Transformation of Corynebacterium diphtheriae, Corynebacterium ulcerans, Corynebacterium glutamicum, and Escherichia coli with the C. diphtheriae plasmid pNG2

Genomic characterization of cocirculating Corynebacterium diphtheriae and non-diphtheritic Corynebacterium species among forcibly displaced Myanmar nationals, 2017-2019

Respiratory diphtheria is a serious infection caused by toxigenic Corynebacterium diphtheriae, and disease transmission mainly occurs through respiratory droplets. Between 2017 and 2019, a large diphtheria outbreak among forcibly displaced Myanmar nationals densely settled in Bangladesh was investigated. Here we utilized whole-genome sequencing (WGS) to characterize recovered isolates of C. diphtheriae and two co-circulating non-diphtheritic Corynebacterium (NDC) species - C. pseudodiphtheriticum and C. propinquum. C. diphtheriae isolates recovered from all 53 positive cases in this study were identified as toxigenic biovar mitis, exhibiting intermediate resistance to penicillin, and formed four phylogenetic clusters circulating among multiple refugee camps. Additional sequenced isolates collected from two patients showed co-colonization with non-toxigenic C. diphtheriae biovar gravis, one of which exhibited decreased susceptibility to the first-line antibiotics and harboured a novel 23-kb multidrug resistance plasmid. Results of phylogenetic reconstruction and virulence-related gene contents of the recovered NDC isolates indicated they were likely commensal organisms, though 80.4 %(45/56) were not susceptible to erythromycin, and most showed high minimum inhibition concentrations against azithromycin. These results demonstrate the high resolution with which WGS can aid molecular investigation of diphtheria outbreaks, through the quantification of bacterial genetic relatedness, as well as the detection of virulence factors and antibiotic resistance markers among case isolates.

Analysis of the HbpA Protein from Corynebacterium diphtheriae Clinical Isolates and Identification of a Putative Hemoglobin-Binding Site on HbpA

The Corynebacterium diphtheriae hemoglobin-binding protein HbpA is critical for the acquisition of iron from the hemoglobin-haptoglobin complex (Hb-Hp). Previous studies using C. diphtheriae strain 1737 showed that large aggregates formed by HbpA are associated with iron transport activity and enhanced binding to Hb-Hp; however, specific regions within HbpA required for Hb-Hp binding or iron uptake have not been identified. In this study, we characterized two clinical isolates from Austria, designated 07-18 and 09-15, which express HbpA proteins that share only 53% and 44% sequence identity, respectively, to the strain 1737 HbpA protein. The HbpA proteins expressed by the Austrian strains had functional and structural properties similar to those of the HbpA protein in strain 1737 despite the limited sequence similarity. These shared characteristics between the HbpA proteins included similar cellular localization, aggregate formation, and Hb and Hb-Hp binding. Additionally, the Austrian strains were able to acquire iron from Hb and Hb-Hp, and deletion of the hbpA gene from these two clinical isolates reduced their ability to use Hb-Hp as an iron source. A sequence comparison between the HbpA proteins from 1737 and the Austrian strains assisted in the identification of a putative Hb-binding site that shared similar characteristics with the Hb-binding regions in Staphylococcus aureus NEAT domains. Amino acid substitutions within this conserved Hb-binding region significantly reduced Hb and Hb-Hp binding and diminished the hemin-iron uptake function of HbpA. These findings represent important advances in our understanding of the interaction of HbpA with human hemoproteins. IMPORTANCE Hemoglobin (Hb) is the primary source of iron in humans, and the acquisition of hemin-iron from Hb is critical for many bacterial pathogens to infect and survive in the human host. In this study, we have examined the C. diphtheriae Hb-binding protein HbpA in two clinical isolates and show that these proteins, despite limited sequence similarity, are functionally equivalent to the previously described HbpA protein in strain 1737. A sequence comparison between these three strains led to the identification of a conserved Hb-binding site, which will further our understanding of how this novel protein functions in hemin-iron transport and, more generally, will expand our knowledge on how Hb interacts with proteins.

