Identification of zinc and Zur-regulated genes in Corynebacterium diphtheriae

Corynebacterium diphtheriae is a Gram-positive bacterial pathogen and the causative agent of diphtheria, a severe disease of the upper respiratory tract of humans. Factors required for C. diphtheriae to survive in the human host are not well defined, but likely include the acquisition of essential metals such as zinc. In C. diphtheriae, zinc-responsive global gene regulation is controlled by the Zinc Uptake Regulator (Zur), a member of the Fur-family of transcriptional regulators. In this study, we use transcriptomics to identify zinc-regulated genes in C. diphtheriae by comparing gene expression of a wild-type strain grown without and with zinc supplementation. Zur-regulated genes were identified by comparing wild-type gene expression with that of an isogenic zur mutant. We observed zinc repression of several putative surface proteins, the heme efflux system hrtBA, various ABC transporters, and the non-ribosomal peptide synthetase/polyketide synthase cluster sidAB. Furthermore, increased gene expression in response to zinc was observed for the alcohol dehydrogenase, adhA. Zinc and Zur regulation were confirmed for several genes by complementing the zur deletion and subsequent RT-qPCR analysis. We used MEME to predict Zur binding sites within the promoter regions of zinc- and Zur-regulated genes, and verified Zur binding by electrophoretic mobility shift assays. Additionally, we characterized cztA (dip1101), which encodes a putative cobalt/zinc/cadmium efflux family protein. Deletion of cztA results in increased sensitivity to zinc, but not to cobalt or cadmium. This study advances our knowledge of changes to Zur-dependent global gene expression in response to zinc in C. diphtheriae. The identification of zinc-regulated ABC transporters herein will facilitate future studies to characterize zinc transport in C. diphtheriae.

Characterization of the second conserved domain in the heme uptake protein HtaA from Corynebacterium diphtheriae

HtaA is a heme-binding protein that is part of the heme uptake system in Corynebacterium diphtheriae. HtaA contains two conserved regions (CR1 and CR2). It has been previously reported that both domains can bind heme; the CR2 domain binds hemoglobin more strongly than the CR1 domain. In this study, we report the biophysical characteristics of HtaA-CR2. UV-visible spectroscopy and resonance Raman experiments are consistent with this domain containing a single heme that is bound to the protein through an axial tyrosine ligand. Mutants of conserved tyrosine and histidine residues (Y361, H412, and Y490) have been studied. These mutants are isolated with very little heme (≤5%) in comparison to the wild-type protein (~20%). Reconstitution after removal of the heme with butanone gave an alternative form of the protein. The HtaA-CR2 fold is very stable; it was necessary to perform thermal denaturation experiments in the presence of guanidinium hydrochloride. HtaA-CR2 unfolds extremely slowly; even in 6.8M GdnHCl at 37°C, the half-life was 5h. In contrast, the apo forms of WT HtaA-CR2 and the aforementioned mutants unfolded at much lower concentrations of GdnHCl, indicating the role of heme in stabilizing the structure and implying that heme transfer is effected only to a partner protein in vivo.

Corynebacterium diphtheriae HmuT: dissecting the roles of conserved residues in heme pocket stabilization

The heme-binding protein HmuT is part of the Corynebacterium diphtheriae heme uptake pathway and is responsible for the delivery of heme to the HmuUV ABC transporter. HmuT binds heme with a conserved His/Tyr heme axial ligation motif. Sequence alignment revealed additional conserved residues of potential importance for heme binding: R237, Y272 and M292. In this study, site-directed mutations at these three positions provided insight into the nature of axial heme binding to the protein and its effect on the thermal stability of the heme-loaded protein fold. UV-visible absorbance, resonance Raman (rR) and thermal unfolding experiments, along with collision-induced dissociation electrospray ionization mass spectrometry, were used to probe the contributions of each mutated residue to the stability of ϖ HmuT. Thermal unfolding and rR experiments revealed that R237 and M292 are important residues for heme binding. Arginine 237 is a hydrogen-bond donor to the phenol side chain of Y235, which serves as an axial heme ligand. Methionine 292 serves a supporting structural role, favoring the R237 hydrogen-bond donation, which elicits a, heretofore, unobserved modulating influence on π donation by the axial tyrosine ligand in the heme carbonyl complex, HmuT-CO.

Heme Binding by Corynebacterium diphtheriae HmuT: Function and Heme Environment

The heme uptake pathway (hmu) of Corynebacterium diphtheriae utilizes multiple proteins to bind and transport heme into the cell. One of these proteins, HmuT, delivers heme to the ABC transporter HmuUV. In this study, the axial ligation of the heme in ferric HmuT is probed by examination of wild-type (WT) HmuT and a series of conserved heme pocket residue mutants, H136A, Y235A, and M292A. Characterization by UV-visible, resonance Raman, and magnetic circular dichroism spectroscopies indicates that H136 and Y235 are the axial ligands in ferric HmuT. Consistent with this assignment of axial ligands, ferric WT and H136A HmuT are difficult to reduce while Y235A is reduced readily in the presence of dithionite. The FeCO Raman shifts in WT, H136A, and Y235A HmuT-CO complexes provide further evidence of the axial ligand assignments. Additionally, these frequencies provide insight into the nonbonding environment of the heme pocket. Ferrous Y235A and the Y235A-CO complex reveal that the imidazole of H136 exists in two forms, one neutral and one with imidazolate character, consistent with a hydrogen bond acceptor on the H136 side of the heme. The ferric fluoride complex of Y235A reveals the presence of at least one hydrogen bond donor on the Y235 side of the heme. Hemoglobin utilization assays showed that the axial Y235 ligand is required for heme uptake in HmuT.

