Regulation of toxinogenesis in Corynebacterium diphtheriae. I. Mutations in bacteriophage beta that alter the effects of iron on toxin production

Corynebacterium diphtheriae: Diphtheria Toxin, the tox Operon, and Its Regulation by Fe2+ Activation of apo-DtxR

Diphtheria is one of the most well studied of all the bacterial infectious diseases. These milestone studies of toxigenic Corynebacterium diphtheriae along with its primary virulence determinant, diphtheria toxin, have established the paradigm for the study of other related bacterial protein toxins. This review highlights those studies that have contributed to our current understanding of the structure-function relationships of diphtheria toxin, the molecular mechanism of its entry into the eukaryotic cell cytosol, the regulation of diphtheria tox expression by holo-DtxR, and the molecular basis of transition metal ion activation of apo-DtxR itself. These seminal studies have laid the foundation for the protein engineering of diphtheria toxin and the development of highly potent eukaryotic cell-surface receptor-targeted fusion protein toxins for the treatment of human diseases that range from T cell malignancies to steroid-resistant graft-versus-host disease to metastatic melanoma. This deeper scientific understanding of diphtheria toxin and the regulation of its expression have metamorphosed the third-most-potent bacterial toxin known into a life-saving targeted protein therapeutic, thereby at least partially fulfilling Paul Erlich's concept of a magic bullet-"a chemical that binds to and specifically kills microbes or tumor cells."

Scaffolding bacterial genomes and probing host-virus interactions in gut microbiome by proximity ligation (chromosome capture) assay

The biochemical activities of microbial communities, or microbiomes, are essential parts of environmental and animal ecosystems. The dynamics, balance, and effects of these communities are strongly influenced by phages present in the population. Being able to characterize bacterium-phage relationships is therefore essential to investigate these ecosystems to the full extent of their complexity. However, this task is currently limited by (i) the ability to characterize complete bacterial and viral genomes from a complex mix of species and (ii) the difficulty to assign phage sequences to their bacterial hosts. We show that both limitations can be circumvented using meta3C, an experimental and computational approach that exploits the physical contacts between DNA molecules to infer their proximity. In a single experiment, dozens of bacterial and phage genomes present in a complex mouse gut microbiota were assembled and scaffolded de novo. The phage genomes were then assigned to their putative bacterial hosts according to the physical contacts between the different DNA molecules, opening new perspectives for a comprehensive picture of the genomic structure of the gut flora. Therefore, this work holds far-reaching implications for human health studies aiming to bridge the virome to the microbiome.

Neuropathy in miniature swine after administration of the mutant diphtheria toxin-based immunotoxin, pCD3-CRM9

BACKGROUND

Effective in vivo T-cell depletion is a critical component of many transplantation tolerance protocols. We have previously demonstrated T-cell depletion in miniature swine using a CRM9-based CD3-immunotoxin, pCD3-CRM9. CRM9 is a mutant form of diphtheria toxin (DT) that binds less efficiently than wild-type DT to the DT receptor (proHB-EGF) of primates. In this report, we describe and characterize the dose-dependent neurotoxicity associated with CRM9-based immunotoxin administration in swine.

METHODS

Miniature swine were treated with varying doses of pCD3-CRM9 followed by daily monitoring for symptoms of neuropathy, including limb weakness, paresis, sluggishness, and/or respiratory distress. Animals demonstrating severe respiratory distress were euthanized and peripheral nerve, spinal cord, and skeletal muscle tissue samples were obtained at autopsy for microscopic examination. Unconjugated CRM9 was administered to one animal to define its toxicity independent of the effects of T-cell depletion.

RESULTS

Excellent T-cell depletion was obtained using doses of pCD3-CRM9 greater than 0.1 mg/kg. However, neurotoxicity was observed at these doses, as manifested by transient muscle weakness or paresis, which in some cases progressed to respiratory failure and death. Dorsal root ganglia samples revealed pathological changes typical of diphtheritic polyneuropathy. The animal receiving unconjugated CRM9 exhibited the same neurotoxic side effects as those receiving the pCD3-CRM9 conjugate.

CONCLUSIONS

Administration of pCD3-CRM9 immunotoxin provides excellent T-cell depletion in miniature swine but is associated with significant dose-dependent neurotoxicity. A possible reason for CRM9-associated neurotoxicity in swine, but not primates, is suggested on the basis of a known amino acid difference in the exodomain of the DT receptor (proHB-EGF) of swine compared with that of primates.