The Corynebacterium diphtheriae HbpA hemoglobin-binding protein contains a domain that is critical for hemoprotein-binding, cellular localization and function

The acquisition of hemin-iron from hemoglobin-haptoglobin (Hb-Hp) by Corynebacterium diphtheriae requires the iron-regulated surface proteins HtaA, ChtA, ChtC, and the recently identified Hb-Hp binding protein HbpA. We previously showed that a purified form of HbpA (HbpA-S), lacking the C-terminal region, was able to bind Hb-Hp. In this study, we show that the C-terminal region of HbpA significantly enhances binding to Hb-Hp. A purified form of HbpA that includes the C-terminal domain (HbpA-FL) exhibits much stronger binding to Hb-Hp than HbpA-S. Size exclusion chromatography (SEC) showed that HbpA-FL as well as HtaA-FL, ChtA-FL, and ChtC-FL exist as high molecular weight complexes, while HbpA-S is present as a monomer, indicating that the C-terminal region is required for formation of large aggregates. Growth studies showed that expression of HbpA-FL in the ΔhbpA mutant restored wild-type levels of growth in low-iron medium that contained Hb-Hp as the sole iron source, while HbpA-S failed to complement the ΔhbpA mutant. Protein localization studies in C. diphtheriae showed that HbpA-FL is present in both in the supernatant and in the membrane fractions, and that the C-terminal region is required for membrane anchoring. Purified HbpA-FL was able to enhance growth of the ΔhbpA mutant when added to culture medium that contained Hb-Hp as a sole iron source, suggesting that secreted HbpA is involved in the use of hemin-iron from Hb-Hp. These studies extend our understanding of this novel Hb-Hp binding protein in this important human pathogen. IMPORTANCE Hemoproteins, such as Hb, are an abundant source of iron in humans and are proposed to be required by numerous pathogens to cause disease. In this report, we expand on our previous studies in further defining the role of HbpA in hemin-iron acquisition in C. diphtheriae. HbpA is unique to C. diphtheriae, and appears to function unlike any previously described bacterial iron-regulated Hb- or Hb-Hp-binding protein. HbpA is both secreted and present in the membrane, and exists as a large aggregate that enhances its ability to bind Hb-Hp and promote hemin-iron uptake. Current studies with HbpA will increase our understanding of iron transport systems in C. diphtheriae.

Building a genome engineering toolbox in nonmodel prokaryotic microbes

The realization of a sustainable bioeconomy requires our ability to understand and engineer complex design principles for the development of platform organisms capable of efficient conversion of cheap and sustainable feedstocks (e.g., sunlight, CO₂ , and nonfood biomass) into biofuels and bioproducts at sufficient titers and costs. For model microbes, such as Escherichia coli, advances in DNA reading and writing technologies are driving the adoption of new paradigms for engineering biological systems. Unfortunately, microbes with properties of interest for the utilization of cheap and renewable feedstocks, such as photosynthesis, autotrophic growth, and cellulose degradation, have very few, if any, genetic tools for metabolic engineering. Therefore, it is important to develop "design rules" for building a genetic toolbox for novel microbes. Here, we present an overview of our current understanding of these rules for the genetic manipulation of prokaryotic microbes and the available genetic tools to expand our ability to genetically engineer nonmodel systems.

Promiscuous plasmid replication in thermophiles: Use of a novel hyperthermophilic replicon for genetic manipulation of Clostridium thermocellum at its optimum growth temperature

Clostridium thermocellum is a leading candidate for the consolidated bioprocessing of lignocellulosic biomass for the production of fuels and chemicals. A limitation to the engineering of this strain is the availability of stable replicating plasmid vectors for homologous and heterologous expression of genes that provide improved and/or novel pathways for fuel production. Current vectors relay on replicons from mesophilic bacteria and are not stable at the optimum growth temperature of C. thermocellum. To develop more thermostable genetic tools for C. thermocellum, we constructed vectors based on the hyperthermophilic Caldicellulosiruptor bescii replicon pBAS2. Autonomously replicating shuttle vectors based on pBAS2 reproducibly transformed C. thermocellum at 60 °C and were maintained in multiple copy. Promoters, selectable markers and plasmid replication proteins from C. bescii were functional in C. thermocellum. Phylogenetic analyses of the proteins contained on pBAS2 revealed that the replication initiation protein RepL is unique among thermophiles. These results suggest that pBAS2 may be a broadly useful replicon for other thermophilic Firmicutes.

Design and testing of a synthetic biology framework for genetic engineering of Corynebacterium glutamicum

BACKGROUND

Synthetic biology approaches can make a significant contribution to the advance of metabolic engineering by reducing the development time of recombinant organisms. However, most of synthetic biology tools have been developed for Escherichia coli. Here we provide a platform for rapid engineering of C. glutamicum, a microorganism of great industrial interest. This bacteria, used for decades for the fermentative production of amino acids, has recently been developed as a host for the production of several economically important compounds including metabolites and recombinant proteins because of its higher capacity of secretion compared to traditional bacterial hosts like E. coli. Thus, the development of modern molecular platforms may significantly contribute to establish C. glutamicum as a robust and versatile microbial factory.