High rate of early virological failure with the once-daily tenofovir/lamivudine/nevirapine combination in naive HIV-1-infected patients

BACKGROUND

The combination of one non-nucleoside reverse transcriptase inhibitor (NNRTI) with two nucleoside reverse transcriptase inhibitors is a validated first-line antiretroviral (ARV) therapy. The once-daily combination of lamivudine, tenofovirDF and nevirapine has not been evaluated in a clinical trial.

METHODS

Randomized, open-label, multicentre, non-inferiority trial comparing lamivudine, tenofovirDF and nevirapine once daily (Group 2) with zidovudine/lamivudine and nevirapine twice daily (Group 1), in naive HIV-1-infected patients with a CD4 count <350/mm(3). We planned to enroll 250 patients.

RESULTS

As of May 2006, 71 patients had been enrolled (35 in Group 1 and 36 in Group 2) and an unplanned interim analysis was done. The groups were comparable at baseline: median CD4 count was 195 and 191/mm(3) and median plasma viral load was 4.9 log(10) and 5.01 log(10), respectively, in Groups 1 and 2. Eight early non-responses (22.2%) were observed, all in Group 2, while two later viral rebounds occurred. Resistance genotypes for the nine Group 2 failing patients showed the mutations M184V/I (n = 3), K65R (n = 6), one or more NNRTI resistance mutations in all cases. At baseline, the nine Group 2 patients who failed had higher median plasma viral load (5.4 log(10)) and lower median CD4 count (110/mm(3)) than the other Group 2 patients (4.7 log(10), P = 0.002 and 223/mm(3), P = 0.004). Nevirapine trough concentrations were not different between the two groups, nor between patients with full viral suppression or those who failed in Group 2. Due to slow recruitment, and those results, the steering committee decided to stop the trial at 12 months.

CONCLUSIONS

In ARV-naive HIV-1-infected patients, the once-daily lamivudine, tenofovirDF and nevirapine regimen resulted in a high rate of early virological failures. The reasons for the failures remain unclear.

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 analysis of hmuO function and expression in Corynebacterium species

We have constructed defined deletions in the hmuO gene from Corynebacterium diphtheriae and Corynebacterium ulcerans and show that the C. ulcerans hmuO mutation results in a significant reduction in hemoglobin-iron utilization, whereas in C. diphtheriae strains, deletion of hmuO caused no or only partial reduction in the utilization of heme as an iron source. We also show that expression from the C. ulcerans hmuO promoter exhibits minimal regulation by iron and heme whereas transcription from the C. diphtheriae hmuO promoter shows both significant iron repression and heme-dependent activation. These findings indicate that variability in HmuO function and expression exists among Corynebacterium species.

Analysis of a heme-dependent signal transduction system in Corynebacterium diphtheriae: deletion of the chrAS genes results in heme sensitivity and diminished heme-dependent activation of the hmuO promoter

The Corynebacterium diphtheriae hmuO gene encodes a heme oxygenase that is involved in the utilization of heme as an iron source. Transcription of hmuO is activated by heme or hemoglobin and repressed by iron and DtxR. Previous studies with Escherichia coli showed that heme-dependent transcriptional activation of an hmuO promoter-lacZ fusion was dependent on the cloned C. diphtheriae chrA and chrS genes (chrAS), which encode the response regulator and sensor kinase, respectively, of a two-component signal transduction system. In this study, nonpolar deletions in the chrAS genes were constructed on the chromosome of C. diphtheriae. Mutations in chrAS resulted in marked reduction in heme-dependent transcription of hmuO, which indicates that the ChrA/S system is a key regulator at the hmuO promoter. However, low but significant levels of heme-specific transcriptional activity were observed at the hmuO promoter in the chrAS mutants, suggesting that an additional heme-dependent activator is involved in hmuO expression. The chrAS mutants were also sensitive to heme, which was observed only in stationary-phase cultures and correlated with reduced cell viability. The heme sensitivity of the mutants was not due to reduced expression of hmuO, and these results suggest that additional factors controlled by the ChrA/S system may be involved in protection against heme toxicity. Transcriptional analysis of the chrAS operon revealed that it was not autoregulated or affected by iron or heme levels.

Analysis of a DtxR-regulated iron transport and siderophore biosynthesis gene cluster in Corynebacterium diphtheriae

This report describes a genetic locus associated with siderophore biosynthesis and transport in Corynebacterium diphtheriae. A BLAST search of the C. diphtheriae genome identified a seven-gene cluster that included four genes, designated ciuA, ciuB, ciuC, and ciuD, whose predicted products are related to ABC-type iron transporters. Downstream from ciuD is the ciuE gene, whose predicted product is similar to the aerobactin biosynthetic enzymes IucA and IucC. The CiuE protein, which has a predicted mass of 121,582 Da and is approximately twice the size of either IucC or IucA, is homologous to each of these proteins in both its N- and C-terminal regions. C. diphtheriae ciuE deletion mutants exhibited a defect in siderophore production, iron uptake, and growth in low-iron medium. Mutations in the ciuA gene, whose predicted product is a lipoprotein component of an iron transport system, resulted in a severe defect in iron uptake and reduced ability to use the C. diphtheriae siderophore as an iron source. Site-directed mutations in irp6A, a gene previously reported to be associated with siderophore transport, had no effect on iron uptake or the utilization of the C. diphtheriae siderophore as an iron source. Transcriptional analysis demonstrated that expression of ciuA and ciuE is DtxR and iron regulated, and DNase I protection experiments confirmed the presence of DtxR binding sites upstream from each of these genes. Thus, this iron- and DtxR-regulated gene cluster is involved in the synthesis and transport of the C. diphtheriae siderophore.