Peritoneal culture alters Streptococcus pneumoniae protein profiles and virulence properties

We have examined the properties of Streptococcus pneumoniae cultured in the murine peritoneal cavity and compared its virulence-associated characteristics to those of cultures grown in vitro. Analysis of mRNA levels for specific virulence factors demonstrated a 2.8-fold increase in ply expression and a 2.2-fold increase in capA3 expression during murine peritoneal culture (MPC). Two-dimensional gels and immunoblots using convalescent-phase patient sera and murine sera revealed distinct differences in protein production in vivo (MPC). MPC-grown pneumococci adhered to A549 epithelial cell lines at levels 10-fold greater than those cultured in vitro.

Biology and molecular epidemiology of diphtheria toxin and the tox gene

Diphtheria toxin (DT) is an extracellular protein of Corynebacterium diphtheriae that inhibits protein synthesis and kills susceptible cells. The gene that encodes DT (tox) is present in some corynephages, and DT is only produced by C. diphtheriae isolates that harbor tox+ phages. The diphtheria toxin repressor (DtxR) is a global regulatory protein that uses Fe2+ as co-repressor. Holo-DtxR represses production of DT, corynebacterial siderophore, heme oxygenase, and several other proteins. Diagnostic tests for toxinogenicity of C. diphtheriae are based either on immunoassays or on bioassays for DT. Molecular analysis of tox and dtxR genes in recent clinical isolates of C. diphtheriae revealed several tox alleles that encode identical DT proteins and multiple dtxR alleles that encode five variants of DtxR protein. Therefore, recent clinical isolates of C. diphtheriae produce a single antigenic type of DT, and diphtheria toxoid continues to be an effective vaccine for immunization against diphtheria.

Characterization of specific nucleotide substitutions in DtxR-specific operators of Corynebacterium diphtheriae that dramatically affect DtxR binding, operator function, and promoter strength

The diphtheria toxin repressor (DtxR) of Corynebacterium diphtheriae uses Fe(2+) as a corepressor. Holo-DtxR inhibits transcription from the iron-regulated promoters (IRPs) designated IRP1 through IRP5 as well as from the promoters for the tox and hmuO genes. DtxR binds to 19-bp operators with the consensus sequence 5'-TTAGGTTAGCCTAACCTAA-3', a perfect 9-bp palindrome interrupted by a single C. G base pair. Among the seven known DtxR-specific operators, IRP3 exhibits the weakest binding to DtxR. The message (sense) strand of the IRP3 operator (5'-TTAGGTGAGACGCACCCAT-3' [nonconsensus nucleotides underlined]) overlaps by 2 nucleotides at its 5' end with the putative -10 sequence of the IRP3 promoter. The underlined C at position +7 from the center of the IRP3 operator [C(+7)] is unique, because T is conserved at that position in other DtxR-specific operators. The present study examined the effects of nucleotide substitutions at position +7 or -7 in the IRP3 operator. In gel mobility shift assays, only the change of C(+7) to the consensus nucleotide T caused a dramatic increase in the binding of DtxR, whereas other nucleotide substitutions for C(+7) or replacements for A(-7) had only small positive or negative effects on DtxR binding. All substitutions for C(+7) or A(-7) except for A(-7)C dramatically decreased IRP3 promoter strength. In contrast, the A(-7)C variant caused increased promoter strength at the cost of nearly eliminating repressibility by DtxR. The message (sense) strand of the IRP1 operator (5'-TTAGGTTAGCCAAACCTTT-3') includes the -35 region of the IRP3 promoter. A T(+7)C variant of the IRP1 operator was also constructed, and it was shown to exhibit decreased binding to DtxR, decreased repressibility by DtxR, and increased promoter strength. The nucleotides at positions +7 and -7 in DtxR-specific operators are therefore important determinants of DtxR binding and repressibility of transcription by DtxR, and they also have significant effects on promoter activity for IRP3 and IRP1.

In vivo T cell depletion in miniature swine using the swine CD3 immunotoxin, pCD3-CRM9

BACKGROUND

Partially inbred miniature swine developed in this laboratory provide a unique preclinical large animal model for studying transplant tolerance. The importance of in vivo T cell depletion for establishing stable mixed hematopoietic cell chimerism using a clinically relevant sublethal regimen has been well documented in murine studies (1). Until now, the lack of an effective in vivo T cell-depleting reagent in swine has limited the progress of studies involving hematopoietic cell transplants.