RESULTS

A plasmid based platform named pTGR was created where all the genetic components are flanked by unique restriction sites to both facilitate the evaluation of regulatory sequences and the assembly of constructs for the expression of multiple genes. The approach was validated by using reporter genes to test promoters, ribosome binding sites, and for the assembly of dual gene operons and gene clusters containing two transcriptional units. Combinatorial assembly of promoter (tac, cspB and sod) and RBS (lacZ, cspB and sod) elements with different strengths conferred clear differential gene expression of two reporter genes, eGFP and mCherry, thus allowing transcriptional "fine-tuning"of multiple genes. In addition, the platform allowed the rapid assembly of operons and genes clusters for co-expression of heterologous genes, a feature that may assist metabolic pathway engineering.

CONCLUSIONS

We anticipate that the pTGR platform will contribute to explore the potential of novel parts to regulate gene expression, and to facilitate the assembly of genetic circuits for metabolic engineering of C. glutamicum. The standardization provided by this approach may provide a means to improve the productivity of biosynthetic pathways in microbial factories for the production of novel compounds.

HtaA is an iron-regulated hemin binding protein involved in the utilization of heme iron in Corynebacterium diphtheriae

Many human pathogens, including Corynebacterium diphtheriae, the causative agent of diphtheria, use host compounds such as heme and hemoglobin as essential iron sources. In this study, we examined the Corynebacterium hmu hemin transport region, a genetic cluster that contains the hmuTUV genes encoding a previously described ABC-type hemin transporter and three additional genes, which we have designated htaA, htaB, and htaC. The hmu gene cluster is composed of three distinct transcriptional units. The htaA gene appears to be part of an iron- and DtxR-regulated operon that includes hmuTUV, while htaB and htaC are transcribed from unique DtxR-regulated promoters. Nonpolar deletion of either htaA or the hmuTUV genes resulted in a reduced ability to use hemin as an iron source, while deletion of htaB had no effect on hemin iron utilization in C. diphtheriae. A comparison of the predicted amino acid sequences of HtaA and HtaB showed that they share some sequence similarity, and both proteins contain leader sequences and putative C-terminal transmembrane regions. Protein localization studies with C. diphtheriae showed that HtaA is associated predominantly with the cell envelope when the organism is grown in minimal medium but is secreted during growth in nutrient-rich broth. HtaB and HmuT were detected primarily in the cytoplasmic membrane fraction regardless of the growth medium. Hemin binding studies demonstrated that HtaA and HtaB are able to bind hemin, suggesting that these proteins may function as cell surface hemin receptors in C. diphtheriae.

Characterization of a new 2.4-kb plasmid of Corynebacterium casei and development of stable corynebacterial cloning vector

A new plasmid pCASE1 was isolated from Gram-positive Corynebacterium casei JCM 12072. It comprised a 2.4-kb nucleotide sequence with three ORFs, two of which were indispensable for autonomous replication in Corynebacterium glutamicum. Homology search identified these two ORFs as repA and repB, areas coding proteins involved in plasmid replication. repA sequence showed high similarity to theta-replicating Escherichia coli ColE2-P9 plasmids and even higher similarity to plasmids derived from Gram-positive bacteria belonging to a subfamily of this ColE2-P9 group. An E. coli-C. glutamicum shuttle vector was constructed with pCASE1 fragment including repA and repB to transform C. glutamicum and showed compatibility with corynebacterial plasmids from different plasmid families. The copy number of the shuttle vector in C. glutamicum was 13 and the vector showed stability for 102 generations with no selective pressure.

Genomics of Actinobacteria: tracing the evolutionary history of an ancient phylum

Actinobacteria constitute one of the largest phyla among bacteria and represent gram-positive bacteria with a high G+C content in their DNA. This bacterial group includes microorganisms exhibiting a wide spectrum of morphologies, from coccoid to fragmenting hyphal forms, as well as possessing highly variable physiological and metabolic properties. Furthermore, Actinobacteria members have adopted different lifestyles, and can be pathogens (e.g., Corynebacterium, Mycobacterium, Nocardia, Tropheryma, and Propionibacterium), soil inhabitants (Streptomyces), plant commensals (Leifsonia), or gastrointestinal commensals (Bifidobacterium). The divergence of Actinobacteria from other bacteria is ancient, making it impossible to identify the phylogenetically closest bacterial group to Actinobacteria. Genome sequence analysis has revolutionized every aspect of bacterial biology by enhancing the understanding of the genetics, physiology, and evolutionary development of bacteria. Various actinobacterial genomes have been sequenced, revealing a wide genomic heterogeneity probably as a reflection of their biodiversity. This review provides an account of the recent explosion of actinobacterial genomics data and an attempt to place this in a biological and evolutionary context.

