Structural basis for heme detoxification by an ATP-binding cassette-type efflux pump in gram-positive pathogenic bacteria.

Bacterial pathogens acquire heme from the host hemoglobin as an iron nutrient for their virulence and proliferation in blood. Concurrently, they encounter cytotoxic-free heme that escapes the heme-acquisition process. To overcome this toxicity, many gram-positive bacteria employ an ATP-binding cassette heme-dedicated efflux pump, HrtBA in the cytoplasmic membranes. Although genetic analyses have suggested that HrtBA expels heme from the bacterial membranes, the molecular mechanism of heme efflux remains elusive due to the lack of protein studies. Here, we show the biochemical properties and crystal structures of Corynebacterium diphtheriae HrtBA, alone and in complex with heme or an ATP analog, and we reveal how HrtBA extracts heme from the membrane and releases it. HrtBA consists of two cytoplasmic HrtA ATPase subunits and two transmembrane HrtB permease subunits. A heme-binding site is formed in the HrtB dimer and is laterally accessible to heme in the outer leaflet of the membrane. The heme-binding site captures heme from the membrane using a glutamate residue of either subunit as an axial ligand and sequesters the heme within the rearranged transmembrane helix bundle. By ATP-driven HrtA dimerization, the heme-binding site is squeezed to extrude the bound heme. The mechanism sheds light on the detoxification of membrane-bound heme in this bacterium.

Kinetics and Optimization of the Lysine-Isopeptide Bond Forming Sortase Enzyme from Corynebacterium diphtheriae.

Site-specifically modified protein bioconjugates have important applications in biology, chemistry, and medicine. Functionalizing specific protein side chains with enzymes using mild reaction conditions is of significant interest, but remains challenging. Recently, the lysine-isopeptide bond forming activity of the sortase enzyme that builds surface pili in Corynebacterium diphtheriae (CdSrtA) has been reconstituted in vitro. A mutationally activated form of CdSrtA was shown to be a promising bioconjugating enzyme that can attach Leu-Pro-Leu-Thr-Gly peptide fluorophores to a specific lysine residue within the N-terminal domain of the SpaA protein (NSpaA), enabling the labeling of target proteins that are fused to NSpaA. Here we present a detailed analysis of the CdSrtA catalyzed protein labeling reaction. We show that the first step in catalysis is rate limiting, which is the formation of the CdSrtA-peptide thioacyl intermediate that subsequently reacts with a lysine ε-amine in NSpaA. This intermediate is surprisingly stable, limiting spurious proteolysis of the peptide substrate. We report the discovery of a new enzyme variant (CdSrtAΔ) that has significantly improved transpeptidation activity, because it completely lacks an inhibitory polypeptide appendage ("lid") that normally masks the active site. We show that the presence of the lid primarily impairs formation of the thioacyl intermediate and not the recognition of the NSpaA substrate. Quantitative measurements reveal that CdSrtAΔ generates its cross-linked product with a catalytic turnover number of 1.4 ± 0.004 h-1 and that it has apparent KM values of 0.16 ± 0.04 and 1.6 ± 0.3 mM for its NSpaA and peptide substrates, respectively. CdSrtAΔ is 7-fold more active than previously studied variants, labeling >90% of NSpaA with peptide within 6 h. The results of this study further improve the utility of CdSrtA as a protein labeling tool and provide insight into the enzyme catalyzed reaction that underpins protein labeling and pilus biogenesis.

Methionine sulfoxide reductase B from Corynebacterium diphtheriae catalyzes sulfoxide reduction via an intramolecular disulfide cascade.

