does altering the spacing between sites. We conclude that binding of DnaA at almost all target sites requires a dimer of DnaA, with each subunit making critical contacts with a DnaA box.

Targeting synthesis of the Chromosome Replication Initiator Protein DnaA by antisense PNA-peptide conjugates in Escherichia coli

Initiation of chromosome replication is an essential stage of the bacterial cell cycle that is controlled by the DnaA protein. With the aim of developing novel antimicrobials, we have targeted the initiation of DNA replication, using antisense peptide nucleic acids (PNAs), directed against DnaA translation. A series of anti-DnaA PNA conjugated to lysine-rich bacterial penetrating peptides (PNA-BPPs) were designed to block DnaA translation. These anti-DnaA PNA-BPPs inhibited growth of wild-type Escherichia coli cells at low micromolar concentrations, and cells exposed to anti-DnaA PNA-BPPs exhibited characteristic hallmarks of chromosome replication inhibition. These results present one of very few compounds successfully targeting initiation of chromosome replication, an essential step in the bacterial cell cycle.

Open Questions about the Roles of DnaA, Related Proteins, and Hyperstructure Dynamics in the Cell Cycle

The DnaA protein has long been considered to play the key role in the initiation of chromosome replication in modern bacteria. Many questions about this role, however, remain unanswered. Here, we raise these questions within a framework based on the dynamics of hyperstructures, alias large assemblies of molecules and macromolecules that perform a function. In these dynamics, hyperstructures can (1) emit and receive signals or (2) fuse and separate from one another. We ask whether the DnaA-based initiation hyperstructure acts as a logic gate receiving information from the membrane, the chromosome, and metabolism to trigger replication; we try to phrase some of these questions in terms of DNA supercoiling, strand opening, glycolytic enzymes, SeqA, ribonucleotide reductase, the macromolecular synthesis operon, post-translational modifications, and metabolic pools. Finally, we ask whether, underpinning the regulation of the cell cycle, there is a physico-chemical clock inherited from the first protocells, and whether this clock emits a single signal that triggers both chromosome replication and cell division.

Multiple mechanisms for overcoming lethal over-initiation of DNA replication

DNA replication is highly regulated and primarily controlled at the step of initiation. In bacteria, the replication initiator DnaA and the origin of replication oriC are the primary targets of regulation. Perturbations that increase or decrease replication initiation can cause a decrease in cell fitness. We found that multiple mechanisms, including an increase in replication elongation and a decrease in replication initiation, can compensate for lethal over-initiation. We found that in Bacillus subtilis, under conditions of rapid growth, loss of yabA, a negative regulator of replication initiation, caused a synthetic lethal phenotype when combined with the dnaA1 mutation that also causes replication over-initiation. We isolated several classes of suppressors that restored viability to dnaA1 ∆yabA double mutants. Some suppressors (relA, nrdR) stimulated replication elongation. Others (dnaC, cshA) caused a decrease in replication initiation. One class of suppressors decreased replication initiation in the dnaA1 ∆yabA mutant by causing a decrease in the amount of the replicative helicase, DnaC. We found that decreased levels of helicase in otherwise wild-type cells were sufficient to decrease replication initiation during rapid growth, indicating that the replicative helicase is limiting for replication initiation. Our results highlight the multiple mechanisms cells use to regulate DNA replication.

Elevated Levels of the Escherichia coli nrdAB-Encoded Ribonucleotide Reductase Counteract the Toxicity Caused by an Increased Abundance of the β Clamp

Expression of the Escherichia coli dnaN-encoded β clamp at ≥10-fold higher than chromosomally expressed levels impedes growth by interfering with DNA replication. A mutant clamp (β^E202K bearing a glutamic acid-to-lysine substitution at residue 202) binds to DNA polymerase III (Pol III) with higher affinity than the wild-type clamp, suggesting that its failure to impede growth is independent of its ability to sequester Pol III away from the replication fork. Our results demonstrate that the dnaN^E202K strain underinitiates DNA replication due to insufficient levels of DnaA-ATP and expresses several DnaA-regulated genes at altered levels, including nrdAB, that encode the class 1a ribonucleotide reductase (RNR). Elevated expression of nrdAB was dependent on hda function. As the β clamp-Hda complex regulates the activity of DnaA by stimulating its intrinsic ATPase activity, this finding suggests that the dnaN^E202K allele supports an elevated level of Hda activity in vivo compared with the wild-type strain. In contrast, using an in vitro assay reconstituted with purified components the β^E202K and wild-type clamp proteins supported comparable levels of Hda activity. Nevertheless, co-overexpression of the nrdAB-encoded RNR relieved the growth defect caused by elevated levels of the β clamp. These results support a model in which increased cellular levels of DNA precursors relieve the ability of elevated β clamp levels to impede growth and suggest either that multiple effects stemming from the dnaN^E202K mutation contribute to elevated nrdAB levels or that Hda plays a noncatalytic role in regulating DnaA-ATP by sequestering it to reduce its availability. IMPORTANCE DnaA bound to ATP acts in initiation of DNA replication and regulates the expression of several genes whose products act in DNA metabolism. The state of the ATP bound to DnaA is regulated in part by the β clamp-Hda complex. The dnaN^E202K allele was identified by virtue of its inability to impede growth when expressed ≥10-fold higher than chromosomally expressed levels. While the dnaN^E202K strain exhibits several phenotypes consistent with heightened Hda activity, the wild-type and β^E202K clamp proteins support equivalent levels of Hda activity in vitro. Taken together, these results suggest that β^E202K-Hda plays a noncatalytic role in regulating DnaA-ATP. This, as well as alternative models, is discussed.

Transcriptional Activity of the Bacterial Replication Initiator DnaA

In bacteria, DnaA is the most conserved DNA replication initiator protein. DnaA is a DNA binding protein that is part of the AAA+ ATPase family. In addition to initiating chromosome replication, DnaA can also function as a transcription factor either as an activator or repressor. The first gene identified to be regulated by DnaA at the transcriptional levels was dnaA. DnaA has been shown to regulate genes involved in a variety of cellular events including those that trigger sporulation, DNA repair, and cell cycle regulation. DnaA's dual functions (replication initiator and transcription factor) is a potential mechanism for DnaA to temporally coordinate diverse cellular events with the onset of chromosome replication. This strategy of using chromosome replication initiator proteins as regulators of gene expression has also been observed in archaea and eukaryotes. In this mini review, we focus on our current understanding of DnaA's transcriptional activity in various bacterial species.