METHODS

The swine CD3 immunotoxin, pCD3-CRM9, was prepared by conjugating our porcine-specific CD3 monoclonal antibody 898H2-6-15 to the diphtheria toxin derivative, CRM9. The resultant immunotoxin was administered i.v. to several miniature swine at doses ranging from 0.15-0.2 mg/kg either in a single dose or two doses 2 days apart. T-cell depletion was monitored in the peripheral blood, mesenteric lymph node, and thymus by flow cytometric analysis and histological examination.

RESULTS

T cells were depleted to less than 1% of their pretreatment levels based on absolute numbers in the peripheral blood. Fluorescence activated cell sorter analysis and histological examination of serial lymph node biopsies confirmed depletion of the CD3+ T cells rather than down modulation or masking of the surface CD3 expression. Depletion of the CD3 bright medullary thymocytes could also be detected by flow cytometry and histological examination after immunotoxin treatment.

CONCLUSIONS

Administration of the immunotoxin i.v. drastically depletes mature T cells from the peripheral blood, lymph node, and thymus compartments of the pig. This first description of an effective in vivo T-cell depleting reagent for the pig provides a valuable tool for studies of transplant tolerance in this large animal model. It also makes possible preclinical studies of T cell depletion with anti-CD3 immunotoxin in this large animal model.

High-yield production of diphtheria toxin mutants by high-density culture of C7 (beta)tox+ strains grown in a non-deferrated medium

A high-density growth approach was utilized to produce mutated diphtheria toxin from two strains of Corynebacterium diphtheria: C7 (beta)(tox-201,tox-9) and C7 (beta)(tox-107). The cross-reacting mutants (CRM) of the diphtheria toxin are CRM9 and CRM107; both of them carry the mutation in their binding site and, as a result, have 1/300 of the systemic toxicity of the wild-type diphtheria toxin. Since iron inhibits diphtheria toxin production, the traditional approach has been to grow the bacteria in a very low iron concentration. The procedure described here involved the use of a modified, non-deferrated, growth medium that provided fast and high-density growth of the bacteria, and which, when associated with simultaneous depletion of glucose and iron, enhanced the toxin production. Oxygen-enriched air was supplied to enable the bacteria to grow to a cell density giving an absorbance of 70 at 600 nm (15-20 g/l dry weight). The maximum toxin concentration in the culture supernatant was 150 mg/l. The CRM products, which remained stable following microfiltration and ultrafiltration, could be easily purified using a two-step chromatography procedure.

Iron, DtxR, and the regulation of diphtheria toxin expression

In recent years considerable advances have been made in the understanding of the molecular basis of iron-mediated regulation of diphtheria toxin expression. The tox gene has been shown to be regulated by the heavy metal ion-activated regulatory element DtxR. In the presence of divalent heavy metal ions, DtxR becomes activated and binds to a 9 bp interrupted palindromic sequence. The consensus-binding site has been determined by both the sequence analysis of DtxR-responsive operators cloned from genomic libraries of Corynebacterium diphtheriae as well as by in vitro genetic methods using cyclic amplification of selected targets (CASTing). It is now clear that DtxR functions as a global iron-sensitive regulatory element in the control of gene expression in C. diphtheriae. In addition, the metal ion-activation domain of DtxR is being characterized by both mutational analysis and determination of the X-ray structure at 3.0 A resolution.

Determination of the minimal essential nucleotide sequence for diphtheria tox repressor binding by in vitro affinity selection

The expression of diphtheria toxin in lysogenic toxigenic strains of Corynebacterium diphtheriae is controlled by the heavy metal ion-activated regulatory protein DtxR. In the presence of divalent heavy metal ions, DtxR specifically binds to the diphtheria tox operator and protects a 27-bp interrupted palindromic sequence from DNase I digestion. To determine the consensus DNA sequence for DtxR binding, we have used gel electrophoresis mobility-shift assay and polymerase chain reaction (PCR) amplification for in vitro affinity selection of DNA binding sequences from a universe of 6.9 x 10(10) variants. After 10 rounds of in vitro affinity selection, each round coupled with 30 cycles of PCR amplification, we isolated and characterized a family of DNA sequences that function as DtxR-responsive genetic elements both in vitro and in vivo. Moreover, these DNA sequences were found to bind activated DtxR with an affinity similar to that of the wild-type tox operator. The DNA sequence analysis of 21 unique in vitro affinity-selected binding sites has revealed the minimal essential nucleotide sequence for DtxR binding to be a 9-bp palindrome separated by a single base pair.