Corynebacterium diphtheriae is a human pathogen that causes diphtheria. In response to immune system-induced oxidative stress, C. diphtheriae expresses antioxidant enzymes, among which are methionine sulfoxide reductase (Msr) enzymes, which are critical for bacterial survival in the face of oxidative stress. Although some aspects of the catalytic mechanism of the Msr enzymes have been reported, several details still await full elucidation. Here, we solved the solution structure of C. diphtheriae MsrB (Cd-MsrB) and unraveled its catalytic and oxidation-protection mechanisms. Cd-MsrB catalyzes methionine sulfoxide reduction involving three redox-active cysteines. Using NMR heteronuclear single-quantum coherence spectra, kinetics, biochemical assays, and MS analyses, we show that the conserved nucleophilic residue Cys-122 is S-sulfenylated after substrate reduction, which is then resolved by a conserved cysteine, Cys-66, or by the nonconserved residue Cys-127. We noted that the overall structural changes during the disulfide cascade expose the Cys-122-Cys-66 disulfide to recycling through thioredoxin. In the presence of hydrogen peroxide, Cd-MsrB formed reversible intra- and intermolecular disulfides without losing its Cys-coordinated Zn2+, and only the nonconserved Cys-127 reacted with the low-molecular-weight (LMW) thiol mycothiol, protecting it from overoxidation. In summary, our structure-function analyses reveal critical details of the Cd-MsrB catalytic mechanism, including a major structural rearrangement that primes the Cys-122-Cys-66 disulfide for thioredoxin reduction and a reversible protection against excessive oxidation of the catalytic cysteines in Cd-MsrB through intra- and intermolecular disulfide formation and S-mycothiolation.

Protein Labeling via a Specific Lysine-Isopeptide Bond Using the Pilin Polymerizing Sortase from Corynebacterium diphtheriae.

Proteins that are site-specifically modified with peptides and chemicals can be used as novel therapeutics, imaging tools, diagnostic reagents and materials. However, there are few enzyme-catalyzed methods currently available to selectively conjugate peptides to internal sites within proteins. Here we show that a pilus-specific sortase enzyme from Corynebacterium diphtheriae (CdSrtA) can be used to attach a peptide to a protein via a specific lysine-isopeptide bond. Using rational mutagenesis we created CdSrtA3M, a highly activated cysteine transpeptidase that catalyzes in vitro isopeptide bond formation. CdSrtA3M mediates bioconjugation to a specific lysine residue within a fused domain derived from the corynebacterial SpaA protein. Peptide modification yields greater than >95% can be achieved. We demonstrate that CdSrtA3M can be used in concert with the Staphylococcus aureus SrtA enzyme, enabling dual, orthogonal protein labeling via lysine-isopeptide and backbone-peptide bonds.

Structural Basis of a Thiol-Disulfide Oxidoreductase in the Hedgehog-Forming Actinobacterium Corynebacterium matruchotii.

The actinobacterium Corynebacterium matruchotii has been implicated in nucleation of oral microbial consortia leading to biofilm formation. Due to the lack of genetic tools, little is known about basic cellular processes, including protein secretion and folding, in this organism. We report here a survey of the C. matruchotii genome, which encodes a large number of exported proteins containing paired cysteine residues, and identified an oxidoreductase that is highly homologous to the Corynebacterium diphtheriae thiol-disulfide oxidoreductase MdbA (MdbACd). Crystallization studies uncovered that the 1.2-Å resolution structure of C. matruchotii MdbA (MdbACm) possesses two conserved features found in actinobacterial MdbA enzymes, a thioredoxin-like fold and an extended α-helical domain. By reconstituting the disulfide bond-forming machine in vitro, we demonstrated that MdbACm catalyzes disulfide bond formation within the actinobacterial pilin FimA. A new gene deletion method supported that mdbA is essential in C. matruchotii Remarkably, heterologous expression of MdbACm in the C. diphtheriae ΔmdbA mutant rescued its known defects in cell growth and morphology, toxin production, and pilus assembly, and this thiol-disulfide oxidoreductase activity required the catalytic motif CXXC. Altogether, the results suggest that MdbACm is a major thiol-disulfide oxidoreductase, which likely mediates posttranslocational protein folding in C. matruchotii by a mechanism that is conserved in ActinobacteriaIMPORTANCE The actinobacterium Corynebacterium matruchotii has been implicated in the development of oral biofilms or dental plaque; however, little is known about the basic cellular processes in this organism. We report here a high-resolution structure of a C. matruchotii oxidoreductase that is highly homologous to the Corynebacterium diphtheriae thiol-disulfide oxidoreductase MdbA. By biochemical analysis, we demonstrated that C. matruchotii MdbA catalyzes disulfide bond formation in vitro Furthermore, a new gene deletion method revealed that deletion of mdbA is lethal in C. matruchotii Remarkably, C. matruchotii MdbA can replace C. diphtheriae MdbA to maintain normal cell growth and morphology, toxin production, and pilus assembly. Overall, our studies support the hypothesis that C. matruchotii utilizes MdbA as a major oxidoreductase to catalyze oxidative protein folding.