The ChrA-ChrS and HrrA-HrrS signal transduction systems are required for activation of the hmuO promoter and repression of the hemA promoter in Corynebacterium diphtheriae

Transcription of the Corynebacterium diphtheriae hmuO gene, which encodes a heme oxygenase involved in heme iron utilization, is activated in a heme- or hemoglobin-dependent manner in part by the two-component system ChrA-ChrS. Mutation of either the chrA or the chrS gene resulted in a marked reduction of hemoglobin-dependent activation at the hmuO promoter in C. diphtheriae; however, it was observed that significant levels of hemoglobin-dependent expression were maintained in the mutants, suggesting that an additional activator is involved in regulation. A BLAST search of the C. diphtheriae genome sequence revealed a second two-component system, encoded by DIP2268 and DIP2267, that shares similarity with ChrS and ChrA, respectively; we have designated these genes hrrS (DIP2268) and hrrA (DIP2267). Analysis of hmuO promoter expression demonstrated that hemoglobin-dependent activity was fully abolished in strains from which both the chrA-chrS and the hrrA-hrrS two-component systems were deleted. Similarly, deletion of the sensor kinase genes chrS and hrrS or the genes encoding both of the response regulators chrA and hrrA also eliminated hemoglobin-dependent activation at the hmuO promoter. We also show that the regulators ChrA-ChrS and HrrA-HrrS are involved in the hemoglobin-dependent repression of the promoter upstream of hemA, which encodes a heme biosynthesis enzyme. Evidence for cross talk between the ChrA-ChrS and HrrA-HrrS systems is presented. In conclusion, these findings demonstrate that the ChrA-ChrS and HrrA-HrrS regulatory systems are critical for full hemoglobin-dependent activation at the hmuO promoter and also suggest that these two-component systems are involved in the complex mechanism of the regulation of heme homeostasis in C. diphtheriae.

Comparative genomics identified two conserved DNA modules in a corynebacterial plasmid family present in clinical isolates of the opportunistic human pathogen Corynebacterium jeikeium

Investigation of 62 clinical isolates of the opportunistic human pathogen Corynebacterium jeikeium revealed that 17 possessed plasmids ranging in size from 7.6 to 14.9 kb. The plasmids formed four groups on DNA restriction analysis. The complete nucleotide sequence of a representative from each group (pK43, pK64, pCJ84, and pB85766) was subsequently determined. Additionally, two plasmids (pCo455 and pCo420) were shown to be derivatives of pK43 and pK64 carrying insertion sequences of the IS3 family. Comparative genomics identified a conserved plasmid backbone consisting of two distinct DNA modules. Conserved motifs in the parAB-repA module indicated that the sequenced plasmids from C. jeikeium are new members of the pNG2 family. Recombinant derivatives of pK43 were shown to replicate in the soil bacterium Corynebacterium glutamicum and in the human pathogen Corynebacterium diphtheriae. The second plasmid module most likely encodes a novel type of DNA invertase. The respective gene is flanked by highly conserved 112-bp inverted repeats. All plasmids are 'loaded' with a characteristic set of genes encoding products of unknown function. Plasmids indistinguishable from pK43 by DNA restriction analysis were identified in different C. jeikeium strains, which revealed 16S-23S rDNA spacer length polymorphisms and specific antibiotic susceptibility profiles, implying a wide dissemination of the plasmid in clinical isolates of C. jeikeium.

Tylosin resistance in Arcanobacterium pyogenes is encoded by an erm X determinant