Analysis of the Corynebacterium diphtheriae DtxR regulon: identification of a putative siderophore synthesis and transport system that is similar to the Yersinia high-pathogenicity island-encoded yersiniabactin synthesis and uptake system

The diphtheria toxin repressor, DtxR, is a global iron-dependent regulatory protein in Corynebacterium diphtheriae that controls gene expression by binding to 19-bp operator sequences. To further define the DtxR regulon in C. diphtheriae, a DtxR repressor titration assay (DRTA) was developed and used to identify 10 previously unknown DtxR binding sites. Open reading frames downstream from seven of the newly identified DtxR binding sites are predicted to encode proteins associated with iron or heme transport. Electrophoretic mobility shift assays indicated that DtxR was able to bind to DNA fragments carrying the 19-bp operator regions, and transcriptional analysis of putative promoter elements adjacent to the binding site sequences revealed that most of these regions displayed iron- and DtxR-regulated activity. A putative siderophore biosynthesis and transport operon located downstream from one of the DtxR binding sites, designated sid, is similar to the yersiniabactin synthesis and uptake genes encoded on the Yersinia pestis high pathogenicity island. The siderophore biosynthetic genes in the sid operon contained a large deletion in the C. diphtheriae C7 strain, but the sid genes were unaffected in four clinical isolates that are representative of the dominant strains from the recent diphtheria epidemic in the former Soviet Union. Mutations in the siderophore biosynthetic genes in a clinical strain had no effect on siderophore synthesis or growth in low-iron conditions; however, a mutation in one of the putative transport proteins, cdtP, resulted in reduced growth in iron-depleted media, which suggests that this system may have a role in iron uptake. The findings from this study indicate that C. diphtheriae contains at least 18 DtxR binding sites and that DtxR may affect the expression of as many as 40 genes.

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.

[Structured therapeutic interruption in patients infected with HIV-1 experiencing a block in their antiretroviral treatment: preliminary results]

Structured therapeutic interruption (STI) has been offered to HIV-1 infected patients with virological failure (viral load > 1500 copies/mL) of potent antiretroviral therapy (ART) (three or four drugs for at least one year). CD4 lymphocyte count, HIV-1 viral load, clinical status, were assessed every month during STI and after ART reintroduction. Genotype analysis by plasma virus sequencing was done before and after treatment interruption. The results of 14 patients who resumed ART for at least two months are presented. Median duration of STI was 7.5 months (range: 2-13 months). Median CD4 count was low (45/mm3) when treatment was stopped, and decreased during STI (-37/mm3 after six months). Several patients exhibited important CD4 diminutions. Viral load slightly increased (+0.83 log at M6). Few clinical events occurred: one: severe HIV-related prurigo and one CMV viremia. Reversion of resistance mutations was only seen in 2/13 (15: 4%) patients (who had previously a major CD4 deficiency, and a long treatment history), a partial reversion occurred in 5/13 (38.5%) subjects, and the mutations didn't change in the other cases (genotyping non interpretable in the last patient). ART reintroduction induced a good immune response: CD4/mm3 after six months, with significant increases in 10/14 subjects. There was an initial viral response (median viral load: -2.34 log at M1), but a quick rebound most often occurred. However, viral load remained < 50 copies/mL in four patients. In conclusion, a rapid and important decline in CD4 cell count can occur when treatment is discontinued, in patients with virological failure of ART, but the clinical risk appears to be limited. Treatment re-initiation induces a good response, but virologically transient in most cases. Patients with a shift to wild-type virus seem to have a better response.

Efavirenz as a substitute for protease inhibitors in HIV-1-infected patients with undetectable plasma viral load on HAART: a median follow-up of 64 weeks

We investigated, in a prospective cohort follow-up study, whether substituting efavirenz (EFV) for protease inhibitors (PIs) could be safe in HIV-infected patients with optimal viral suppression achieved on PI-containing regimens. In patients with undetectable plasma viral load (pVL) <50 copies/ml who were naive to therapy with nonnucleoside reverse transcriptase inhibitors (NNRTIs), PIs were replaced by EFV whereas associated nucleoside analogs (NAs) were retained. 62 patients were enrolled. Median follow-up on EFV was 64 weeks (2-88 weeks). Side effects due to EFV occurred in 48 patients. Two patients experienced a high level viral rebound due to diminished compliance; 55 (88.7%) maintained a pVL <50 copies/ml; 3 showed one episode of viremia (52-89 copies/ml); 2 stopped EFV before any VL control. Mean CD4 cell count did not change significantly. One AIDS patient experienced a single cutaneous recurrence of Kaposi's sarcoma after 40 weeks on EFV. Replacing PI with EFV in patients with optimal pVL suppression appears to be safe both virologically and immunologically.

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.

Zidovudine resensitization and dual HIV-1 resistance to zidovudine and lamivudine in the delta lamivudine roll-over study

OBJECTIVE

To study zidovudine resensitization and dual resistance to zidovudine/lamivudine in HIV-1 isolates from nucleoside reverse transcriptase (RT) inhibitor-experienced patients during selective pressure exerted by zidovudine/lamivudine combination therapy.

DESIGN AND METHODS

HIV-1 isolates from 29 patients receiving zidovudine/lamivudine combination therapy in the Delta roll-over study were analysed at entry and during a 1 year follow-up period for phenotypic susceptibility to zidovudine and lamivudine in the ANRS PBMC assay. The RT gene from codon 20 to 230 and at codon 333 was analysed by nucleotide sequencing of the corresponding isolates.