The glyceraldehyde-3-phosphate dehydrogenase GapDH of Corynebacterium diphtheriae is redox-controlled by protein S-mycothiolation under oxidative stress.

Mycothiol (MSH) is the major low molecular weight (LMW) thiol in Actinomycetes and functions in post-translational thiol-modification by protein S-mycothiolation as emerging thiol-protection and redox-regulatory mechanism. Here, we have used shotgun-proteomics to identify 26 S-mycothiolated proteins in the pathogen Corynebacterium diphtheriae DSM43989 under hypochlorite stress that are involved in energy metabolism, amino acid and nucleotide biosynthesis, antioxidant functions and translation. The glyceraldehyde-3-phosphate dehydrogenase (GapDH) represents the most abundant S-mycothiolated protein that was modified at its active site Cys153 in vivo. Exposure of purified GapDH to H2O2 and NaOCl resulted in irreversible inactivation due to overoxidation of the active site in vitro. Treatment of GapDH with H2O2 or NaOCl in the presence of MSH resulted in S-mycothiolation and reversible GapDH inactivation in vitro which was faster compared to the overoxidation pathway. Reactivation of S-mycothiolated GapDH could be catalyzed by both, the Trx and the Mrx1 pathways in vitro, but demycothiolation by Mrx1 was faster compared to Trx. In summary, we show here that S-mycothiolation can function in redox-regulation and protection of the GapDH active site against overoxidation in C. diphtheriae which can be reversed by both, the Mrx1 and Trx pathways.

A thiol-disulfide oxidoreductase of the Gram-positive pathogen Corynebacterium diphtheriae is essential for viability, pilus assembly, toxin production and virulence.

The Gram-positive pathogen Corynebacterium diphtheriae exports through the Sec apparatus many extracellular proteins that include the key virulence factors diphtheria toxin and the adhesive pili. How these proteins attain their native conformations after translocation as unfolded precursors remains elusive. The fact that the majority of these exported proteins contain multiple cysteine residues and that several membrane-bound oxidoreductases are encoded in the corynebacterial genome suggests the existence of an oxidative protein-folding pathway in this organism. Here we show that the shaft pilin SpaA harbors a disulfide bond in vivo and alanine substitution of these cysteines abrogates SpaA polymerization and leads to the secretion of degraded SpaA peptides. We then identified a thiol-disulfide oxidoreductase (MdbA), whose structure exhibits a conserved thioredoxin-like domain with a CPHC active site. Remarkably, deletion of mdbA results in a severe temperature-sensitive cell division phenotype. This mutant also fails to assemble pilus structures and is greatly defective in toxin production. Consistent with these defects, the ΔmdbA mutant is attenuated in a guinea pig model of diphtheritic toxemia. Given its diverse cellular functions in cell division, pilus assembly and toxin production, we propose that MdbA is a component of the general oxidative folding machine in C. diphtheriae.

Corynebacterium diphtheriae methionine sulfoxide reductase a exploits a unique mycothiol redox relay mechanism.

Methionine sulfoxide reductases are conserved enzymes that reduce oxidized methionines in proteins and play a pivotal role in cellular redox signaling. We have unraveled the redox relay mechanisms of methionine sulfoxide reductase A of the pathogen Corynebacterium diphtheriae (Cd-MsrA) and shown that this enzyme is coupled to two independent redox relay pathways. Steady-state kinetics combined with mass spectrometry of Cd-MsrA mutants give a view of the essential cysteine residues for catalysis. Cd-MsrA combines a nucleophilic cysteine sulfenylation reaction with an intramolecular disulfide bond cascade linked to the thioredoxin pathway. Within this cascade, the oxidative equivalents are transferred to the surface of the protein while releasing the reduced substrate. Alternatively, MsrA catalyzes methionine sulfoxide reduction linked to the mycothiol/mycoredoxin-1 pathway. After the nucleophilic cysteine sulfenylation reaction, MsrA forms a mixed disulfide with mycothiol, which is transferred via a thiol disulfide relay mechanism to a second cysteine for reduction by mycoredoxin-1. With x-ray crystallography, we visualize two essential intermediates of the thioredoxin relay mechanism and a cacodylate molecule mimicking the substrate interactions in the active site. The interplay of both redox pathways in redox signaling regulation forms the basis for further research into the oxidative stress response of this pathogen.