Characterization of mutations that inactivate the diphtheria toxin repressor gene (dtxR)

The diphtheria toxin repressor (DtxR) is an iron-dependent regulator of diphtheria toxin production and iron uptake in Corynebacterium diphtheriae. It is activated in vitro by divalent metal ions including Fe2+, Cd2+, Co2+, Mn2+, Ni2+, and Zn2+. We characterized 20 different mutations in dtxR induced by bisulfite mutagenesis, 18 of which caused single-amino-acid substitutions in DtxR and two of which were chain-terminating mutations. Six of the amino acid replacements were clustered between residues 39 and 52 in a predicted helix-turn-helix motif that exhibits homology with several other repressors and is identified as the putative DNA-binding domain of DtxR. Three substitutions occurred within a predicted alpha-helical region with the sequence His-98-X3-Cys-102-X3-His-106 that resembles metal-binding motifs in several other proteins and is identified as the putative metal-binding site of DtxR. Several purified variants of DtxR with decreased repressor activity failed to bind in gel retardation assays to DNA fragments that contained the tox operator. A quantitative assay for binding of DtxR to 63Ni2+ was also developed. Scatchard analysis revealed that DtxR has a single class of high-affinity 63Ni(2+)-binding sites with a Kd of 2.11 x 10(-6) M and a maximum binding capacity of approximately 1.2 atoms of Ni2+ per DtxR monomer. The P39L, T40I, T44I, and R47H variants of DtxR exhibited normal to slightly decreased 63Ni(2+)-binding activity, but H106Y, which has an amino acid substitution in the presumed metal-binding domain, exhibited markedly decreased 63Ni(2+)-binding activity.

Cloning, sequence, and footprint analysis of two promoter/operators from Corynebacterium diphtheriae that are regulated by the diphtheria toxin repressor (DtxR) and iron

DtxR is an iron-dependent sequence-specific DNA-binding protein that binds to the tox operator, an inverted-repeat nucleotide sequence located upstream from the diphtheria toxin gene. In this study, two additional iron-regulated promoter/operator sequences (IRP1 and IRP2) that are controlled by DtxR were cloned from the chromosome of Corynebacterium diphtheriae and characterized. Operon fusions to lacZ were used to analyze expression from IRP1 and IRP2 in Escherichia coli. Transcription from both promoters was strongly repressed in high-iron medium in the presence of the cloned dtxR gene; however, transcription in the absence of dtxR was 50- to 100-fold greater, regardless of the iron concentration. Purified DtxR altered the electrophoretic mobility of DNA fragments carrying IRP1 or IRP2, and the nucleotide sequences of the two promoter/operator regions indicated that they are both homologous with the tox operator. DtxR protected an approximately 30-bp region on both IRP1 and IRP2 from DNase I digestion. A 19-bp consensus DtxR-binding site was derived from a comparison of the various DtxR-regulated operator/promoter sequences. Footprinting experiments using hydroxyl radicals and dimethyl sulfate demonstrated that DtxR interacted with these operators in a symmetrical manner, probably as a dimer or multimer. The deduced amino acid sequence of an open reading frame (ORF1) located downstream from IRP1 was homologous with a family of periplasmic proteins involved in iron transport in gram-negative bacteria and with the ferrichrome receptor, FhuD, from Bacillus subtilis. These findings suggest that ORF1 encodes a membrane-associated lipoprotein that may serve as the receptor for a ferric-siderophore complex in C. diphtheriae.

Characterization of an iron-regulated promoter involved in desferrioxamine B synthesis in Streptomyces pilosus: repressor-binding site and homology to the diphtheria toxin gene promoter

Desferrioxamine B is the main siderophore of Streptomyces pilosus. Its production is induced in response to iron limitation. Two genes involved in desferrioxamine production have been cloned and were found to be translated from a polycistronic mRNA that is produced only under conditions of iron limitation (T. Schupp, C. Toupet, and M. Divers, Gene 64:179-188, 1988). Here we report the nucleotide sequence of the desferrioxamine (des) operon promoter region. The transcriptional start site was localized by S1 nuclease mapping. Deletion analysis defined a 71-bp region downstream of the -35 region that is sufficient for iron regulation in the original host, S. pilosus, and also in Streptomyces lividans. Site-directed mutagenesis was used to create a mutation that abolishes iron repression. Two iron-independent mutants were obtained by deletion of part of a 19-bp region with dyad symmetry which overlaps the -10 promoter region and the transcriptional start site. The putative repressor-binding site identified by these constitutive mutations is not homologous to the consensus binding site of the Escherichia coli central iron repressor, Fur (ferric uptake regulation), but is similar to the DtxR-binding site in the iron-regulated promoter of the corynebacterial diphtheria toxin gene.