RESULTS

HIV-1 isolates from 23 of the 29 patients were phenotypically resistant to zidovudine at baseline; 61% of these patients showed significant zidovudine resensitization during follow-up. The zidovudine IC50 value correlated positively with log10 plasma HIV-1 RNA (P = 0.02) and negatively with the CD4 cell count (P = 0.004). Zidovudine resensitization (related to acquisition of the M184V mutation) was transient, with evolution towards dual resistance to zidovudine and lamivudine in 20 of the 29 patients. The phenotype of certain dually resistant isolates coincided with the emergence of multiple mutations in the 5' part of the RT gene.

CONCLUSIONS

M184V-mediated zidovudine resensitization of HIV-1 is transient in most patients who are given zidovudine/lamivudine combination therapy when zidovudine resistance has already emerged. The subsequent evolution towards dual phenotypic resistance to zidovudine/lamivudine corresponds to complex genotypic profiles.

Increasing the number of hepatitis B vaccine injections augments anti-HBs response rate in HIV-infected patients. Effects on HIV-1 viral load

Preventing hepatitis B by vaccination is essential in HIV-infected patients (higher progression rate of HBV infection to chronicity, lower rate of serum HBe Ag loss). However, it has been shown a decreased anti-HBs response in these individuals after a standard vaccination (3 doses of 20 micrograms). Thus, we tested the hypothesis that doubling the number of hepatitis B vaccine injections might increase anti-HBs response rate. HIV-infected patients with CD4 > 200/microliter, who were on stable antiretroviral treatment, as well as seronegative for HBV markers, and who have never been vaccinated against HBV, were given 3 intramuscular injections of Genhevac B 20 micrograms at 1 month intervals. Initial non responders were given 3 additional monthly injections. Anti-HBs titer was followed. We also evaluated the effects on HIV-1 viral load. Twenty patients with a median CD4 cell count of 470/microliter were enrolled. The response rate after three 20 micrograms injections was 55% (11/20), lower in individuals with CD4 between 200 and 500/microliter (4/12 = 33.3%), compared to patients with CD4 above 500/microliter (7/8 = 87.5%, P = 0.02). Among 9 initial non-responders, only 2 did not respond to 3 additional doses; thus, the overall response rate was 90% (18/20). Geometric mean titers of anti-HBs were 133 IU/l and 77.5 IU/l, after 3 and 6 Genhevac doses, respectively (P = 0.38). One year later, only 10/17 (58.8%) patients had protective anti-HBs. Five patients experienced a significant viral load increase, transient in 3 cases. These preliminary results suggest that doubling the number of hepatitis B vaccinations in HIV-infected patients might significantly improve anti-HBs response rate; however, close monitoring of anti-HBs is necessary because of its short-lived persistence. The effects on HIV-1 viral load are limited.

Identification of a two-component signal transduction system from Corynebacterium diphtheriae that activates gene expression in response to the presence of heme and hemoglobin

Corynebacterium diphtheriae, the causative agent of diphtheria, utilizes various host compounds to acquire iron. The C. diphtheriae hmuO gene encodes a heme oxygenase that is involved in the utilization of heme and hemoglobin as iron sources. Transcription of the hmuO gene in C. diphtheriae is controlled under a dual regulatory mechanism in which the diphtheria toxin repressor protein (DtxR) and iron repress expression while either heme or hemoglobin is needed to activate transcription. In this study, two clones isolated from a C. diphtheriae chromosomal library were shown to activate transcription from the hmuO promoter in Escherichia coli. Sequence analysis revealed that these activator clones each carried distinct genes whose products had significant homology to response regulators of two-component signal transduction systems. Located upstream from each of these response regulator homologs are partial open reading frames that are predicted to encode the C-terminal portions of sensor kinases. The full-length sensor kinase gene for each of these systems was cloned from the C. diphtheriae chromosome, and constructs each carrying one complete sensor kinase gene and its cognate response regulator were constructed. One of these constructs, pTSB20, which carried the response regulator (chrA) and its cognate sensor kinase (chrS), was shown to strongly activate transcription from the hmuO promoter in a heme-dependent manner in E. coli. A mutation in chrA (chrAD50N), which changed a conserved aspartic acid residue at position 50, the presumed site of phosphorylation by ChrS, to an asparagine, abolished heme-dependent activation. These findings suggest that the sensor kinase ChrS is involved in the detection of heme and the transduction of this signal, via a phosphotransfer mechanism, to the response regulator ChrA, which then activates transcription of the hmuO promoter. This is the first report of a bacterial two-component signal transduction system that controls gene expression through a heme-responsive mechanism.

Zidovudine Resensitization and Dual HIV-1 Resistance to Zidovudine and Lamivudine in the Delta Lamivudine Roll-Over Study

Objective

To study zidovudine resensitization and dual resistance to zidovudine/lamivudine in HIV-1 isolates from nucleoside reverse transcriptase (RT) inhibitor-experienced patients during selective pressure exerted by zidovudine/lamivudine combination therapy.

Design and methods

HIV-1 isolates from 29 patients receiving zidovudine/lamivudine combination therapy in the Delta roll-over study were analysed at entry and during a 1 year follow-up period for phenotypic susceptibility to zidovudine and lamivudine in the ANRS PBMC assay. The RT gene from codon 20 to 230 and at codon 333 was analysed by nucleotide sequencing of the corresponding isolates.

Results

HIV-1 isolates from 23 of the 29 patients were phenotypically resistant to zidovudine at baseline; 61% of these patients showed significant zidovudine resensitization during follow-up. The zidovudine IC50 value correlated positively with log10 plasma HIV-1 RNA ( P=0.02) and negatively with the CD4 cell count ( P=0.004). Zidovudine resensitization (related to acquisition of the M184V mutation) was transient, with evolution towards dual resistance to zidovudine and lamivudine in 20 of the 29 patients. The phenotype of certain dually resistant isolates coincided with the emergence of multiple mutations in the 5’ part of the RT gene.