Arcanobacterium pyogenes, a commensal on the mucous membranes of many economically important animal species, is also a pathogen, causing abscesses of the skin, joints, and visceral organs as well as mastitis and abortion. In food animals, A. pyogenes is exposed to antimicrobial agents used for growth promotion, prophylaxis, and therapy, notably tylosin, a macrolide antibiotic used extensively for the prevention of liver abscessation in feedlot cattle in the United States. Of 48 A. pyogenes isolates, 11 (22.9%) exhibited inducible or constitutive resistance to tylosin (MIC of > or = 128 microg/ml). These isolates also exhibited resistance to other macrolide and lincosamide antibiotics, suggesting a macrolide-lincosamide resistance phenotype. Of the 11 resistant isolates, genomic DNA from nine hybridized to an erm(X)-specific probe. Cloning and nucleotide sequencing of the A. pyogenes erm(X) gene indicated that it was >95% similar to erm(X) genes from Corynebacterium and Propionibacterium spp. Eight of the erm(X)-containing A. pyogenes isolates exhibited inducible tylosin resistance, which was consistent with the presence of a putative leader peptide upstream of the erm(X) open reading frame. For at least one A. pyogenes isolate, 98-4277-2, erm(X) was present on a plasmid, pAP2, and was associated with the insertion sequence IS6100. pAP2 also carried genes encoding the repressor-regulated tetracycline efflux system determinant Tet 33. The repA gene from pAP2 was nonfunctional in Escherichia coli and at least one A. pyogenes isolate, suggesting that there may be host-encoded factors required for replication of this plasmid.

Host-vector system for phenol-degrading Rhodococcus erythropolis based on Corynebacterium plasmids

The strain Rhodococcus erythropolis CCM2595, which was shown to degrade phenol, was chosen for genetic studies. To facilitate strain improvement using the methods of gene manipulation, the technique of genetic transfer was introduced and cloning vectors were constructed. Using the plasmid pFAJ2574, an electrotransformation procedure yielding up to 7x10(4) transformants/microg DNA was optimized. Escherichia coli- R. erythropolis shuttle vectors were constructed using the replicons pSR1 and pGA1 from Corynebacterium glutamicum. The small vector pSRK21 (5.8 kb) provides six unique cloning sites and selection of recombinant clones using alpha-complementation of beta-galactosidase in E. coli. This vector, exhibiting high segregational stability under non-selective conditions in R. erythropolis CCM2595, was applied to cloning and efficient expression of the gene coding for green fluorescent protein (gfpuv).

Insights into the genetic organization of the Corynebacterium diphtheriae erythromycin resistance plasmid pNG2 deduced from its complete nucleotide sequence

The complete nucleotide sequence of the erythromycin resistance plasmid pNG2 from the human pathogen Corynebacterium diphtheriae S601 was determined. The plasmid has a total size of 15,100 bp and contains at least 17 coding regions. Comparative genomics identified conserved motifs within replication initiator proteins of corynebacterial plasmids and a novel nucleotide sequence feature, termed 22-bp box, located downstream of the repA gene. The erythromycin resistance determinant erm(X) is flanked by inverted repeats of the novel insertion sequence IS3504, which may be responsible for a spontaneous deletion of the antibiotic resistance gene region. Furthermore, pNG2 encodes a putative conjugative relaxase, a membrane protein of the natural resistance-associated macrophage protein (Nramp) family and a protein with Nudix hydrolase signature. Expression of the predicted coding regions of pNG2 in Escherichia coli JM109 was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) assays. The detailed annotation of the entire pNG2 sequence provided genetic information regarding its molecular evolution and its role in dissemination of antibiotic resistance genes by horizontal gene transfer.

Analysis of a DtxR-like metalloregulatory protein, MntR, from Corynebacterium diphtheriae that controls expression of an ABC metal transporter by an Mn(2+)-dependent mechanism

The DtxR protein is a global iron-dependent repressor in Corynebacterium diphtheriae that regulates transcription from multiple promoters. A search of the partially completed C. diphtheriae genome identified a gene, mntR, whose predicted product has significant homology with the DtxR repressor protein. The mntR gene is the terminal gene in a five-gene operon that also carries the mntABCD genes, whose predicted products are homologous to ABC metal transporters. Transcription of this genetic system, as measured by expression of an mntA-lacZ reporter fusion, is strongly repressed by Mn(2+). The divalent metals Fe(2+), Cu(2+), and Zn(2+) did not repress expression of the mntA-lacZ construct. A mutation in the mntR gene abolished Mn(2+)-dependent repression of the mntA-lacZ fusion, demonstrating that MntR is essential for the Mn(2+)-dependent regulation of this promoter. Footprinting experiments showed that MntR protects from DNase I digestion an approximately 73-bp AT-rich region that includes the entire mntA promoter. This large region protected from DNase I suggests that as many as three MntR dimer pairs may bind to this region. Binding studies also revealed that DtxR failed to bind to the MntR binding site and that MntR exhibited weak and diffuse binding at the DtxR binding site at the tox promoter. A C. diphtheriae mntA mutant grew as well as the wild type in a low-Mn(2+) medium, which suggests that the mntABCD metal transporter is not required for growth in a low-Mn(2+) medium and that additional Mn(2+) transport systems may be present in C. diphtheriae. This study reports the characterization of MntR, a Mn(2+)-dependent repressor, and the second member of the family of DtxR-like metalloregulatory proteins to be identified in C. diphtheriae.