RNase I Modulates Escherichia coli Motility, Metabolism, and Resistance

Bacteria are constantly adapting to their environment by sensing extracellular factors that trigger production of intracellular signaling molecules, known as second messengers. Recently, 2',3'-cyclic nucleotide monophosphates (2',3'-cNMPs) were identified in Escherichia coli and have emerged as possible novel signaling molecules. 2',3'-cNMPs are produced through endonucleolytic cleavage of short RNAs by the T2 endoribonuclease, RNase I; however, the physiological roles of RNase I remain unclear. Our transcriptomic analysis suggests that RNase I is involved in modulating numerous cellular processes, including nucleotide metabolism, motility, acid sensitivity, metal homeostasis, and outer membrane morphology. Through a combination of deletion strain and inhibitor studies, we demonstrate that RNase I plays a previously unknown role in E. coli stress resistance by affecting pathways that are part of the defense mechanisms employed by bacteria when introduced to external threats, including antibiotics. Thus, this work provides insight into the emerging roles of RNase I in bacterial signaling and physiology and highlights the potential of RNase I as a target for antibacterial adjuvants.

Insufficient levels of the nrdAB-encoded ribonucleotide reductase underlie the severe growth defect of the Δhda E. coli strain

The ATP-bound form of the Escherichia coli DnaA replication initiator protein remodels the chromosomal origin of replication, oriC, to load the replicative helicase. The primary mechanism for regulating the activity of DnaA involves the Hda and β clamp proteins, which act together to dramatically stimulate the intrinsic DNA-dependent ATPase activity of DnaA via a process termed Regulatory Inactivation of DnaA. In addition to hyperinitiation, strains lacking hda function also exhibit cold sensitive growth at 30°C. Strains impaired for the other regulators of initiation (i.e., ΔseqA or ΔdatA) fail to exhibit cold sensitivity. The goal of this study was to gain insight into why loss of hda function impedes growth. We used a genetic approach to isolate 9 suppressors of Δhda cold sensitivity, and characterized the mechanistic basis by which these suppressors alleviated Δhda cold sensitivity. Taken together, our results provide strong support for the view that the fundamental defect associated with Δhda is diminished levels of DNA precursors, particularly dGTP and dATP. We discuss possible mechanisms by which the suppressors identified here may regulate dNTP pool size, as well as similarities in phenotypes between the Δhda strain and hda⁺ strains exposed to the ribonucleotide reductase inhibitor hydroxyurea.

NrdR Transcription Regulation: Global Proteome Analysis and Its Role in Escherichia coli Viability and Virulence

Bacterial ribonucleotide reductases (RNRs) play an important role in the synthesis of dNTPs and their expression is regulated by the transcription factors, NrdR and Fur. Recent transcriptomic studies using deletion mutants have indicated a role for NrdR in bacterial chemotaxis and in the maintenance of topoisomerase levels. However, NrdR deletion alone has no effect on bacterial growth or virulence in infected flies or in human blood cells. Furthermore, transcriptomic studies are limited to the deletion strain alone, and so are inadequate for drawing biological implications when the NrdR repressor is active or abundant. Therefore, further examination is warranted of changes in the cellular proteome in response to both NrdR overexpression, as well as deletion, to better understand its functional relevance as a bacterial transcription repressor. Here, we profile bacterial fate under conditions of overexpression and deletion of NrdR in E. coli. Biochemical assays show auxiliary zinc enhances the DNA binding activity of NrdR. We also demonstrate at the physiological level that increased nrdR expression causes a significant reduction in bacterial growth and fitness even at normal temperatures, and causes lethality at elevated temperatures. Corroborating these direct effects, global proteome analysis following NrdR overexpression showed a significant decrease in global protein expression. In parallel, studies on complementary expression of downregulated essential genes polA, eno and thiL showed partial rescue of the fitness defect caused by NrdR overexpression. Deletion of downregulated non-essential genes ygfK and trxA upon NrdR overexpression resulted in diminished bacterial growth and fitness suggesting an additional role for NrdR in regulating other genes. Moreover, in comparison with NrdR deletion, E. coli cells overexpressing NrdR showed significantly diminished adherence to human epithelial cells, reflecting decreased bacterial virulence. These results suggest that elevated expression of NrdR could be a suitable means to retard bacterial growth and virulence, as its elevated expression reduces bacterial fitness and impairs host cell adhesion.

Nucleotides, Nucleosides, and Nucleobases

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

H-NS is a novel transcriptional modulator of the ribonucleotide reductase genes in Escherichia coli

Ribonucleotide reductases (RNRs) are essential enzymes for DNA synthesis because they are responsible for the production of the four deoxyribonucleotides (dNTPs) from their corresponding ribonucleotides. Escherichia coli contains two classes of aerobic RNRs, encoded by the nrdAB (class Ia) and nrdHIEF (class Ib) operons, and a third RNR class, which is functional under anaerobic conditions and is encoded by the nrdDG (class III) operon. Because cellular imbalances in the amounts of the four dNTPs cause an increase in the rate of mutagenesis, the activity and the expression of RNRs must be tightly regulated during bacterial chromosome replication. The transcriptional regulation of these genes requires several transcription factors (including DnaA, IciA, FIS [factor for inversion stimulation], Fnr, Fur, and NrdR), depending on the RNR class; however, the factors that dictate the expression of some RNR genes in response to different environmental conditions are not known. We show that H-NS modulates the expression of the nrdAB and nrdDG operons. H-NS represses expression both in aerobically and in anaerobically growing cells. Under aerobic conditions, repression occurs at the exponential phase of growth as well as at the transition from the exponential to the stationary phase, a period when no dNTPs are needed. Under anoxic conditions, repression occurs mainly in exponentially growing cells. Electrophoretic mobility assays performed with two DNA fragments from the regulatory region of the nrdAB operon demonstrated the direct interaction of H-NS with these sequences.

The tRNA thiolation pathway modulates the intracellular redox state in Escherichia coli

We have performed a screening of hydroxyurea (HU)-sensitive mutants using a single-gene-deletion mutant collection in Escherichia coli K-12. HU inhibits ribonucleotide reductase (RNR), an enzyme that catalyzes the formation of deoxyribonucleotides. Unexpectedly, seven of the mutants lacked genes that are required for the incorporation of sulfur into a specific tRNA modification base, 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U), via persulfide relay. We found that the expression of RNR in the mutants was reduced to about one-third both in the absence and presence of HU, while sufficient deoxynucleoside triphosphate (dNTP) was maintained in the mutants in the absence of HU but a shortage occurred in the presence of HU. Trans-supply of an RNR R2 subunit rescued the HU sensitivity of these mutants. The mutants showed high intracellular ATP/ADP ratios, and overexpression of Hda, which catalyzes the conversion of DnaA-ATP to DnaA-ADP, rescued the HU sensitivity of the mutants, suggesting that DnaA-ATP represses RNR expression. The high intracellular ATP/ADP ratios were due to high respiration activity in the mutants. Our data suggested that intracellular redox was inclined toward the reduced state in these mutants, which may explain a change in RNR activity by reduction of the catalytically formed disulfide bond and high respiration activity by the NADH reducing potential. The relation between persulfide relay and intracellular redox is discussed.