Conclusions

M184V-mediated zidovudine resensitization of HIV-1 is transient in most patients who are given zidovudine/lamivudine combination therapy when zidovudine resistance has already emerged. The subsequent evolution towards dual phenotypic resistance to zidovudine/lamivudine corresponds to complex genotypic profiles.

Expression and characterization of a heme oxygenase (Hmu O) from Corynebacterium diphtheriae. Iron acquisition requires oxidative cleavage of the heme macrocycle

A full-length heme oxygenase gene from the pathogenic bacterium Corynebacterium diphtheriae has been subcloned and expressed in Escherichia coli. The enzyme is expressed at high levels as a soluble catalytically active protein that results in the accumulation of biliverdin within the E. coli cells. The purified heme oxygenase forms a 1:1 complex with heme (Kd = 2.5 +/- 1 microM) and has hemeprotein spectra similar to those previously reported for the purified eukaryotic heme oxygenases. In the presence of an E. coli NADPH-dependent reductase isolated during the purification of Hmu O, the heme-Hmu O complex is catalytically turned over to yield biliverdin IXalpha and carbon monoxide. A number of redox partners were investigated for their ability to reconstitute Hmu O activity in vitro. Of these the most efficient appeared to be the recombinant NADH-dependent putidaredoxin/putidaredoxin reductase from Pseudomonas putida. As with the E. coli NADPH-dependent reductase the final products of the reaction were biliverdin IXalpha and carbon monoxide. This is the first bacterial heme oxygenase to be described to date. The close relationship between iron acquisition and pathogenesis suggests that the release of iron from heme by heme oxygenase may play a crucial role in the pathogenicity of C. diphtheriae.

Characterization of lipoprotein IRP1 from Corynebacterium diphtheriae, which is regulated by the diphtheria toxin repressor (DtxR) and iron

The Corynebacterium diphtheriae irp1 gene is negatively regulated by DtxR and iron. The nucleotide sequence of irp1 revealed that it has homology with genes involved in iron acquisition. Expression of the irp1 gene showed that it encodes a lipoprotein (IRP1) with a predicted size of 38 kDa. Northern blot experiments indicated that transcription from the irp1 promoter is repressed in high-iron medium and suggested that irp1 is part of an iron-regulated operon.

Transcription of the Corynebacterium diphtheriae hmuO gene is regulated by iron and heme

The hmuO gene is required for the utilization of heme and hemoglobin as iron sources by Corynebacterium diphtheriae. The product of hmuO has homology to eukaryotic heme oxygenases which are involved in the degradation of heme and the release of iron. To investigate the mechanism of hmuO regulation, a promoterless lacZ gene present on the promoter-probe vector pCM502 was placed under transcriptional control of the hmuO promoter. In C. diphtheriae C7, optimal expression from the hmuO promoter was obtained only in the presence of heme or hemoglobin under low-iron conditions. Expression of hmuO in high-iron medium containing heme was repressed five- to sixfold from that seen under low-iron conditions in the presence of heme. Transcription from the hmuO promoter in the absence of heme or hemoglobin was fully repressed in high-iron medium and was expressed at very low levels in iron-depleted conditions. Expression studies with tile hmuO-lacZ fusion construct in C7hm723, a dtxR mutant of C7, and in a hmuO mutant of C. diphtheriae HC1 provided further evidence that transcription of the hmuO promoter is repressed by DtxR and iron and activated by heme. In Escherichia coli, the hmuO promoter was expressed at very low levels under all conditions examined. Gel mobility shift assays and DNase I footprinting experiments indicated that DtxR binds in a metal-dependent manner to a sequence that overlaps the putative hmuO promoter. Total cellular RNA isolated from C. diphtheriae was used to identify the transcriptional start site for the hmuO gene. Northern blot analysis suggested that the hmuO mRNA was monocistronic and that transcription was heme inducible.

Identification and characterization of three new promoter/operators from Corynebacterium diphtheriae that are regulated by the diphtheria toxin repressor (DtxR) and iron

DtxR is a dimeric, sequence-specific, DNA-binding protein that functions as an iron-dependent, negative global regulator in Corynebacterium diphtheriae. Under high-iron conditions, DtxR represses the synthesis of diphtheria toxin, corynebacterial siderophore, and other components of the high-affinity iron uptake system. Three DtxR-regulated promoter/operators designated tox, IRP1, and IRP2 were reported previously. In this study, we identified and characterized three additional DtxR-regulated promoter/operators from C. diphtheriae designated IRP3, IRP4, and IRP5. When beta-galactosidase was expressed from these three new promoter/ operators in Escherichia coli containing dtxR+ on pDSK29, enzyme levels were 5- to 30-fold lower during high-iron growth than during low-iron growth. In gel shift assays, the mobility of DNA fragments containing each promoter/operator decreased in the presence of purified DtxR and Co2+. In footprinting assays, DtxR protected 36-, 35-, and 30-bp regions of IRP3, IRP4, and IRP5, respectively, from cleavage by DNase I. In the 19-bp core of each promoter/operator, 12 or 13 bp matched the consensus for the DtxR-binding site. The putative polypeptides encoded by the open reading frames (ORFs) downstream from IRP3 and IRP4 were homologous, respectively, to several bacterial transcriptional regulators and to the deduced polypeptide encoded by an ORF located between the E. coli genes for primosomal replication protein N and adenine phosphoribosyltransferase. The putative polypeptide encoded by the ORF downstream from IRP5 was not homologous to any sequence in the protein database at the National Center for Biotechnology Information. When the ORFs downstream from IRP3 and IRP4 were expressed under the control of the phage T7 promoter in E. coli, polypeptide products of the predicted sizes were detected in small amounts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

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.