Role of iron in regulation of virulence genes

The abilities of bacterial pathogens to adapt to the environment within the host are essential to their virulence. Microorganisms have adapted to the iron limitation present in mammalian hosts by evolving diverse mechanisms for the assimilation of iron sufficient for growth. In addition, many bacterial pathogens have used the low concentration of iron present in the host as an important signal to enhance the expression of a wide variety of bacterial toxins and other virulence determinants. The molecular basis of coordinate regulation by iron has been most thoroughly studied in Escherichia coli. In this organism, coordinate regulation of gene expression by iron depends on the regulatory gene, fur. Regulation of gene expression by iron in a number of pathogenic organisms is coordinated by proteins homologous to the Fur protein of E. coli. Additional regulatory proteins may be superimposed on the Fur repressor to provide the fine-tuning necessary for the precise regulation of individual virulence genes in response to iron and other environmental signals. Studies of the mechanisms of regulation of iron acquisition systems and virulence determinants by iron should lead to a better understanding of the adaptive response of bacteria to the low-iron environment of the host and its importance in virulence.

Purification and characterization of the diphtheria toxin repressor

The diphtheria toxin repressor gene (dtxR) encodes a protein (DtxR) that regulates transcription of the diphtheria toxin gene (tox) by an iron-dependent mechanism. Cloned dtxR was expressed in Escherichia coli from the phage T7 gene 10 promoter, and DtxR was purified. Specific binding of DtxR to the tox+ operator was dependent on reduction of DtxR and the presence of ferrous ions. DtxR protected a sequence of approximately 30 nucleotide pairs, partially overlapping the tox promoter and containing a region of dyad symmetry, from digestion by DNase I. DtxR exhibited very little binding to the mutant tox-201 operator region and failed to bind to the promoter/operator region of the ferric uptake regulation (fur) gene of E. coli.

Transcription analysis and nucleotide sequence of tox promoter/operator mutants of corynebacteriophage beta

The production of diphtheria toxin (DT) by Corynebacterium diphtheriae C7 (beta) is transcriptionally regulated by the iron-dependent diphtheria toxin repressor, DtxR. Transcription of the tox gene was studied in wild-type C. diphtheriae C7 (beta) and in lysogens carrying mutants of beta that determine insensitivity to inhibition of DT production by iron. Under low iron conditions in all strains, tox-specific mRNA appeared and DT production began during late-log phase, and they increased to maximal levels at stationary phase. Under high iron conditions, tox-specific mRNA and DT production were strongly repressed in C7 (beta) but only partially repressed in C7 (beta tox-202) and C7 (beta tox-201). Under high and low iron conditions, DT production and tox-specific mRNA levels were greater in C7 (beta tox-201) and C7 (beta tox-202) than in wild-type C7 (beta). Addition of iron or rifampicin to low iron cultures of C. diphtheriae C7 (beta) repressed tox-mRNA production promptly and with a similar time course. In contrast, repression of tox-mRNA synthesis in C. diphtheriae C7 (beta tox-201) occurred promptly after addition of rifampicin but more slowly after addition of iron. Nucleotide sequence analysis revealed single G to A mutations at positions -47 and -48, within the preferred '-10' sequence of the tox promoter, in beta tox-201 and beta tox-202, respectively. The single nucleotide substitutions in the tox-201 and tox-202 regulatory alleles, therefore, have pleiotropic effects, causing increased activity of the promoter and partial resistance of the operator to iron-dependent repression.

Specific binding of the diphtheria tox regulatory element DtxR to the tox operator requires divalent heavy metal ions and a 9-base-pair interrupted palindromic sequence

The structural gene for diphtheria toxin, tox, is carried by a family of closely related corynebacteriophages; however, the regulation of tox expression is controlled by a Corynebacterium diphtheriae-encoded regulatory element, dtxR. The molecular cloning and sequence analysis of dtxR was recently described. Previous studies have suggested that DtxR-mediated regulation of the diphtheria tox operator involves the formation of an iron-repressor complex, which specifically binds to the tox operator. We have expressed and purified DtxR from recombinant Escherichia coli. Immunoblot analysis shows DtxR to be a single M(r) 28,000 protein band in both recombinant E. coli and the C7(-) and C7hm723(-) strains of C. diphtheriae. In addition, we demonstrate that the binding of DtxR to a diphtheria tox promoter/operator probe requires the addition of Mn2+ to the reaction mixture; however, binding may be blocked by addition of the chelator 2,2'-dipyridyl, anti-DtxR antiserum, and excess unlabeled probe to the reaction mixture. Deletion of one of the 9-base-pair inverted repeat sequences from the tox operator results in a loss of DtxR binding. The results presented here demonstrate that regulation of diphtheria toxin expression by DtxR requires direct interaction between this regulatory factor and the tox operator in the presence of a divalent heavy metal ion.