Novel mutator mutants of E. coli nrdAB ribonucleotide reductase: insight into allosteric regulation and control of mutation rates

Ribonucleotide reductase (RNR) is the enzyme critically responsible for the production of the 5'-deoxynucleoside-triphosphates (dNTPs), the direct precursors for DNA synthesis. The dNTP levels are tightly controlled to permit high efficiency and fidelity of DNA synthesis. Much of this control occurs at the level of the RNR by feedback processes, but a detailed understanding of these mechanisms is still lacking. Using a genetic approach in the bacterium Escherichia coli, a paradigm for the class Ia RNRs, we isolated 23 novel RNR mutants displaying elevated mutation rates along with altered dNTP levels. The responsible amino-acid substitutions in RNR reside in three different regions: (i) the (d)ATP-binding activity domain, (ii) a novel region in the small subunit adjacent to the activity domain, and (iii) the dNTP-binding specificity site, several of which are associated with different dNTP pool alterations and different mutational outcomes. These mutants provide new insight into the precise mechanisms by which RNR is regulated and how dNTP pool disturbances resulting from defects in RNR can lead to increased mutation.

The Caulobacter crescentus chromosome replication origin evolved two classes of weak DnaA binding sites

The Caulobacter crescentus replication initiator DnaA and essential response regulator CtrA compete to control chromosome replication. The C. crescentus replication origin (Cori) contains five strong CtrA binding sites but only two apparent DnaA boxes, termed G-boxes (with a conserved second position G, TGATCCACA). Since clusters of DnaA boxes typify bacterial replication origins, this discrepancy suggested that C. crescentus DnaA recognizes different DNA sequences or compensates with novel DNA-binding proteins. We searched for novel DNA sites by scanning mutagenesis of the most conserved Cori DNA. Autonomous replication assays showed that G-boxes and novel W-boxes (TCCCCA) are essential for replication. Further analyses showed that C. crescentus DnaA binds G-boxes with moderate and W-boxes with very weak affinities significantly below DnaA's capacity for high-affinity Escherichia coli-boxes (TTATCCACA). Cori has five conserved W-boxes. Increasing W-box affinities increases or decreases autonomous replication depending on their strategic positions between the G-boxes. In vitro, CtrA binding displaces DnaA from proximal G-boxes and from distal W-boxes implying CtrA-DnaA competition and DnaA-DnaA cooperation between G-boxes and W-boxes. Similarly, during cell cycle progression, CtrA proteolysis coincides with DnaA binding to Cori. We also observe highly conserved W-boxes in other replication origins lacking E. coli-boxes. Therefore, strategically weak DnaA binding can be a general means of replication control.

Structural and thermodynamic signatures of DNA recognition by Mycobacterium tuberculosis DnaA

An essential protein, DnaA, binds to 9-bp DNA sites within the origin of replication oriC. These binding events are prerequisite to forming an enigmatic nucleoprotein scaffold that initiates replication. The number, sequences, positions, and orientations of these short DNA sites, or DnaA boxes, within the oriCs of different bacteria vary considerably. To investigate features of DnaA boxes that are important for binding Mycobacterium tuberculosis DnaA (MtDnaA), we have determined the crystal structures of the DNA binding domain (DBD) of MtDnaA bound to a cognate MtDnaA-box (at 2.0 Å resolution) and to a consensus Escherichia coli DnaA-box (at 2.3 Å). These structures, complemented by calorimetric equilibrium binding studies of MtDnaA DBD in a series of DnaA-box variants, reveal the main determinants of DNA recognition and establish the [T/C][T/A][G/A]TCCACA sequence as a high-affinity MtDnaA-box. Bioinformatic and calorimetric analyses indicate that DnaA-box sequences in mycobacterial oriCs generally differ from the optimal binding sequence. This sequence variation occurs commonly at the first 2 bp, making an in vivo mycobacterial DnaA-box effectively a 7-mer and not a 9-mer. We demonstrate that the decrease in the affinity of these MtDnaA-box variants for MtDnaA DBD relative to that of the highest-affinity box TTGTCCACA is less than 10-fold. The understanding of DnaA-box recognition by MtDnaA and E. coli DnaA enables one to map DnaA-box sequences in the genomes of M. tuberculosis and other eubacteria.

DnaA and ORC: more than DNA replication initiators

Mutations in DNA replication initiator genes in both prokaryotes and eukaryotes lead to a pleiotropic array of phenotypes, including defects in chromosome segregation, cytokinesis, cell cycle regulation and gene expression. For years, it was not clear whether these diverse effects were indirect consequences of perturbed DNA replication, or whether they indicated that DNA replication initiator proteins had roles beyond their activity in initiating DNA synthesis. Recent work from a range of organisms has demonstrated that DNA replication initiator proteins play direct roles in many cellular processes, often functioning to coordinate the initiation of DNA replication with essential cell-cycle activities. The aim of this review is to highlight these new findings, focusing on the pathways and mechanisms utilized by DNA replication initiator proteins to carry out a diverse array of cellular functions.

Ribonucleotide reductases of Salmonella typhimurium: transcriptional regulation and differential role in pathogenesis

Ribonucleotide reductases (RNRs) are essential enzymes that carry out the de novo synthesis of deoxyribonucleotides by reducing ribonucleotides. There are three different classes of RNRs (I, II and III), all having different oxygen dependency and biochemical characteristics. Salmonella enterica serovar Typhimurium (S. Typhimurium) harbors class Ia, class Ib and class III RNRs in its genome. We have studied the transcriptional regulation of these three RNR classes in S. Typhimurium as well as their differential function during infection of macrophage and epithelial cells. Deletion of both NrdR and Fur, two main transcriptional regulators, indicates that Fur specifically represses the class Ib enzyme and that NrdR acts as a global repressor of all three classes. A Fur recognition sequence within the nrdHIEF promoter has also been described and confirmed by electrophoretic mobility shift assays (EMSA). In order to elucidate the role of each RNR class during infection, S. Typhimurium single and double RNR mutants (as well as Fur and NrdR mutants) were used in infection assays with macrophage and epithelial cell lines. Our results indicate class Ia to be mainly responsible for deoxyribonucleotide production during invasion and proliferation inside macrophages and epithelial cells. Neither class Ib nor class III seem to be essential for growth under these conditions. However, class Ib is able to maintain certain growth in an nrdAB mutant during the first hours of macrophage infection. Our results suggest that, during the early stages of macrophage infection, class Ib may contribute to deoxyribonucleotide synthesis by means of both an NrdR and a Fur-dependent derepression of nrdHIEF due to hydrogen peroxide production and DNA damage associated with the oxidative burst, thus helping to overcome the host defenses.