Role of active-site residues Tyr55 and Tyr114 in catalysis and substrate specificity of Corynebacterium diphtheriae C-S lyase.

In recent years, there has been increased interest in bacterial methionine biosynthesis enzymes as antimicrobial targets because of their pivotal role in cell metabolism. C-S lyase from Corynebacterium diphtheriae is a pyridoxal 5'-phosphate-dependent enzyme in the transsulfuration pathway that catalyzes the α,ß-elimination of sulfur-containing amino acids, such as L-cystathionine, to generate ammonia, pyruvate, and homocysteine, the immediate precursor of L-methionine. In order to gain deeper insight into the functional and dynamic properties of the enzyme, mutants of two highly conserved active-site residues, Y55F and Y114F, were characterized by UV-visible absorbance, fluorescence, and CD spectroscopy in the absence and presence of substrates and substrate analogs, as well as by steady-state kinetic studies. Substitution of Tyr55 with Phe apparently causes a 130-fold decrease in K(d)(PLP) at pH 8.5 providing evidence that Tyr55 plays a role in cofactor binding. Moreover, spectral data show that the mutant accumulates the external aldimine intermediate suggesting that the absence of interaction between the hydroxyl moiety and PLP-binding residue Lys222 causes a decrease in the rate of substrate deprotonation. Mutation of Tyr114 with Phe slightly influences hydrolysis of L-cystathionine, and causes a change in substrate specificity towards L-serine and O-acetyl-L-serine compared to the wild type enzyme. These findings, together with computational data, provide useful insights in the substrate specificity of C-S lyase, which seems to be regulated by active-site architecture and by the specific conformation in which substrates are bound, and will aid in development of inhibitors.

New diphtheria toxin repressor types depicted in a Romanian collection of Corynebacterium diphtheriae isolates.

Corynebacterium diphtheriae is the etiological agent of diphtheria, a potential fatal disease caused by a corynephage toxin. The expression of this diphtheria toxin is controlled via an iron-dependent repressor with various functions (DtxR). Some mutations in the dtxR gene are associated with diminished activity or even with total loss of DtxR function. We conducted a molecular study to characterize the dtxR alleles harbored by 34 isolates of C. diphtheriae recovered from Romanian patients between 1961 and 2007. Three of the seven alleles identified in this study have not previously been described. Two new DtxR types were identified, one of which has an unusual polypeptide length. All the new DtxR types were found in toxigenic isolates, suggesting that they effectively regulate the expression of diphtheria toxin. Furthermore, one of the new DtxR identified was also found in a non-toxigenic isolate, making it a potential source of toxigenic isolates after lysogenic conversion.

Characterization of C-S Lyase from C. diphtheriae: a possible target for new antimicrobial drugs.

The emergence of antibiotic resistance in microbial pathogens requires the identification of new antibacterial drugs. The biosynthesis of methionine is an attractive target because of its central importance in cellular metabolism. Moreover, most of the steps in methionine biosynthesis pathway are absent in mammals, lowering the probability of unwanted side effects. Herein, detailed biochemical characterization of one enzyme required for methionine biosynthesis, a pyridoxal-5'-phosphate (PLP-) dependent C-S lyase from Corynebacterium diphtheriae, a pathogenic bacterium that causes diphtheria, has been performed. We overexpressed the protein in E. coli and analyzed substrate specificity, pH dependence of steady state kinetic parameters, and ligand-induced spectral transitions of the protein. Structural comparison of the enzyme with cystalysin from Treponema denticola indicates a similarity in overall folding. We used site-directed mutagenesis to highlight the importance of active site residues Tyr55, Tyr114, and Arg351, analyzing the effects of amino acid replacement on catalytic properties of enzyme. Better understanding of the active site of C. diphtheriae C-S lyase and the determinants of substrate and reaction specificity from this work will facilitate the design of novel inhibitors as antibacterial therapeutics.