In vivo T-cell ablation by a holo-immunotoxin directed at human CD3

We have evaluated the in vivo efficacy of anti-CD3-CRM9, a holo-immunotoxin constructed with a diphtheria toxin binding-site mutant. Eighty percent of established human T-cell subcutaneous tumors in nude mice completely regressed following intraperitoneal injection of immunotoxin at a dose set at half the minimum lethal dose assayed in toxin-sensitive animals. Similar regressions produced by a 137Cs source required a dose in excess of 500 cGy. The high degree of in vivo T-cell ablation produced by this immunotoxin is apparently due to maintenance of the toxin translocation function provided by CRM9 and a necessary intracellular routing function supplied by CD3. This immunotoxin may be useful in treating conditions caused by pathologic oligoclonal T-cell expansion such as graft-versus-host disease, autoimmune diseases, and possibly AIDS.

DNA sequences and characterization of dtxR alleles from Corynebacterium diphtheriae PW8(-), 1030(-), and C7hm723(-)

The structural gene encoding DtxR, an iron-dependent diphtheria tox regulatory element, has recently been cloned and sequenced from the C7(-) strain of Corynebacterium diphtheriae (J. M. Boyd, M. Oza, and J. R. Murphy, Proc. Natl. Acad. Sci. USA 87:5972, 1990). We report here the molecular cloning, DNA sequence analysis, and characterization of DtxR from the PW8(-), 1030(-), and C7hm723 strains of C. diphtheriae. While the sequence of dtxR from PW8(-) is identical to that of the C7(-) allele, the sequence of dtxR from the 1030(-) strain is only 91.4% identical; however, the deduced amino acid sequence of DtxR from 1030(-) differs by only 6 of 678 amino acids. Moreover, DtxR from all three strains is shown to regulate expression of beta-galactosidase from a tox promoter-operator (toxPO)-lacZ transcriptional fusion. In contrast, the dtxR allele from the iron-insensitive tox constitutive mutant C7hm723 was found to have a single G----A transition, resulting in a substitution of Arg-47 to His and the loss of tox regulatory activity in recombinant Escherichia coli.

Characterization of a defective diphtheria toxin repressor (dtxR) allele and analysis of dtxR transcription in wild-type and mutant strains of Corynebacterium diphtheriae

The production of diphtheria toxin and siderophore by the Corynebacterium diphtheriae regulatory mutant C7(beta)hm723 is resistant to the inhibitory effects of iron, and the mutant strain is defective for function of the regulatory gene dtxR. A 2.8-kb HindIII fragment carrying the C7(beta)hm723 dtxR allele was cloned and characterized in Escherichia coli. The restriction endonuclease maps of the 2.8-kb HindIII fragment from C7(beta)hm723 and the corresponding fragment from wild-type C. diphtheriae C7 were identical. RNA dot blot analysis with total RNA isolated from wild-type C. diphtheriae C7 and C7(beta)hm723 indicated that the dtxR gene was transcribed at very low but equivalent levels in both strains and was not regulated by iron. beta-Galactosidase synthesis from a tox-lacZ translational fusion construct in E. coli in high-iron medium was not repressed by the C7(beta)hm723dtxR allele, but was strongly repressed by the wild-type dtxR gene. The 28- to 29-kDa polypeptide expressed from the mutant dtxR allele in E. coli had the same electrophoretic mobility as the wild-type dtxR gene product in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The nucleotide sequence of the coding region and the 5' upstream region of the C7(beta)hm723 dtxR allele was determined and compared with the wild-type nucleotide sequence. The dtxR allele from C7(beta)hm723 contained a single-base change located 140 nucleotides from the 5' start of the gene, which resulted in replacement of arginine in the wild-type sequence by histidine in the mutant protein. These data demonstrate that C7(beta)hm723 expresses a mutant DtxR repressor protein that is severely defective in repressor activity.