DnaA-ATP acts as a molecular switch to control levels of ribonucleotide reductase expression in Escherichia coli

Ribonucleotide reductase (RNR) is the bottleneck enzyme in the synthesis of dNTPs required for DNA replication. In order to avoid the mutagenic effects of imbalances in dNTPs the amount and activity of RNR enzyme in the cell is tightly regulated. RNR expression from the nrdAB operon is thus coupled to coincide with the initiation of DNA replication. However, the mechanism for the co-ordination of gene transcription and DNA replication remains to be elucidated. The timing and synchrony of DNA replication initiation in Escherichia coli is controlled in part by the binding of the DnaA protein to the origin of replication. DnaA is also a transcription factor of the nrdAB operon and could thus be the link between these two processes. Here we show that RNA polymerase can form a stable transcription initiation complex at the nrdAB promoter by direct interaction with the far upstream sites required for the timing of expression as a function of DNA replication. In addition, we show that the binding of DnaA on the promoter can either activate or repress transcription as a function of its concentration and its nucleotide-bound state. However, transcription regulation by DnaA does not significantly affect the timing of expression of RNR from the nrdAB operon.

Role of decreased levels of Fis histone-like protein in Crohn's disease-associated adherent invasive Escherichia coli LF82 bacteria interacting with intestinal epithelial cells

The interaction of Crohn's disease (CD)-associated adherent-invasive Escherichia coli (AIEC) strain LF82 with intestinal epithelial cells depends on surface appendages, such as type 1 pili and flagella. Histone-like proteins operate as global regulators to control the expression of these virulence factors. We evaluated the role of histone-like proteins in AIEC reference strain LF82 during infection of intestinal epithelial cells, Intestine-407, and observed that the fis mRNA level was decreased. The role of Fis in AIEC LF82 was determined by studying the phenotype of an LF82 fis::Km mutant. This was the first mutant of strain LF82 that has been described thus far that is unable to express flagellin but still able to produce type 1 pili. The cyclic-di-GMP pathway linking flagella and type 1 pilus expression is not involved in Fis-mediated regulation, and we identified in the present study Fis-binding sites located upstream of the fimE gene and in the intergenic region between fimB and nanC of the fim operon encoding type 1 pili. The major consequence of decreased Fis expression in AIEC bacteria in contact with host cells is a direct downregulation of fimE expression, leading to the preferential ON phase of the fimS element. Thus, by maintaining type 1 pilus expression, AIEC bacteria, which interact with the gut mucosa, have greater ability to colonize and to induce inflammation in CD patients.

Functional analysis of the Streptomyces coelicolor NrdR ATP-cone domain: role in nucleotide binding, oligomerization, and DNA interactions

Ribonucleotide reductases (RNRs) are essential enzymes in all living cells, providing the only known de novo pathway for the biosynthesis of deoxyribonucleotides (dNTPs), the immediate precursors of DNA synthesis and repair. RNRs catalyze the controlled reduction of all four ribonucleotides to maintain a balanced pool of dNTPs during the cell cycle. Streptomyces species contain genes, nrdAB and nrdJ, coding for oxygen-dependent class I and oxygen-independent class II RNRs, either of which is sufficient for vegetative growth. Both sets of genes are transcriptionally repressed by NrdR. NrdR contains a zinc ribbon DNA-binding domain and an ATP-cone domain similar to that present in the allosteric activity site of many class I and class III RNRs. Purified NrdR contains up to 1 mol of tightly bound ATP or dATP per mol of protein and binds to tandem 16-bp sequences, termed NrdR-boxes, present in the upstream regulatory regions of bacterial RNR operons. Previously, we showed that the ATP-cone domain alone determines nucleotide binding and that an NrdR mutant defective in nucleotide binding was unable to bind to DNA probes containing NrdR-boxes. These observations led us to propose that when NrdR binds ATP/dATP it undergoes a conformational change that affects DNA binding and hence RNR gene expression. In this study, we analyzed a collection of ATP-cone mutant proteins containing changes in residues inferred to be implicated in nucleotide binding and show that they result in pleiotrophic effects on ATP/dATP binding, on protein oligomerization, and on DNA binding. A model is proposed to integrate these observations.

The relationship between dNTP pool levels and mutagenesis in an Escherichia coli NDP kinase mutant

Loss of nucleoside diphosphate kinase (Ndk) function in Escherichia coli results in an increased frequency of spontaneous mutation and an imbalance in dNTP pool levels. It is presumed that the imbalance in dNTP pool levels is responsible for the mutator phenotype of an E. coli ndk mutant. A human homologue of Ndk and potential suppressor of tumor metastasis, nm23-H2, can complement the mutagenic phenotype of an E. coli ndk mutant. Here, we show that the antimutagenic property of nm23-H2 in E. coli is independent of dNTP pool levels, indicating that dNTP pool imbalance is not responsible for the mutator phenotype associated with the loss of ndk function. We have identified multiple genetic interactions between ndk and genes involved in the metabolism of dUTP, a potentially mutagenic precursor of thymidine biosynthesis. We show that loss of ndk function is synergistic with a dut-1 mutation and synthetically lethal with the loss of thymidine kinase function. Our results suggest that Ndk prevents the accumulation of dUTP in vivo. Based on these results and biochemical studies of Ndk, we propose that the mutagenic phenotype of an ndk mutant is caused by excess misincorporation of uracil in place of thymidine combined with a defect in the uracil base excision pathway.

Once in a lifetime: strategies for preventing re-replication in prokaryotic and eukaryotic cells

DNA replication is an extremely accurate process and cells have evolved intricate control mechanisms to ensure that each region of their genome is replicated only once during S phase. Here, we compare what is known about the processes that prevent re-replication in prokaryotic and eukaryotic cells by using the model organisms Escherichia coli and Schizosaccharomyces pombe as examples. Although the underlying molecular details are different, the logic behind the control mechanisms is similar. For example, after initiation, crucial molecules required for the loading of replicative helicases in both prokaryotes and eukaryotes are inactivated until the next cell cycle. Furthermore, in both systems the beta-clamp of the replicative polymerase associates with enzymatic activities that contribute to the inactivation of the helicase loaders. Finally, recent studies suggest that the control mechanism that prevents re-replication in both systems also increases the synthesis of DNA building blocks.