Cell surface display of minor pilin adhesins in the form of a simple heterodimeric assembly in Corynebacterium diphtheriae.

Pilus assembly in Gram-positive bacteria occurs by a two-step mechanism, whereby pilins are polymerized and then covalently anchored to the cell wall. In Corynebacterium diphtheriae, the pilin-specific sortase SrtA catalyses polymerization of the SpaA-type pilus, consisting of the shaft pilin SpaA, tip pilin SpaC and minor pilin SpaB. Cell wall anchoring of the SpaA polymers is triggered when SrtA incorporates SpaB into the pilus base via lysine-mediated transpeptidation; anchoring to the cell wall peptidoglycan is subsequently catalysed by the housekeeping sortase SrtF. Here we show that SpaB and SpaC formed a heterodimer independent of SpaA polymerization. SrtA was absolutely required for the formation of the SpaBC heterodimer, while SrtF facilitated the optimal cell wall anchoring of this heterodimer. Alanine substitution of the SpaB lysine residue K139 or truncation of the SpaB cell wall-sorting signal (CWSS) abolished assembly of the SpaBC heterodimer, hence underscoring SpaB function in transpeptidation and cell wall linkage. Importantly, sortase specificity for the cell wall-anchoring step was found to be dependent on the LAFTG motif within the SpaB CWSS. Thus, C. diphtheriae employs a common sortase-catalysed mechanism involving lysine-mediated transpeptidation to generate both adhesive pilus and simple heterodimeric structures on the bacterial the cell wall.

Acyl enzyme intermediates in sortase-catalyzed pilus morphogenesis in gram-positive bacteria.

In gram-positive bacteria, covalently linked pilus polymers are assembled by a specific transpeptidase enzyme called pilus-specific sortase. This sortase is postulated to cleave the LPXTG motif of a pilin precursor between threonine and glycine and to form an acyl enzyme intermediate with the substrate. Pilus polymerization is believed to occur through the resolution of this intermediate upon specific nucleophilic attack by the conserved lysine located within the pilin motif of another pilin monomer, which joins two pilins with an isopeptide bond formed between threonine and lysine. Here, we present evidence for sortase reaction intermediates in Corynebacterium diphtheriae. We show that truncated SrtA mutants that are loosely bound to the cytoplasmic membrane form high-molecular-weight complexes with SpaA polymers secreted into the extracellular milieu. These complexes are not formed with SpaA pilin mutants that have alanine substitutions in place of threonine in the LPXTG motif or lysine in the pilin motif. The same phenotype is observed with alanine substitutions of either the conserved cysteine or histidine residue of SrtA known to be required for catalysis. Remarkably, the assembly of SpaA pili, or the formation of intermediates, is abolished with a SrtA mutant missing the membrane-anchoring domain. We infer that pilus polymerization involves the formation of covalent pilin-sortase intermediates, which occurs within a molecular platform on the exoplasmic face of the cytoplasmic membrane that brings together both sortase and its cognate substrates in close proximity to each other, likely surrounding a secretion apparatus. We present electron microscopic data in support of this picture.

DNase test as a novel approach for the routine screening of Corynebacterium diphtheriae.

AIMS: To examine the value of the DNase test as an alternative procedure for differentiating Corynebacterium diphtheriae from Corynebacterium-like colonies. METHODS AND RESULTS: DNase test medium was inoculated by spotting a loopful of bacterial growth and incubated aerobically at 37 degrees C. The DNase production was detectable following both 24 and 48 h incubation periods. The DNase activity was detected in all 91 C. diphtheriae (37 toxigenic and 54 nontoxigenic) strains examined, previously identified by both conventional biochemical methods and API Coryne System. Conversely, DNase test results were negative in 93.9% of the 564 nondiphtherial Gram-positive rod clinical strains. CONCLUSIONS: The DNase test emerged as an easily interpretable and cost-effective alternative screening procedure for C. diphtheriae laboratory identification. SIGNIFICANCE AND IMPACT OF THE STUDY: The method should facilitate routine laboratory diagnosis of toxigenic and nontoxigenic C. diphtheriae.

Housekeeping sortase facilitates the cell wall anchoring of pilus polymers in Corynebacterium diphtheriae.