Iron-dependent regulation of diphtheria toxin and siderophore expression by the cloned Corynebacterium diphtheriae repressor gene dtxR in C. diphtheriae C7 strains

A regulatory gene (dtxR) responsible for iron-dependent repression of the toxin (tox) and siderophore genes in Corynebacterium diphtheriae was cloned and characterized. A DNA fragment carrying dtxR repressed expression of a tox-lacZ gene fusion in Escherichia coli DH5 alpha in a high-iron environment but not under low-iron conditions. A protein with mobility corresponding to approximately 28 to 29 kDa was identified as the product of the dtxR gene by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A shuttle vector designated pCM2.6 was constructed which carries the origin of replication from C. diphtheriae plasmid pNG2 and confers resistance to chloramphenicol in E. coli and C. diphtheriae. DNA fragments carrying dtxR were cloned into pCM2.6, and the hybrid shuttle plasmids were transformed by electroporation into wild-type C. diphtheriae C7(beta) and the regulatory mutant C7(beta)hm723, which produces toxin and siderophore constitutively under high-iron conditions. Expression of the cloned dtxR determinant did not affect the phenotype of C. diphtheriae C7(beta). In C. diphtheriae C7(beta)hm723, expression of cloned dtxR restored full repression of siderophore production and partial repression of diphtheria toxin production during growth in a high-iron environment.

Coordinate regulation of siderophore and diphtheria toxin production by iron in Corynebacterium diphtheriae

Iron is an environmental signal which regulates the coordinate expression of genes associated with virulence in many pathogenic bacteria. In response to iron-deprivation, lysogenic Corynebacterium diphtheriae C7 (beta) synthesizes and secretes diphtheria toxin and siderophore and induces a high-affinity iron uptake system. Diphtheria toxin is encoded by beta phage, but genes for siderophore production are encoded on the bacterial chromosome. Diphtheria toxin and siderophore production were shown to be coordinately induced during late logarithmic phase growth of wild-type C7(beta) in iron-limited medium. C. diphtheriae mutant C7hm723 produced siderophore and toxin constitutively under low-iron and high-iron conditions, but in mutants HC1, HC3, HC4, and HC5 their synthesis was partially repressed under high-iron conditions. The phenotypes of HC1, HC3, HC4, and HC5 are consistent with their severe defects in iron uptake, but the phenotype of C7hm723 is more likely to be explained by inactivation of the repressor for the iron regulon of C. diphtheriae.

Molecular cloning and DNA sequence analysis of a diphtheria tox iron-dependent regulatory element (dtxR) from Corynebacterium diphtheriae

Although the structural gene for diphtheria toxin, tox, is carried by a family of closely related corynebacteriophages, the regulation of tox expression is controlled, to a large extent, by its bacterial host Corynebacterium diphtheriae. Optimal yields of tox gene products are obtained only when iron becomes the growth-rate-limiting substrate. Previous studies suggest that regulation of tox expression is mediated through an iron-binding aporepressor. To facilitate molecular cloning of the tox regulatory element from genomic libraries of C. diphtheriae, we constructed a tox promoter/operator (toxPO)-lacZ transcriptional fusion in Escherichia coli strain DH5 alpha. We report the molecular cloning and nucleic acid sequence of a diphtheria tox iron-dependent regulatory element, dtxR, and demonstrate that expression of beta-galactosidase from the toxPO-lacZ fusion is regulated by dtxR-encoded protein in an iron-sensitive manner. In addition, we show that expression of the toxPO-lacZ fusion is not affected by the E. coli iron-regulatory protein Fur and that the dtxR protein does not inhibit expression of fur-regulated outer-membrane proteins.

Evidence for direct regulation of diphtheria toxin gene transcription by an Fe2+-dependent DNA-binding repressor, DtoxR, in Corynebacterium diphtheriae

Previous studies provided indirect evidence that in Corynebacterium diphtheriae regulation of diphtheria toxin gene (tox) transcription by iron is mediated by a bacterial repressor. By performing in vitro protein-DNA binding experiments, we establish that a corynebacterial Fe2+-sensitive protein, named DtoxR, can bind to a palindromic motif present in the tox promoter region. Binding of this factor prevents the interaction of the transcription initiation machinery with presumptive critical promoter elements, providing evidence that DtoxR is responsible for the repression of toxinogenesis observed in iron-containing growth medium.

Highly toxinogenic but avirulent Park-Williams 8 strain of Corynebacterium diphtheriae does not produce siderophore

The highly toxinogenic Park-Williams 8 strain of Corynebacterium diphtheriae grows slowly in vitro and is avirulent. C. diphtheriae Park-Williams 8 is defective in iron uptake and does not produce the corynebacterial siderophore corynebactin. Addition of partially purified corynebactin stimulated iron uptake and growth of iron-deprived C. diphtheriae Park-Williams 8 cells.