Effects of morphine on purified human blood monocytes. Modifications of properties involved in antiviral defences

It has been demonstrated that morphine stimulates the replication of human immunodeficiency virus in peripheral blood mononuclear cells as well as in Kupffer cells. Since the mechanism of action of this drug is still unknown, we have studied its effects on different properties of isolated human blood monocytes. In the presence of morphine, cultured monocytes showed an increase in the fluidity of their membranes as well as an inhibition in their capacity to differentiate into macrophages. Furthermore, the response of the cells to interferon-gamma was significantly decreased and the release of superoxide anions was altered. Finally the production of interferon-alpha and of prostaglandin E2 induced by stimulation of the cells with endotoxin (LPS) was diminished. We conclude that morphine decreases the functions of monocytes that are essential for their antiviral defence and inhibits their response to activating stimuli, which may explain the increased multiplication of HIV in morphine treated monocytes.

Changes in sexual behaviour of patients attending an HIV testing centre: a prospective study 1988-1994

OBJECTIVE

To evaluate the sexual behaviour changes in patients attending HIV testing during the period July 1988 to June 1994.

DESIGN

In a prospective study, 6824 face-to-face interviews were carried out before the HIV test was performed. The frequency of condom use and the number of sexual partners during the 6 previous months were recorded annually from July to June. The data were analysed according to gender, age class and sexual orientation.

SETTING

Strasbourg, Bas-Rhin, France.

SUBJECTS

Patients attending the HIV testing centre of Strasbourg.

RESULTS

There was a striking increase in the number of attenders of this centre from 358 patients in 1988/89 to 2421 in 1993/94. We observed a significant decrease of homosexuals having more than five partners (p < 0.05) whereas multipartner sex remained unchanged in heterosexuals. There was no change in the proportion of patients having only one partner, except a slight raise in patients under 20 years. All groups showed a very significant increase in condom use, which was especially marked in young heterosexuals aged under 30 years. Nevertheless, condom use remained higher in homosexuals than in heterosexuals in 1993/94. In addition, there was a striking fall of past sexually transmitted disease in heterosexual patients under 20 years during the study period, and a fall in the HIV positivity rate from 1.96% to 0.42%.

CONCLUSIONS

A major increase was noted in condom use in all groups and a reduction of multipartner sex in some patients. These data are encouraging in the younger patients, but prevention efforts should also be concentrated on middle aged patients who did not show major sexual changes, although having important risk factors for HIV infection.

Characterization of an iron-dependent regulatory protein (IdeR) of Mycobacterium tuberculosis as a functional homolog of the diphtheria toxin repressor (DtxR) from Corynebacterium diphtheriae

The DtxR protein from Corynebacterium diphtheriae is an iron-dependent repressor that regulates transcription from the tox, IRP1, and IRP2 promoters. A gene from virulent Mycobacterium tuberculosis H37Rv was recently shown to encode a protein, here designated iron-dependent regulator (IdeR), that is almost 60% homologous to DtxR from C. diphtheriae. A 750-bp PCR-derived DNA fragment carrying the M. tuberculosis ideR allele was subcloned to both high- and low-copy-number vectors. In Escherichia coli, transcription from the C. diphtheriae tox, IRP1, and IRP2 promoters was strongly repressed by ideR under high-iron conditions, and ideR restored normal iron-dependent expression of the corynebacterial siderophore in the C. diphtheriae dtxR mutant C7(beta)hm723. The M. tuberculosis IdeR protein was overexpressed in E. coli and purified to near homogeneity by nickel affinity chromatography. Gel mobility shift experiments revealed that IdeR bound to a DNA fragment that carried the C. diphtheriae tox promoter/operator sequence. DNAse I footprint analysis demonstrated that IdeR, in the presence of Cd2+, Co2+, Fe2+, Mn2+, Ni2+, or Zn2+, protected an approximately 30-bp region on DNA fragments carrying the tox, IRP1, or IRP2 promoter/operator sequences. IdeR reacted very weakly in Western blots (immunoblots) with antiserum against the C. diphtheriae DtxR protein, suggesting that the immunodominant epitopes of DtxR may be located in its poorly conserved carboxyl-terminal domain.

Three-dimensional structure of the diphtheria toxin repressor in complex with divalent cation co-repressors

BACKGROUND

When Corynebacterium diphtheriae encounters an environment with a low concentration of iron ions, it initiates the synthesis of several virulence factors, including diphtheria toxin. The diphtheria toxin repressor (DtxR) plays a key role in this iron-dependent, global regulatory system and is the prototype for a new family of iron-dependent repressor proteins in Gram-positive bacteria. This study aimed to increase understanding of the general regulatory principles of cation binding to DtxR.

RESULTS

The crystal structure of dimeric DtxR holo-repressor in complex with different transition metals shows that each subunit comprises an amino-terminal DNA-binding domain, an interface domain (which contains two metal-binding sites) and a third, very flexible carboxy-terminal domain. Each DNA-binding domain contains a helix-turn-helix motif and has a topology which is very similar to catabolite gene activator protein (CAP). Molecular modeling suggests that bound DNA adopts a bent conformation with helices alpha 3 of DtxR interacting with the major grooves. The two metal-binding sites lie approximately 10 A apart. Binding site 2 is positioned at a potential hinge region between the DNA-binding and interface domains. Residues 98-108 appear to be crucial for the functioning of the repressor; these provide four of the ligands of the two metal-binding sites and three residues at the other side of the helix which are at the heart of the dimer interface.