Genome-wide analysis of Fis binding in Escherichia coli indicates a causative role for A-/AT-tracts

We determined the genome-wide distribution of the nucleoid-associated protein Fis in Escherichia coli using chromatin immunoprecipitation coupled with high-resolution whole genome-tiling microarrays. We identified 894 Fis-associated regions across the E. coli genome. A significant number of these binding sites were found within open reading frames (33%) and between divergently transcribed transcripts (5%). Analysis indicates that A-tracts and AT-tracts are an important signal for preferred Fis-binding sites, and that A(6)-tracts in particular constitute a high-affinity signal that dictates Fis phasing in stretches of DNA containing multiple and variably spaced A-tracts and AT-tracts. Furthermore, we find evidence for an average of two Fis-binding regions per supercoiling domain in the chromosome of exponentially growing cells. Transcriptome analysis shows that approximately 21% of genes are affected by the deletion of fis; however, the changes in magnitude are small. To address the differential Fis bindings under growth environment perturbation, ChIP-chip analysis was performed using cells grown under aerobic and anaerobic growth conditions. Interestingly, the Fis-binding regions are almost identical in aerobic and anaerobic growth conditions-indicating that the E. coli genome topology mediated by Fis is superficially identical in the two conditions. These novel results provide new insight into how Fis modulates DNA topology at a genome scale and thus advance our understanding of the architectural bases of the E. coli nucleoid.

Distribution of stable DnaA-binding sites on the Bacillus subtilis genome detected using a modified ChIP-chip method

We developed a modified ChIP-chip method, designated ChAP-chip (Chromatin Affinity Precipitation coupled with tiling chip). The binding sites of Bacillus subtilis Spo0J determined using this technique were consistent with previous findings. A DNA replication initiator protein, DnaA, formed stable complexes at eight intergenic regions on the B. subtilis genome. Characterization of the binding sequences suggested that two factors—the local density of DnaA boxes and their affinities for DnaA—are critical for stable binding. We further showed that in addition to autoregulation, DnaA directly modulate the expression of sda in a positive, and ywlC and yydA in a negative manner. Examination of possible stable DnaA-binding sequences in other Bacillus species suggested that DnaA-dependent regulation of those genes is maintained in most bacteria examined, supporting their biological significance. In addition, a possible stable DnaA-binding site downstream of gcp is also suggested to be conserved. Furthermore, potential DnaA-binding sequences specific for each bacterium have been identified, generally in close proximity to oriC. These findings suggest that DnaA plays several additional roles, such as control of the level of effective initiator, ATP-DnaA, and/or stabilization of the domain structure of the genome around oriC for the proper initiation of chromosome replication.

Ribonucleotide reductase and the regulation of DNA replication: an old story and an ancient heritage

All organisms that synthesize their own DNA have evolved mechanisms for maintaining a constant DNA/cell mass ratio independent of growth rate. The DNA/cell mass ratio is a central parameter in the processes controlling the cell cycle. The co-ordination of DNA replication with cell growth involves multiple levels of regulation. DNA synthesis is initiated at specific sites on the chromosome termed origins of replication, and proceeds bidirectionally to elongate and duplicate the chromosome. These two processes, initiation and elongation, therefore determine the total rate of DNA synthesis in the cell. In Escherichia coli, initiation depends on the DnaA protein while elongation depends on a multiprotein replication factory that incorporates deoxyribonucleotides (dNTPs) into the growing DNA chain. The enzyme ribonucleotide reductase (RNR) is universally responsible for synthesizing the necessary dNTPs. In this review we examine the role RNR plays in regulating the total rate of DNA synthesis in E. coli and, hence, in maintaining constant DNA/cell mass ratios during normal growth and under conditions of DNA stress.

Common and variable contributions of Fis residues to high-affinity binding at different DNA sequences

Fis is a nucleoid-associated protein that interacts with poorly related DNA sequences with a high degree of specificity. A difference of more than 3 orders of magnitude in apparent Kd values was observed between specific (Kd, approximately 1 to 4 nM) and nonspecific (Kd, approximately 4 microM) DNA binding. To examine the contributions of Fis residues to the high-affinity binding at different DNA sequences, 13 alanine substitutions were generated in or near the Fis helix-turn-helix DNA binding motif, and the resulting proteins were purified. In vitro binding assays at three different Fis sites (fis P II, hin distal, and lambda attR) revealed that R85, T87, R89, K90, and K91 played major roles in high-affinity DNA binding and that R85, T87, and K90 were consistently vital for binding to all three sites. Other residues made variable contributions to binding, depending on the binding site. N84 was required only for binding to the lambda attR Fis site, and the role of R89 was dramatically altered by the lambda attR DNA flanking sequence. The effects of Fis mutations on fis P II or hin distal site binding in vitro generally correlated with their abilities to mediate fis P repression or DNA inversion in vivo, demonstrating that the in vitro DNA-binding effects are relevant in vivo. The results suggest that while Fis is able to recognize a minimal common set of DNA sequence determinants at different binding sites, it is also equipped with a number of residues that contribute to the binding strength, some of which play variable roles.

A novel regulatory mechanism couples deoxyribonucleotide synthesis and DNA replication in Escherichia coli

We present evidence for a complex regulatory interplay between the initiation of DNA replication and deoxyribonucleotide synthesis. In Escherichia coli, the ATP-bound DnaA protein initiates chromosomal replication. Upon loading of the beta-clamp subunit (DnaN) of the replicase, DnaA is inactivated as its intrinsic ATPase activity is stimulated by the protein Hda. The beta-subunit acts as a matchmaker between Hda and DnaA. Chain elongation of DNA requires a sufficient supply of deoxyribonucleotides (dNTPs), which are produced by ribonucleotide reductase (RNR). We present evidence suggesting that the molecular switch from ATP-DnaA to ADP-DnaA is a critical step coordinating DNA replication with increased deoxyribonucleotide synthesis. Characterization of dnaA and dnaN mutations that result in a constitutively high expression of RNR reveal this mechanism. We propose that the nucleotide bound state of DnaA regulates the transcription of the genes encoding ribonucleotide reductase (nrdAB). Accordingly, the conversion of ATP-DnaA to ADP-DnaA after initiation and loading of the beta-subunit DnaN would allow increased nrdAB expression, and consequently, coordinated RNR synthesis and DNA replication during the cell cycle.

DnaA coordinates replication initiation and cell cycle transcription in Caulobacter crescentus

The level of DnaA, a key bacterial DNA replication initiation factor, increases during the Caulobacter swarmer-to-stalked transition just before the G1/S transition. We show that DnaA coordinates DNA replication initiation with cell cycle progression by acting as a global transcription factor. Using DnaA depletion and induction in synchronized cell populations, we have analysed global transcription patterns to identify the differential regulation of normally co-expressed genes. The DnaA regulon includes genes encoding several replisome components, the GcrA global cell cycle regulator, the PodJ polar localization protein, the FtsZ cell division protein, and nucleotide biosynthesis enzymes. In cells depleted of DnaA, the G1/S transition is temporally separated from the swarmer-to-stalked cell differentiation, which is normally coincident. In the absence of DnaA, the CtrA master regulator is cleared by proteolysis during the swarmer-to-stalked cell transition as usual, but DNA replication initiation is blocked. In this case, expression of gcrA, which is directly repressed by CtrA, does not increase in conjunction with the disappearance of CtrA until DnaA is subsequently induced, showing that gcrA expression requires DnaA. DnaA boxes are present upstream of many genes whose expression requires DnaA, and His6-DnaA binds to the promoters of gcrA, ftsZ and podJ in vitro. This redundant control of gcrA transcription by DnaA (activation) and CtrA (repression) forms a robust switch controlling the decision to proceed through the cell cycle or to remain in the G1 stage.