Many surface proteins in Gram-positive bacteria are covalently linked to the cell wall through a transpeptidation reaction catalysed by the enzyme sortase. Corynebacterium diphtheriae encodes six sortases, five of which are devoted to the assembly of three distinct types of pilus fibres--SrtA for the SpaA-type pilus, SrtB/SrtC for the SpaD-type pilus, and SrtD/SrtE for the SpaH-type pilus. We demonstrate here the function of SrtF, the so-called housekeeping sortase, in the cell wall anchoring of pili. We show that a multiple deletion mutant strain expressing only SrtA secretes a large portion of SpaA polymers into the culture medium, with concomitant decrease in the cell wall-linked pili. The same phenotype is observed with the mutant that is missing SrtF alone. By contrast, a strain that expresses only SrtF displays surface-linked pilins but no polymers. Therefore, SrtF can catalyse the cell wall anchoring of pilin monomers as well as pili, but it does not polymerize pilins. We show that SrtA and SrtF together generate wild-type levels of the SpaA-type pilus on the bacterial surface. Furthermore, by regulating the expression of SpaA in the cell, we demonstrate that the SrtF function becomes critical when the SpaA level is sufficiently high. Together, these findings provide key evidence for a two-stage model of pilus assembly: pilins are first polymerized by a pilus-specific sortase, and the resulting fibre is then attached to the cell wall by either the cognate sortase or the housekeeping sortase.

Analysis of secreted protein profile and enzymatic activities from Corynebacterium diphtheriae and Bordetella pertussis on production batch media using peptide quenched fluorescent substrates.

Proteases were identified and characterized from the culture supernatant of the C. diphtheriae and B. pertussis bacteria. The proteases were secreted in the media and detected at the end of the exponential growth phase. Activity was detected in some fluorescent substrates, based on selected protein sequences such as insuline beta-chain, bradykinin, and synaptobrevin. The proteases were purified by means of gel filtration chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the purified proteins indicated, for the main secreted proteins, an estimated molecular mass of 30 kDa in C. diphtheriae and 69 kDa in B. pertussis culture media. The proteases were stable and presented enzymatic activity at 37 degrees C. These proteases were not related to the main toxic compounds described in these two bacteria, but could represent good markers for the fermentation process when the enzyme activity was measured with the fluorescent substrates.

Diatomic ligand discrimination by the heme oxygenases from Neisseria meningitidis and Pseudomonas aeruginosa.

Heme oxygenases have an increased binding affinity for O2 relative to CO. Such discrimination is critical to the function of HO enzymes because one of the main products of heme catabolism is CO. Kinetic studies of mammalian and bacterial HO proteins reveal a significant decrease in the dissociation rate of O2 relative to other heme proteins such as myoglobin. Here we report the kinetic rate constants for the binding of O2 and CO by the heme oxygenases from Neisseria meningitidis (nmHO) and Pseudomonas aeruginosa (paHO). A combination of stopped-flow kinetic and laser flash photolysis experiments reveal that nmHO and paHO both maintain a similar degree of ligand discrimination as mammalian HO-1 and the HO from Corynebacterium diphtheriae. However, in addition to the observed decrease in dissociation rate for O2 by both nmHO and paHO, kinetic analyses show an increase in dissociation rate for CO by these two enzymes. The crystal structures of nmHO and paHO both contain significant differences from the mammalian HO-1 and bacterial C. diphtheriae HO structures, which suggests a structural basis for ligand discrimination in nmHO and paHO.

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.

Use of synthetic peptides and site-specific antibodies to localize a diphtheria toxin sequence associated with ADP-ribosyltransferase activity.

Diphtheria toxin (DT) and Pseudomonas aeruginosa exotoxin A have the same molecular mechanism of toxicity; both toxins ADP-ribosylate a modified histidine residue in elongation factor 2. To help identify amino acids involved in this reaction, sequences in DT that share homology with P. aeruginosa exotoxin A were synthesized and examined for a role in the ADP-ribosyltransferase reaction. By using this approach, residues 32 to 54 of DT were found to define an epitope associated with antibody-mediated inhibition of DT enzyme activity. This lends further support to the notion that residues in this region of DT are involved in the enzymatic reaction.