CONCLUSIONS

The crystal structure of the DtxR holorepressor suggests that the divalent cation co-repressor controls motions of the DNA-binding domain. In this way the metal co-repressor governs the distance between operator recognition elements in the two subunits and, consequently, DNA recognition.

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.

Analysis of diphtheria toxin repressor-operator interactions and characterization of a mutant repressor with decreased binding activity for divalent metals

The diphtheria toxin repressor (DtxR) is an Fe(2+)-activated protein with sequence-specific DNA-binding activity for the diphtheria toxin (tox) operator. Under high-iron conditions in Corynebacterium diphtheriae, DtxR represses toxin and siderophore biosynthesis as well as iron uptake. DtxR and a mutant repressor with His-47 substituted for Arg-47, designated DtxR-R47H, were purified and compared. Six different divalent cations (Cd2+, Co2+, Fe2+, Mn2+, Ni2+, and Zn2+) activated the sequence-specific DNA-binding activity of DtxR and enabled it to protect the tox operator from DNase I digestion, but Cu2+ failed to activate DtxR. Hydroxyl radical footprinting experiments indicated that DtxR binds symmetrically about the dyad axis of the tox operator. Methylation protection experiments demonstrated that DtxR binding alters the susceptibility to methylation of three G residues within the AT-rich tox operator. These findings suggest that two or more monomers of DtxR are involved in binding to the tox operator, with symmetrical DNA-protein interactions occurring at each end of the palindromic operator. In this regard, DtxR resembles several other well-characterized prokaryotic repressor proteins but differs dramatically from the Fe(2+)-activated ferric uptake repressor protein (Fur) of Escherichia coli. The concentration of Co2+ required to activate DtxR-R47H was at least 10-fold greater than that needed to activate DtxR, but the sequence-specific DNA binding of activated DtxR-R47H was indistinguishable from that of wild-type DtxR. The markedly deficient repressor activity of DtxR-R47H is consistent with a significant decrease in its binding activity for divalent cations.

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.

HIV-1 sensitivity to zidovudine: a consensus culture technique validated by genotypic analysis of the reverse transcriptase

In order to select and standardize a reliable assay for the analysis of sensitivity of HIV isolates to AZT, we have compared two culture methods. The first assay (Cell-Associated Isolate Sensitivity Assay: CAISA) quantified AZT-resistant HIV isolates by end-point dilution cultures of peripheral blood mononuclear cells (PBMCs) in the presence of various concentrations of AZT. In the second assay (Cell-Free Isolate Sensitivity Assay: CFISA), following a conventional isolation of HIV, dilutions of infected cell-free supernatants were cultivated with fresh normal donor PBMCs in the presence of increasing concentrations of AZT. Samples from 64 untreated and AZT-treated patients were studied by CAISA (41), CFISA (43) or both assays (20). The CFISA, which allows the determination of titration parameters with respect to various kinetics patterns of viral replication was selected, and some of the CFISA phenotypically characterized isolates were further studied by nucleotide sequence analysis of the reverse transcriptase gene. CFISA showed that isolates from untreated patients were susceptible to AZT while the frequency of resistance increased with the duration of therapy. Genotypic analysis of CFISA-resistant isolates exhibited mutations at crucial positions, particularly at residue 215. We consider CFISA as a consensus culture technique for longitudinal studies of isolates from patients receiving AZT or other analogs of nucleosides.

Primary cultures of endothelial cells from the human liver sinusoid are permissive for human immunodeficiency virus type 1

Human endothelial cells isolated from hepatic sinusoids were infected in vitro with human immunodeficiency virus type 1 (HIV-1). An early sign of infection occurring in the culture was the formation of multinucleated cells. By double-labeling immunofluorescence, 5-15% of the cells recognized as endothelial cells owing to the presence of von Willebrand factor were found to contain HIV p24 and gp120 antigens after 2 weeks. Reverse transcriptase activity was released into the medium, and different steps in the process of viral budding were observed by electron microscopy. The virus produced by the endothelial cells was found to be infectious for CEM cells, a human T-cell line. CD4 molecules are present at the surface of the endothelial cells, as demonstrated by immunogold-silver staining and backscattered electron imaging. Treatment with an anti-CD4 antibody abolished productive infection of the sinusoidal endothelial cells. The possibility that endothelial cells of the liver sinusoid are infected in vivo with HIV remains to be clearly shown.

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.

Morphine stimulates HIV replication in primary cultures of human Kupffer cells

Intravenous drug abusers represent a high risk group for HIV infection in Europe and North America. Although the use of blood-contaminated needles undoubtedly constitutes the main factor of transmission of the virus, an effect of the drug itself either on the immune system or on virus replication, thus favouring the initiation of the infection, may not be excluded. We have formerly established that primary cultures of human Kupffer cells (KC) are permissive for HIV1. In this paper, we describe the effect of morphine hydrochloride on the multiplication of different isolates of HIV1 in cultured human KC. KC were obtained by dissociation of human liver fragments with collagenase and purified by centrifugal elutriation. Five-day-old KC were infected with HIV1; at different intervals, the production of virus was quantitated by the reverse transcriptase activity associated with the particles present in the culture medium. In primary cultures of KC preincubated for 48 h and maintained in the presence of morphine, the production of viral particles was increased. This enhancing effect was found with 3 different HIV1 isolates. Treatment of KC with morphine prior to infection was not required for the stimulation to take place, which indicated that the enhancing effect was not related to a more efficient adsorption of the virus to the KC plasma membrane. Stimulation of HIV1 production was observed for all the concentrations of morphine used (0.05 to 0.5 mg/ml). These results, if confirmed in vivo, may shed new light on the risk factors related to the intravenous administration of heroin.