Construction and characterization of transposon insertion mutations in Corynebacterium diphtheriae that affect expression of the diphtheria toxin repressor (DtxR)

Transcription of the bacteriophage-borne diphtheria toxin gene tox is negatively regulated, in response to intracellular Fe(2+) concentration, by the chromosomally encoded diphtheria toxin repressor (DtxR). Due to a scarcity of tools, genetic analysis of Corynebacterium diphtheriae has primarily relied on analysis of chemically induced and spontaneously occurring mutants and on the results of experiments with C. diphtheriae genes cloned in Escherichia coli or analyzed in vitro. We modified a Tn5-based mutagenesis technique for use with C. diphtheriae, and we used it to construct the first transposon insertion libraries in the chromosome of this gram-positive pathogen. We isolated two insertions that affected expression of DtxR, one 121 bp upstream of dtxR and the other within an essential region of the dtxR coding sequence, indicating for the first time that dtxR is a dispensable gene in C. diphtheriae. Both mutant strains secrete diphtheria toxin when grown in medium containing sufficient iron to repress secretion of diphtheria toxin by wild-type C. diphtheriae. The upstream insertion mutant still produces DtxR in decreased amounts and regulates siderophore secretion in response to iron in a manner similar to its wild-type parent. The mutant containing the transposon insertion within dtxR does not produce DtxR and overproduces siderophore in the presence of iron. Differences in the ability of the two mutant strains to survive oxidative stress also indicated that the upstream insertion retained slight DtxR activity, whereas the insertion within dtxR abolished DtxR activity. This is the first evidence that DtxR plays a role in protecting the cell from oxidative stress.

Inducible macrolide resistance in Corynebacterium jeikeium

Corynebacterium jeikeium is an opportunistic pathogen primarily of immunocompromised (neutropenic) patients. Broad-spectrum resistance to antimicrobial agents is a common feature of C. jeikeium clinical isolates. We studied the profiles of susceptibility of 20 clinical strains of C. jeikeium to a range of antimicrobial agents. The strains were separated into two groups depending on the susceptibility to erythromycin (ERY), with one group (17 strains) representing resistant organisms (MIC > 128 microg/ml) and the second group (3 strains) representing susceptible organisms (MIC < or = 0.25 microg/ml). The ERY resistance crossed to other members of the macrolide-lincosamide-streptogramin B (MLSb) group. Furthermore, this resistance was inducible with MLSb agents but not non-MLSb agents. Expression of ERY resistance was linked to the presence of an allele of the class X erm genes, erm(X)cj, with >93% identity to other erm genes of this class. Our evidence indicates that erm(X)cj is integrated within the chromosome, which contrasts with previous reports for the plasmid-associated erm(X) genes found in C. diphtheriae and C. xerosis. In 40% of C. jeikeium strains, erm(X)cj is present within the transposon, Tn5432. However, in the remaining strains, the components of Tn5432 (i.e., the erm and transposase genes) have separated within the chromosome. The rearrangement of Tn5432 leads to the possibility that the other drug resistance genes have become included in a new composite transposon bound by the IS1249 elements.

Construction and consequences of directed mutations affecting the hemin receptor in pathogenic Corynebacterium species

Genes encoding an ATP-binding cassette transporter system involved in hemin iron utilization from Corynebacterium ulcerans were cloned and characterized. The genes are homologous to a hemin transport system previously identified in Corynebacterium diphtheriae. Disruption of the hmuT gene, which encodes the putative hemin receptor, resulted in greatly reduced ability of C. ulcerans to use hemin or hemoglobin as an iron source. Inactivation of hmuT in C. diphtheriae by site-specific recombination had no effect on hemin utilization, which suggests that C. diphtheriae has an additional system for transporting hemin.