A transcriptional response to replication status mediated by the conserved bacterial replication protein DnaA

Organisms respond to perturbations in DNA replication. We characterized the global transcriptional response to inhibition of DNA replication in Bacillus subtilis. We focused on changes that were independent of the known recA-dependent global DNA damage (SOS) response. We found that overlapping sets of genes are affected by perturbations in replication elongation or initiation and that this transcriptional response serves to inhibit cell division and maintain cell viability. Approximately 20 of the operons (>50 genes) affected have potential DnaA-binding sites and are probably regulated directly by DnaA, the highly conserved replication initiation protein and transcription factor. Many of these genes have homologues and recognizable DnaA-binding sites in other bacteria, indicating that a DnaA-mediated response, elicited by changes in DNA replication status, may be conserved.

Genome of bacteriophage P1

P1 is a bacteriophage of Escherichia coli and other enteric bacteria. It lysogenizes its hosts as a circular, low-copy-number plasmid. We have determined the complete nucleotide sequences of two strains of a P1 thermoinducible mutant, P1 c1-100. The P1 genome (93,601 bp) contains at least 117 genes, of which almost two-thirds had not been sequenced previously and 49 have no homologs in other organisms. Protein-coding genes occupy 92% of the genome and are organized in 45 operons, of which four are decisive for the choice between lysis and lysogeny. Four others ensure plasmid maintenance. The majority of the remaining 37 operons are involved in lytic development. Seventeen operons are transcribed from sigma(70) promoters directly controlled by the master phage repressor C1. Late operons are transcribed from promoters recognized by the E. coli RNA polymerase holoenzyme in the presence of the Lpa protein, the product of a C1-controlled P1 gene. Three species of P1-encoded tRNAs provide differential controls of translation, and a P1-encoded DNA methyltransferase with putative bifunctionality influences transcription, replication, and DNA packaging. The genome is particularly rich in Chi recombinogenic sites. The base content and distribution in P1 DNA indicate that replication of P1 from its plasmid origin had more impact on the base compositional asymmetries of the P1 genome than replication from the lytic origin of replication.

Escherichia coli prereplication complex assembly is regulated by dynamic interplay among Fis, IHF and DnaA

Initiator DnaA and DNA bending proteins, Fis and IHF, comprise prereplication complexes (pre-RC) that unwind the Escherichia coli chromosome's origin of replication, oriC. Loss of either Fis or IHF perturbs synchronous initiation from oriC copies in rapidly growing E. coli. Based on dimethylsulphate (DMS) footprinting of purified proteins, we observed a dynamic interplay among Fis, IHF and DnaA on supercoiled oriC templates. Low levels of Fis inhibited oriC unwinding by blocking both IHF and DnaA binding to low affinity sites. As the concentration of DnaA was increased, Fis repression was relieved and IHF rapidly redistributed DnaA to all unfilled binding sites on oriC. This behaviour in vitro is analogous to observed assembly of pre-RC in synchronized E. coli. We propose that as new DnaA is synthesized in E. coli, opposing activities of Fis and IHF ensure an abrupt transition from a repressed complex with unfilled weak affinity DnaA binding sites to a completely loaded unwound complex, increasing both the precision of DNA replication timing and initiation synchrony.

Interactions of glutaredoxins, ribonucleotide reductase, and components of the DNA replication system of Escherichia coli

A strain of Escherichia coli missing three members of the thioredoxin superfamily, thioredoxins 1 and 2 and glutaredoxin 1, is unable to grow, a phenotype presumed to be due to the inability of cells to reduce the essential enzyme ribonucleotide reductase. Two classes of mutations can restore growth to such a strain. First, we have isolated a collection of mutations in the gene for the protein glutaredoxin 3 that suppress the growth defect. Remarkably, all eight independent mutations alter the same amino acid, methionine-43, changing it to valine, isoleucine, or leucine. From the position of the amino acid changes and their effects, we propose that these alterations change the protein so that its properties are closer to those of glutaredoxin 1. The second means of suppressing the growth defects of the multiply mutant strain was by mutations in the DNA replication genes, dnaA and dnaN. These mutations substantially increase the expression of ribonucleotide reductase, most likely by altering the interaction of the regulatory protein DnaA with the ribonucleotide reductase promoter. Our results suggest that this increase in the concentration of ribonucleotide reductase in the cell allows more effective interaction with glutaredoxin 3, thus restoring an effective pool of deoxyribonucleotides. Our studies present direct evidence that ribonucleotide reductase is the only essential enzyme that requires the three reductive proteins missing in our strains. Our results also suggest an unexpected regulatory interaction between the DnaA and DnaN proteins.

FNR-mediated oxygen-responsive regulation of the nrdDG operon of Escherichia coli

Transcription of the nrdDG operon, which encodes the class III nucleotide reductase, which is only active under anaerobic conditions, was strongly induced after a shift to anaerobiosis. The induction was completely dependent on the transcriptional activator FNR and was independent of the ArcA-ArcB two-component response regulator system. The nrdD transcript start site was mapped to a position immediately downstream of two FNR binding sites. Transcription of the other two nucleotide reductase operons, nrdAB and nrdEF, did not respond to oxygen conditions in a wild-type background, but nrdAB expression was increased in the fnr mutant under anaerobic conditions.

Expression analysis of the nrdHIEF operon from Escherichia coli. Conditions that trigger the transcript level in vivo

Escherichia coli has two aerobic ribonucleotide reductases encoded by the nrdAB and nrdHIEF operons. While NrdAB is active during aerobiosis, NrdEF is considered a cryptic enzyme with no obvious function. Here, we present evidence that nrdHIEF expression might be important under certain circumstances. Basal transcript levels were dramatically enhanced (25-75-fold), depending on the growth-phase and the growth-medium composition. Likewise, a large increase of >100-fold in nrdHIEF mRNA was observed in bacteria lacking Trx1 and Grx1, the two main NrdAB reductants. Moreover, nrdHIEF expression was triggered in response to oxidative stress, particularly in mutants missing hydroperoxidase I and alkyl-hydroperoxide reductase activities (69.7-fold) and in cells treated with oxidants (up to 23.4-fold over the enhanced transcript level possessed by cells grown on minimal medium). The mechanism(s) that triggers nrdHIEF expression remains unknown, but our findings exclude putative global regulators like RpoS, Fis, cAMP, OxyR, SoxR/S, or RecA. What we have learned about nrdHIEF expression indicates strong differences between its regulation and that of the nrdAB operon and of genes coding for components of both thioredoxin/glutaredoxin pathways. We propose that E. coli might optimize the responses to different stimuli by co-evolving the expression levels for its multiple reductases and electron donors.