Genetic analysis of the enterobactin gene cluster in Shigella flexneri

The genes for transport and synthesis of the phenolate siderophore enterobactin are present on the chromosomes of both Ent+ and Ent- clinical isolates of Shigella flexneri. To determine why Ent- S. flexneri isolates fail to express a functional enterobactin system, the structure and expression of enterobactin genes were examined. Several alterations may be responsible for the inability of S. flexneri to express enterobactin. (i) The mRNA levels produced from the entC and fepB genes were not derepressed in low-iron media. (ii) DNA sequence analysis of the entC-fepB intergenic region revealed an 83-bp noncontiguous deletion in the putative fepB leader sequence. The deleted sequences are in a region which would be capable of forming extensive stem-and-loop structures. (iii) An amber codon in the 5' portion of the entC gene was also detected. (iv) An IS1 element, previously mapped to the Ent- S. flexneri enterobactin gene cluster, was found to lie within a potential transcriptional termination sequence in the entF-fepE intergenic region. (v) A mutation responsible for the inactivation of the entF gene was mapped to the entF coding region by using entF hybrid gene fusions. (vi) A comparison of outer membrane profiles from an E. coli strain harboring the cloned fepA gene from either an Ent+ or Ent- Shigella isolate revealed that the Ent- FepA protein is present in the outer membrane but at greatly reduced levels than that of the Ent+ FepA protein. This observation, along with additional studies, suggests that the Ent- FepA may be defective in translation and/or translocation.

Interaction of cultured human Kupffer cells with HIV-infected CEM cells: an electron microscopic study

Evidence has accumulated indicating that macrophages could play a role in the physiopathology of AIDS. We recently demonstrated that cultured human liver macrophages, the so-called Kupffer cells (KC), are permissive for HIV. Their infection in vivo would lead these cells to constitute a target for the virus and a reservoir as well. Since they occupy a strategic position within the liver sinusoid, their opportunity to interact with blood-borne virus or already infected T lymphocytes may be very high. In the present study, we investigated the possibility for KC to be infected via HIV-infected CEM cells, a lymphoid cell line. Therefore, we cocultured both cell types for various times before fixing them for electron microscopy. Syncytia appeared within 20 h of infection as well as a large amount of virus particles. HIV in the way of budding was also easily observed. This has to be compared to the direct infection of KC with free virus which needs, at least, about 10 days to give the same results.

Permissivity of primary cultures of human Kupffer cells for HIV-1

Kupffer cells (liver macrophages) represent the largest reservoir of fixed macrophages in the body. Accordingly, we have undertaken a study to evaluate their susceptibility to human immunodeficiency virus type 1 (HIV-1). Five-day-old primary cultures of Kupffer cells (KC) were infected with HIV-1, and as the infection progressed, syncytia appeared. Within the cells, viral proteins were detected by immunofluorescence using monoclonal antibodies directed against gp120 and p24. Electron microscopic examinations revealed the presence of typical Lentivirinae particles. The particles released from KC in the extracellular medium showed reverse transcriptase activity and p24 antigen; they could infect lymphocytic cells and were neutralized by a HIV+ patient's serum or an anti-gp120 monoclonal antibody. Our results thus demonstrate that the interaction of HIV-1 with KC in vitro leads to a productive infection. They suggest that the KC may be involved in the pathogenesis of HIV-1 infection and may (i) participate in the transmission of the infection to the peripheral blood cells, (ii) play a role in the depletion of uninfected CD4+ cells.

Multiplication of human immunodeficiency virus in primary cultures of human Kupffer cells--possible role of liver macrophage infection in the physiopathology of AIDS

In primary cultures of Kupffer cells obtained from surgical biopsies of human liver by collagenase perfusion followed by centrifugal elutriation and infected with HIV, the virus multiplied abundantly, as attested by the appearance of a reverse transcriptase activity in the medium. Examined by electron microscopy, the cells were found to contain viral particles with typical features of Lentivirinae. Furthermore, the virus could be revealed by immunofluorescence using an HIV+ patient serum. HIV antibodies also neutralized the infectivity of the Kupffer cell-produced virus. Our results demonstrate that the cells constituting the largest fraction of fixed macrophages in the body may be infected by HIV, thereby suggesting that the Kupffer cells may play a role in the physiopathology of the disease, namely as a reservoir for the virus.

Sequence of a conditionally essential region of bacteriophage T3, including the primary origin of DNA replication

The 3526 base-pair nucleotide sequence from near the end of bacteriophage T3 gene 1 to within the coding sequence of gene 2.5 is given. It includes the complete coding sequences for nine known or presumptive proteins, most of which are only conditionally essential for phage growth. The sequence includes five promoters for the phage RNA polymerase, the terminator for early (host enzyme-catalyzed) transcription, and two recognition sites for RNAase III. The primary origin of T3 DNA replication that is utilized by the phage in vivo has been localized to a 142 base-pair region. It has several features in common with the phage T7 origin of DNA replication, and exhibits considerable homology to recognition sites for the mRNA processing enzyme RNAase III. It is proposed that the primary origin of T3 DNA replication may have evolved directly from an RNAase III recognition site. The deletions present in a number of T3 mutant strains and the location of the nucleotide changes in several T3 strains that are defective in their ability to grow on F+-containing strains or on optA mutant hosts have been determined. We discuss how T3 may have become genetically isolated from its relatives in the T7-T3 group and simultaneously acquired novel biological and biochemical properties.