Corynebacterium diphtheriae genes required for acquisition of iron from haemin and haemoglobin are homologous to ABC haemin transporters

Corynebacterium diphtheriae and Corynebacterium ulcerans use haemin and haemoglobin as essential sources of iron during growth in iron-depleted medium. C. diphtheriae and C. ulcerans mutants defective in haemin iron utilization were isolated and characterized. Four clones from a C. diphtheriae genomic library complemented several of the Corynebacteria haemin utilization mutants. The complementing plasmids shared an approximately 3 kb region, and the nucleotide sequence of one of the plasmids revealed five open reading frames that appeared to be organized in a single operon. The first three genes, which we have termed hmuT, hmuU and hmuV, shared striking homology with genes that are known to be required for haemin transport in Gram-negative bacteria and are proposed to be part of an ABC (ATP-binding cassette) transport system. The hmuT gene encodes a 37 kDa lipoprotein that is associated with the cytoplasmic membrane when expressed in Escherichi coli and C. diphtheriae. HmuT binds in vitro to haemin- and haemoglobin-agarose, suggesting that it is capable of binding both haemin and haemoglobin and may function as the haemin receptor in C. diphtheriae. This study reports the first genetic characterization of a transport system that is involved in the utilization of haemin and haemoglobin as iron sources by a Gram-positive bacterium.

Electroporation-mediated transformation of Arcanobacterium (Actinomyces) pyogenes

Plasmids derived from pNG2 or RSF1010 were introduced into strains of Arcanobacterium (Actinomyces) pyogenes by electroporation. Electroporation conditions were varied systematically to give a maximum electroporation frequency of 3.7 x 10(5) CFU/microgram DNA at 1.5 kV/cm and 246 omega, resulting in a time constant of approximately 10 ms. The A. pyogenes transformants expressed plasmid-encoded resistance to chloramphenicol, erythromycin, kanamycin, and streptomycin. The source of incoming DNA affected the growth rate of transformants, but not the electroporation efficiency. This is the first report of genetic transformation of the veterinary pathogen A. pyogenes.

Mycotic acid composition of Corynebacterium glutamicum and its cell surface mutants: effects of growth with glycine and isonicotinic acid hydrazide

Auxotrophic mutants of Corynebacterium glutamicum strain ATCC 13059 (parent of AS019, a rifampicin-resistant variant), which were morphologically distinct from the parent and formed protoplasts more readily, had been isolated previously. Mutants MLB130-133 and MLB194 were more sensitive to growth inhibition by isonicotinic acid hydrazide (INH) and glycine, which caused branching and budding. Fatty acid and mycolic acid (MA) profiles were determined after growth in LBG (Luria broth plus glucose), LBG-glycine (LBG- and LBG-INH (LBG-I). The fatty acid profiles of all strains were similar, except that mutant MLB133 showed some increase in stearic acid (C18:0), normally a minor component, late in the growth cycle and oleic acid proportionately decreased. All strains had five major types of MAs (C32:0, C34:0, C34:1, C36:1, C36:2) but the relative proportion of each varied with the strain, age of culture and medium composition. Mutants MLB133 and MLB194 showed slightly higher levels of non-covalentiy bound MAs than the parent and normally showed a higher proportion of longer-chained, unsaturated MAs. The proportion of extracellular MAs increased with culture age for these mutants. Typically, by late stationary phase, mycolic acids in culture fluids increased to 6.5% of the total MAs for MLB194 and 7.9% for MLB133 compared with 3.5% for the parent strain grown in LBG. The main effect of glycine (2%, w/v) addition was to increase the proportion of mycolic acids found in extracellular fluids (16.1 % for AS019 and 31% for MLB133). The most significant effects of INH were seen when strains were cultured in LBG with 8 mg INH ml−1. When harvested at late stationary phase, strains MLB133 and MLB194 had 18.8% and 21.2% extracellular mycolic acids respectively, with a significant increase in the relative proportion of unsaturated mycolic acids. This effect was not as marked for AS019, which also showed a similar decrease in C32:0 relative to increases in the proportion of C34:1 and C36:2 plus a corresponding increase in the overall proportion of unsaturated mycolic acids and increased extracellular mycolates (8.5%). These results suggest that the mutations in strains MLB133 and MLB194 are associated with synthesis of specific mycolic acids (e.g. C32:0) and attachment of mycolic acids to the cell surface, both of which are likely target sites for glycine and INH action for cell-surface modifications. In addition to previously reported targeting of the peptidoglycan cross-linking, these results show that glycine affects mycolic acid attachment to the cell surface of C. glutamicum.