Growth-phase regulation of the Escherichia coli thioredoxin gene

The two promoters of Escherichia coli trxA gene were separately cloned into pKO100 as well as pJEL170. Galactokinase expression in cells containing the pKO100 derivatives was found to be negatively correlated with growth rate and was 6- to 20-fold higher in stationary cultures than in exponential cultures. The expression of trxA-galK was induced by amino acid starvation in a RelA(+) strain but not in an isogenic Rel(-) strain indicating that the control involves guanosine 3',5'-bispyrophosphate (ppGpp). RpoS, which appears to be essential for expression of most stationary phase expressed genes, is not required for trxA expression. Increased expression of relA, which increases ppGpp concentration, increases trxA expression.

Interactions of the Streptomyces lividans initiator protein DnaA with its target

The Streptomyces lividans DnaA protein (73 kDa) consists, like other bacterial DnaA proteins, of four domains; it binds to 19 DnaA boxes in the complex oriC region. The S. lividans DnaA protein differs from others in that it contains an additional stretch of 120 predominantly acidic amino acids within domain II. Interactions between the DnaA protein and the two DnaA boxes derived from the promoter region of the S. lividans dnaA gene were analysed in vitro using three independent methods: Dnase-I-footprinting experiments, mobility-shift assay and surface plasmon resonance (SPR). The Dnase-I-footprinting analysis showed that the wild-type DnaA protein binds to both DnaA boxes. Thus, as in Escherichia coli and Bacillus subtilis, the S. lividans dnaA gene may be autoregulated. SPR analysis showed that the affinity of the DnaA protein for a DNA fragment containing both DnaA boxes from the dnaA promoter region (KD = 1.25 nM) is 10 times higher than its affinity for the single 'strong' DnaA box (KD = 12.0 nM). The mobility-shift assay suggests the presence of at least two classes of complex containing different numbers of bound DnaA molecules. The above data reveal that the DnaA protein binds to the two DnaA boxes in a cooperative manner. To deduce structural features of the Streptomyces domain II of DnaA protein, the amino acid DnaA sequences of three Streptomyces species were compared. However, according to the secondary structure prediction, Streptomyces domain II does not contain any common relevant secondary structural element(s). It can be assumed that domain II of DnaA protein can play a role as a flexible protein spacer between the N-terminal domain I and the highly conserved C-terminal part of DnaA protein containing ATP-binding domain III and DNA-binding domain IV.

Ribonucleotide reductases

Ribonucleotide reductases provide the building blocks for DNA replication in all living cells. Three different classes of enzymes use protein free radicals to activate the substrate. Aerobic class I enzymes generate a tyrosyl radical with an iron-oxygen center and dioxygen, class II enzymes employ adenosylcobalamin, and the anaerobic class III enzymes generate a glycyl radical from S-adenosylmethionine and an iron-sulfur cluster. The X-ray structure of the class I Escherichia coli enzyme, including forms that bind substrate and allosteric effectors, confirms previous models of catalytic and allosteric mechanisms. This structure suggests considerable mobility of the protein during catalysis and, together with experiments involving site-directed mutants, suggests a mechanism for radical transfer from one subunit to the other. Despite large differences between the classes, common catalytic and allosteric mechanisms, as well as retention of critical residues in the protein sequence, suggest a similar tertiary structure and a common origin during evolution. One puzzling aspect is that some organisms contain the genes for several different reductases.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

In vivo transcription of nrdAB operon and of grxA and fpg genes is triggered in Escherichia coli lacking both thioredoxin and glutaredoxin 1 or thioredoxin and glutathione, respectively

We have previously described () that Escherichia coli maintains a balanced supply of deoxyribonucleotides by a regulatory mechanism that up-regulates the levels of ribonucleotide reductase with the lack of its main hydrogen donors thioredoxin, glutaredoxin 1, and glutathione (GSH). By using a semi-quantitative reverse transcription/multiplex polymerase chain reaction fluorescent procedure that enables simultaneous analysis of up to seven mRNA species, we now demonstrate that regulation operates at the transcriptional level. Double mutant cells lacking both thioredoxin and glutaredoxin 1 had increased transcription of the nrdAB operon, as compared with the corresponding wild type parent (maximal induction of 10- and 9-fold for mRNA of nrdA and nrdB genes, respectively). Likewise, a dramatic increase of 36-fold in grxA mRNA was observed in bacteria simultaneously deficient in thioredoxin and GSH (the physiological reductant of all glutaredoxins). The increased expression of the grxA gene in trxA gshA double mutant bacteria was mimicked in trxA single mutant cells by depletion of GSH with diethylmaleate (DEM). This induction of grxA transcription was rapid since maximal increase was detected upon 10 min of DEM exposure. Like grxA expression, the basal level of fpg mRNA, encoding formamidopyrimidine-DNA glycosylase, was increased (about 4-fold) in a trxA gshA double mutant strain; this expression was also induced upon exposure to DEM (11-fold maximal induction). These results suggest that transcription of grxA might share common redox regulatory mechanism(s) with that of the fpg gene, involved in the repair of 8-oxoguanine in DNA.

A 45 bp inverted repeat is required for cell cycle regulation of the Escherichia coli nrd operon

Expression of beta-galactosidase from a nrd-lacZ fusion was used to determine the role in nrd regulation of an inverted sequence upstream of the promoter. Removal or replacement of a 45bp inverted repeat with an altered sequence including a 48bp perfect inverted repeat resulted in a mutant phenotype that was low in nrd expression in an exponentially growing culture and that did not increase during DNA synthesis inhibition. Changing the 22 bp in the upstream half of the inverted repeat resulted in the same phenotype, whereas changing the 22 bp in the downstream half of the inverted repeat decreased nrd expression to a lesser extent in an exponentially growing culture and had only a smaller effect on nrd expression during DNA synthesis inhibition. As other mutants with the phenotype of the upstream inverted repeat mutant were found to lack cell cycle regulation, expression of nrd-lac mRNA produced from a plasmid with this mutation in the nrd-lacZ fusion gene was compared with nrd mRNA produced from the chromosomal nrd gene in a synchronized culture. The results indicated that the upstream half of the nrd inverted repeat contains a cis-acting element essential for nrd cell cycle regulation.