Genetic and biochemical evidence for a siderophore-dependent iron transport system in Corynebacterium diphtheriae

During growth under conditions of iron deprivation, Corynebacterium diphtheriae secreted a siderophore into the culture medium. This extracellular siderophore was necessary for rates of iron uptake at pH 8.0 by C. diphtheriae C7 and related strains. We isolated a mutant of C. diphtheriae C7(beta), strain HC6, which did not make the corynebacterial siderophore. Strain HC6 grew very poorly, even under high-iron conditions, and had a severe defect in iron transport. Both growth and iron uptake by strain HC6 were greatly stimulated by the corynebacterial siderophore. We used strain HC6 to develop a bioassay for the corynebacterial siderophore and to look for other potential siderophores for C. diphtheriae. Among the purified phenolate and hydroxamate siderophores tested, only aerobactin was able to stimulate the growth of strain HC6. Partial purification of the corynebacterial siderophore was achieved. The siderophore did not give positive reactions in the Arnow test for phenolates or the Csaky test for hydroxamates and may have a novel chemical structure.

Regulation of toxinogenesis in Corynebacterium diphtheriae: mutations in the bacterial genome that alter the effects of iron on toxin production

Mutants of Corynebacterium diphtheriae C7(beta) that are resistant to the inhibitory effects of iron on toxinogenesis were identified by their ability to form colonies surrounded by toxin-antitoxin halos on agar medium containing both antitoxin and a high concentration of iron. Chromosomal mutations were essential for the altered phenotypes of four independently isolated mutant strains. During growth in deferrated liquid medium containing various amounts of added iron, these mutants differed from wild-type C. diphtheriae C7(beta) in several ways. Their growth rates were slower under low-iron conditions and were stimulated to various degrees under high-iron conditions. The concentrations of iron at which optimal toxin production occurred were higher for the mutants than for wild-type C. diphtheriae C7(beta). Toxin production by the mutants during growth in low-iron medium occurred throughout the period of exponential growth at nearly constant rates that were proportional to the bacterial growth rates. In contrast, toxin production by wild-type C. diphtheriae C7(beta) in similar low-iron cultures occurred predominantly during the late exponential phase, when iron was a growth-limiting nutrient. Additional studies demonstrated that these mutants had severe defects in their transport systems for ferric iron. We propose that the altered regulation of toxinogenesis by iron in our mutants was caused by the severe defects in their iron transport systems. As a consequence, the mutants exhibited a low-iron phenotype during growth under conditions that permitted wild-type C. diphtheriae C7(beta) to exhibit a high-iron phenotype.

Integration of corynebacteriophages beta tox+, omega tox+, and gamma tox- into two attachment sites on the Corynebacterium diphtheriae chromosome

The bacterial attachment sites of independently isolated Corynebacterium diphtheriae strains C7s and (belfanti)1030 lysogenic for corynebacteriophages beta tox+, omega tox+, and gamma tox- were determined by Southern blot analysis. Both corynebacterial strains contained two distinct bacterial attachment sites (attB1 and attB2). We found that infection by any of the three closely related corynebacteriophages may give rise to single, double, and triple lysogens. In the case of toxigenic C. diphtheriae strains C7s(beta tox+) and C7s(omega tox+), the final yields of diphtheria toxin produced under optimal conditions were equivalent and varied by one-, two-, or threefold depending upon the number of integrated prophage.

Regulation of toxinogenesis in Corynebacterium diphtheriae. II. Genetic mapping of a tox regulatory mutation in bacteriophage beta

The structural gene for diphtherial toxin is present in corynebacteriophage beta. Previous studies located several point mutations within the tox locus and determined the orientation of transcription of the toxin structural gene. The production of maximal quantities of toxin by Corynebacterium diphtheriae C7(beta) occurs only when the bacteria are iron deficient. Mutations in phage beta can affect this control of toxin production by iron. The tox-201 mutation regulates expression of the toxin structural gene in a cis-dominant manner and permits large amounts of toxin to be made under high-iron conditions when phage beta tox-201 infects C. diphtheriae C7. In this study tox-201 was found to be closely linked to the structural gene for toxin. We performed a series of multifactor matings to determine the relative positions of tox-201 and several point mutations within the toxin structural gene. The order of these markers on the vegetative genetic map of phage beta was tox-201-tox-4-tox-2-tox-30. These findings establish that the tox-201 regulatory site is closely linked to the end of the toxin structural gene corresponding to the origin of transcription. This location is consistent with our hypothesis that tox-201 defines a cis-dominant regulatory element, such as an operator, promoter, or attenuator, involved in control of toxinogenesis in C. diphtheriae C7(beta).