Plasmid pGA1 from Corynebacterium glutamicum codes for a gene product that positively influences plasmid copy number

The complete nucleotide sequence (4,826 bp) of the cryptic plasmid pGA1 from Corynebacterium glutamicum was determined. DNA sequence analysis revealed four putative coding regions (open reading frame A [ORFA], ORFA2, ORFB, and ORFC). ORFC was identified as a rep gene coding for an initiator of plasmid replication (Rep) according to the high level of homology of its deduced amino acid sequence with the Rep proteins of plasmids pSR1 (from C. glutamicum) and pNG2 (from Corynebacterium diphtheriae). This function was confirmed by deletion mapping of the minimal replicon of pGA1 (1.7 kb) which contains only ORFC. Deletion derivatives of pGA1 devoid of ORFA exhibited significant decreases in the copy number in C. glutamicum cells and displayed segregational instability. Introduction of ORFA in trans into the cells harboring these deletion plasmids dramatically increased their copy number and segregational stability. The ORFA gene product thus positively influences plasmid copy number. This is the first report on such activity associated with a nonintegrating bacterial plasmid. The related plasmids pGA1, pSR1, and pNG2 lacking significant homology with any other plasmid seem to be representatives of a new group of plasmids replicating in the rolling-circle mode.

Utilization of host iron sources by Corynebacterium diphtheriae: identification of a gene whose product is homologous to eukaryotic heme oxygenases and is required for acquisition of iron from heme and hemoglobin

Corynebacterium diphtheriae was examined for the ability to utilize various host compounds as iron sources. C. diphtheriae C7(-) acquired iron from heme, hemoglobin, and transferrin. A siderophore uptake mutant of strain C7 was unable to utilize transferrin but was unaffected in acquisition of iron from heme and hemoglobin, which suggests that C. diphtheriae possesses a novel mechanism for utilizing heme and hemoglobin as iron sources. Mutants of C. diphtheriae and Corynebacterium ulcerans that are defective in acquiring iron from heme and hemoglobin were isolated following chemical mutagenesis and streptonigrin enrichment. A recombinant clone, pCD293, obtained from a C7(-) genomic plasmid library complemented several of the C. ulcerans mutants and three of the C. diphtheriae mutants. The nucleotide sequence of the gene (hmuO) required for complementation was determined and shown to encode a protein with a predicted mass of 24,123 Da. Sequence analysis revealed that HmuO has 33% identity and 70% similarity with the human heme oxygenase enzyme HO-1. Heme oxygenases, which have been well characterized in eukaryotes but have not been identified in prokaryotes, are involved in the oxidation of heme and subsequent release of iron from the heme moiety. It is proposed that the HmuO protein is essential for the utilization of heme as an iron source by C. diphtheriae and that the heme oxygenase activity of HmuO is involved in the release of iron from heme. This is the first report of a bacterial gene whose product has homology to heme oxygenases.

Gene replacement by homologous recombination in Mycobacterium bovis BCG

Gene replacement by homologous recombination is a powerful tool for fundamental studies of gene function, as well as allowing specific attenuation of pathogens, but has proved difficult to achieve for Mycobacterium tuberculosis. We have used a plasmid-based test system to demonstrate the occurrence of homologous recombination in the tuberculosis vaccine strain Mycobacterium bovis BCG, and we have successfully replaced a target gene in BCG by homologous recombination, using a shuttle plasmid. Specific inactivation of selected genes will facilitate study of virulence factors and drug resistance as well as allowing rational attenuation of M. tuberculosis for the production of new vaccines.

Molecular analysis and characterization of a broad-host-range plasmid, pEP2

Plasmid pEP2 was found to encode a protein, RepA, which is essential and rate limiting for its replication in Escherichia coli and Corynebacterium pseudotuberculosis. Mutations which altered the rate of synthesis of this protein in E. coli affected the copy number and segregational stability of pEP2 in the two hosts. RepA contains 483 amino acid residues and has the calculated molecular weight of 53,925. It shows 45% amino acid residue identity with open reading frame ORF2 of pSR1, a plasmid isolated from Corynebacterium glutamicum (J. A. C. Archer and A. J. Sinskey, J. Gen. Microbiol. 139:1753-1759, 1993). Plasmid pEP2 was shown to accumulate single-stranded DNA corresponding to the RepA coding strand during its replication in E. coli and C. pseudotuberculosis, suggesting that it may replicate by a rolling circle mechanism. However, RepA has no significant sequence homology with the replication initiator proteins of plasmids known to use this mode of replication.

Transformation of the gram-positive bacterium Clavibacter xyli subsp. cynodontis by electroporation with plasmids from the IncP incompatibility group

We report the transformation of a gram-positive bacterium, Clavibacter xyli subsp. cynodontis, with several plasmids in the IncP incompatibility group from gram-negative bacteria. Our results suggest that IncP plasmids may be transferable to other gram-positive organisms. After optimizing electroporation parameters, we obtained a maximum of 2 x 10(5) transformants per microgram of DNA. The availability of a transformation system for this bacteria will facilitate its use in indirectly expressing beneficial traits in plants.

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.