Multiple cis-acting sites positively regulate Escherichia coli nrd expression

Regulation of nrd expression in Escherichia coli by cis-acting elements was found to be more complex than previously reported. At least five upstream sites appear to positively regulate nrd expression including a Fis binding site, a DnaA binding site, an AT-rich region, an inverted repeat and a 10 bp site between the AT-rich region and the inverted repeat. Double mutants defective in these sites indicate that all sites tested act independently when regulating nrd expression. As the decrease in nrd expression in exponentially growing cultures paralleled the decrease observed in DNA synthesis-inhibited cultures for all single and double mutants, we concluded that nrd is regulated by the same mechanism in these physiological states. As mutants unable to induce nrd expression during inhibition of DNA synthesis also fail to exhibit cell cycle-regulated nrd expression, we conclude that cell cycle nrd regulation is controlled by these same sites. Site-directed mutagenesis was used to show that the absence of an increase in nrd expression during DNA inhibition previously observed for deletion of the AT-rich region results from deletion of both the Fis binding site and the AT-rich region.

Information analysis of Fis binding sites

Originally discovered in the bacteriophage Mu DNA inversion system gin, Fis (Factor for Inversion Stimulation) regulates many genetic systems. To determine the base frequency conservation required for Fis to locate its binding sites, we collected a set of 60 experimentally defined wild-type Fis DNA binding sequences. The sequence logo for Fis binding sites showed the significance and likely kinds of base contacts, and these are consistent with available experimental data. Scanning with an information theory based weight matrix within fis, nrd, tgt/sec and gin revealed Fis sites not previously identified, but for which there are published footprinting and biochemical data. DNA mobility shift experiments showed that a site predicted to be 11 bases from the proximal Salmonella typhimurium hin site and a site predicted to be 7 bases from the proximal P1 cin site are bound by Fis in vitro. Two predicted sites separated by 11 bp found within the nrd promoter region, and one in the tgt/sec promoter, were also confirmed by gel shift analysis. A sequence in aldB previously reported to be a Fis site, for which information theory predicts no site, did not shift. These results demonstrate that information analysis is useful for predicting Fis DNA binding.

Deletion analysis of the fis promoter region in Escherichia coli: antagonistic effects of integration host factor and Fis

Fis is a small DNA-binding and -bending protein in Escherichia coli that is involved in several different biological processes, including stimulation of specialized DNA recombination events and regulation of gene expression. fis protein and mRNA levels rapidly increase during early logarithmic growth phase in response to a nutritional upshift but become virtually undetectable during late logarithmic and stationary phases. We present evidence that the growth phase-dependent fis expression pattern is not determined by changes in mRNA stability, arguing in favor of regulation at the level of transcription. DNA deletion analysis of the fis promoter (fis P) region indicated that DNA sequences from -166 to -81, -36 to -26, and +107 to +366 relative to the transcription start site are required for maximum expression. A DNA sequence resembling the integration host factor (IHF) binding site centered approximately at -114 showed DNase I cleavage protection by IHF. In ihf cells, maximum cellular levels of fis mRNA were decreased more than 3-fold and transcription from fis P on a plasmid was decreased about 3.8-fold compared to those in cells expressing wild-type IHF. In addition, a mutation in the ihf binding site resulted in a 76 and 61% reduction in transcription from fis P on a plasmid in the presence or absence of Fis, respectively. Insertions of 5 or 10 bp between this ihf site and fis P suggest that IHF functions in a position-dependent manner. We conclude that IHF plays a role in stimulating transcription from fis P by interacting with a site centered approximately at -114 relative to the start of transcription. We also showed that although the fis P region contains six Fis binding sites, Fis site II (centered at -42) played a predominant role in autoregulation, Fis sites I and III (centered at +26 and -83, respectively) seemingly played smaller roles, and no role in negative autoregulation could be attributed to Fis sites IV, V, and VI (located upstream of site III). The fis P region from -36 to +7, which is not directly regulated by either IHF or Fis, retained the characteristic fis regulation pattern in response to a nutritional upshift.

Threonine 435 of Escherichia coli DnaA protein confers sequence-specific DNA binding activity

The Escherichia coli DnaA protein, as a sequence-specific DNA binding protein, promotes the initiation of chromosomal replication by binding to four asymmetric 9-mer sequences termed DnaA boxes in oriC. Characterization of N-terminal, C-terminal, and internal in-frame deletion mutants identified residues near the C terminus of DnaA protein required for DNA binding. Furthermore, genetic and biochemical characterization of 11 missense mutations mapping within the C-terminal 89 residues indicated that they were defective in DNA binding. Detailed biochemical characterization of one mutant protein bearing a threonine to methionine substitution at position 435 (T435M) revealed that it retained only nonspecific DNA binding activity, suggesting that threonine 435 imparts specificity in binding. Finally, T435M was inactive on its own for in vitro replication of an oriC plasmid but was able to augment limiting levels of wild type DnaA protein, consistent with the proposal that not all of the DnaA monomers in the initial complex are bound specifically to oriC and that direct interaction occurs among monomers.

Fis binding in the dnaA operon promoter region

The region between the rpmH and dnaA genes contains five promoters that divergently express the ribosomal protein L34 and the proteins of the dnaA operon, including DnaA, the beta clamp of DNA polymerase III holoenzyme, and RecF. The DNA-binding protein Fis was shown by the band shift assay to bind near the rpmHp2 and dnaAp2 promoters and by DNase I footprinting to bind to a single site in the dnaAp2 promoter overlapping the -35 and spacer sequences. There were no observable differences in Fis affinity or the angle of bending induced by Fis between methylated and unmethylated DNA fragments containing the Fis binding site in the dnaAp2 promoter. Fis directly or indirectly represses the expression of DnaA protein and the beta clamp of DNA polymerase III. A fis null mutant containing a dnaA-lacZ in-frame fusion had twofold greater beta-galactosidase activity than a fis wild-type strain, and induced expression of Fis eliminated the increase in activity of the fusion protein. A two- to threefold increase in the levels of DnaA and beta clamp proteins was found in a fis null mutant by immunoblot gel analysis.

Two different operons for the same function: comparison of the Salmonella typhimurium nrdAB and nrdEF genes

By using a P22 phage-mediated cloning system, the nrdAB genes of Salmonella typhimurium (St), encoding a ribonucleotide reductase (RR) of class I, have been isolated. The coding regions of the St nrdAB operon show a very high identity with those of the homologous operon of Escherichia coli (Ec). Nevertheless, there are significant differences in their promoter regions since, although the promoters of both operons present two DnaA boxes, these boxes are located downstream from the transcription start point in St, being upstream in Ec. Moreover, the deduced amino-acid sequences of the St nrdAB showed a very limited overall identity (28%) with the products of St nrdEF, which encode a second class-I RR. Expression of St nrdAB and nrdEF is inducible by hydroxyurea, an inhibitor of RR activity. Alignment of the promoter regions of the nrdAB and nrdEF operons of both St and Ec reveals the presence of a consensus sequence. St is the first organism from which two different RR belonging to the same biochemical class are known.