Synthesis and properties of delta-N-(phosphonacetyl)-L-ornithine. A transition-state analog inhibitor of ornithine transcarbamylase

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.

N5-phosphonoacetyl-L-ornithine (PALO): a convenient synthesis and investigation of influence on regulation of amino acid biosynthetic genes in Saccharomyces cerevisiae

A scalable four-step synthesis of the ornithine transcarbamylase inhibitor N(5)-phosphonoacetyl-l-ornithine (PALO) is achieved through boroxazolidinone protection of ornithine. Investigations in the model organism Saccharomyces cerevisiae found that, in contrast to a previous report, PALO did not influence growth rate or expression of genes involved in arginine metabolism.

Mechanism of inactivation of ornithine transcarbamoylase by Ndelta -(N'-Sulfodiaminophosphinyl)-L-ornithine, a true transition state analogue? Crystal structure and implications for catalytic mechanism

The crystal structure is reported at 1.8 A resolution of Escherichia coli ornithine transcarbamoylase in complex with the active derivative of phaseolotoxin from Pseudomonas syringae pv. phaseolicola, N(delta)-(N'-sulfodiaminophosphinyl)-l-ornithine. Electron density reveals that the complex is not a covalent adduct as previously thought. Kinetic data confirm that N(delta)-(N'-sulfodiaminophosphinyl)-l-ornithine exhibits reversible inhibition with a half-life in the order of approximately 22 h and a dissociation constant of K(D) = 1.6 x 10(-12) m at 37 degrees C and pH 8.0. Observed hydrogen bonding about the chiral tetrahedral phosphorus of the inhibitor is consistent only with the presence of the R enantiomer. A strong interaction is also observed between Arg(57) Nepsilon and the P-N-S bridging nitrogen indicating that imino tautomers of N(delta)-(N'-sulfodiaminophosphinyl)-l-ornithine are present in the bound state. An imino tautomer of N(delta)-(N'-sulfodiaminophosphinyl)-l-ornithine is structurally analogous to the proposed reaction transition state. Hence, we propose that N(delta)-(N'-sulfodiaminophosphinyl)-l-ornithine, with its three unique N-P bonds, represents a true transition state analogue for ornithine transcarbamoylases, consistent with the tight binding kinetics observed.

alpha-Functionalized phosphonylphosphinates: synthesis and evaluation as transcarbamoylase inhibitors

Diverse alpha-methyl-substituted phosphonylphosphinates (P-C-P-C-X) are accessible from a protected, pentafluorophenylsulfonated phosphonylphosphinate via nucleophilic displacement. The utility of this route is demonstrated with several nitrogen nucleophiles. The resulting amine and amino acid phosphonylphosphinate derivatives were evaluated as inhibitors of Streptococcus faecalis ornithine transcarbamoylase (OTC). Compared with the structurally related phosphonoacetyl-L-ornithine (L-PALO), a known inhibitor of OTCs from various sources, the phosphonylphosphinates are surprisingly poor inhibitors, binding several orders of magnitude less tightly to the enzyme. These results suggest that the tetrahedral intermediate formed in the normal transcarbamoylase reaction is poorly mimicked by a tetrahedral and anionic phosphonate, either because of directly unfavorable interactions with a hydrogen-bond acceptor within the active site or because transition-state analogues are unable to induce the protein conformation changes that normally accompany reaction.

Structural similarity between ornithine and aspartate transcarbamoylases of Escherichia coli: characterization of the active site and evidence for an interdomain carboxy-terminal helix in ornithine transcarbamoylase

Predictions of tertiary structures of proteins from their amino acid sequences are facilitated greatly when the structures of homologous proteins are known. On this basis, structural features of Escherichia coli ornithine transcarbamoylase (OTCase) were investigated by site-directed mutagenesis experiments based on the known tertiary structure of the catalytic (c) chain of E. coli aspartate transcarbamoylase (ATCase). In ATCase, each c chain is composed of two globular domains connected by two interdomain helices, one of which is near the C-terminus and is critical for the in vivo folding of the chains and their assembly into trimers. Each active site is located at the interface between two chains and requires the participation of residues from each of the adjacent chains. OTCase, a trimeric enzyme, has been proposed to be similar in structure to the ATCase trimer on the basis of sequence identity (32%), the nature of the reaction catalyzed by the enzyme, and secondary structure predictions. As shown here, analysis of OTCase and ATCase sequences revealed extensive evolutionary conservation in portions corresponding to the ATCase active site and the C-terminal helix. Truncations and substitutions within the predicted C-terminal helix of OTCase had effects on activity and thermal stability strikingly similar to those caused by analogous alterations in ATCase. Similarly, substitutions at either of two conserved residues, Ser 55 and Lys 86, in the proposed active site of OTCase had deleterious effects parallel to those caused by the analogous ATCase substitutions. Hybrid trimers comprised of chains from both these relatively inactive OTCase mutants exhibited dramatically increased activity, as predicted for shared active sites located at the chain interfaces. These results strongly support the hypothesis that the tertiary and quaternary structures of the two enzymes are similar.

Kinetic properties of carbamoyl-phosphate synthase (ammonia) and ornithine carbamoyltransferase in permeabilized mitochondria

Previous studies using intact rat liver mitochondria have shown that the soluble matrix enzymes carbamoyl-phosphate synthase (ammonia) (CPS) and ornithine carbamoyltransferase (OCT) display some kinetic properties which would not be observed if they were homogeneously distributed in the matrix. In the present work we have extended these studies, using toluene-treated mitochondria which are fully permeable to substrates and inhibitors, yet retain 90% of their soluble enzymes. The results provide evidence of functional organization of CPS and OCT in situ. The major findings are as follows. (1) The apparent Km values of matrix OCT for carbamoyl phosphate and ornithine are respectively 8 and 2 times those measured for the soluble enzyme. delta-N-Phosphonacetyl-L-ornithine inhibits OCT in situ less than in solution, especially when carbamoyl phosphate is synthesized in the mitochondria rather than added to the medium. (2) During citrulline synthesis from endogenously generated carbamoyl phosphate, the concentration of the latter in permeabilized mitochondria is more than 10 times that in the medium, although the mitochondria are freely permeable to added molecules of this size. (3) Endogenously formed carbamoyl phosphate is used preferentially by OCT in situ; addition of a 200-fold excess of unlabelled carbamoyl phosphate has little effect on the conversion of labelled endogenously formed carbamoyl phosphate into citrulline by matrix OCT. (4) The synthesis de novo of carbamoyl phosphate from NH3, HCO3- and ATPMg is the same in the presence and absence of ornithine. (5) Studies with co-immobilized CPS and OCT gave results concordant with some of the above observations and with previous ones with intact mitochondria.

High-level expression of a mitochondrial enzyme, ornithine transcarbamylase from rat liver, in a baculovirus expression system

The mitochondrial enzyme, ornithine transcarbamylase (OTC) from rat liver was expressed in Spodoptera frugiperda (Sf) insect cells using a baculovirus vector. When insect cells were infected with recombinant Autographica californica nuclear polyhedrosis virus (AcNPV) containing a cDNA encoding the precursor form of OTC (pOTC) inserted into the polyhedrin gene, they expressed catalytically active enzyme at levels of approximately 2.5 micrograms/10(6) cells. About 25% of the active enzyme was a novel, partially processed product of pOTC containing four extra amino acids at the amino terminus of OTC. The most abundant protein found in mitochondria from infected insect cells was the normal processing intermediate iOTC, which contains 8 extra amino acids at the amino terminus of OTC. Whereas this species, present at 20 micrograms/10(6) cells, was not active and did not bind the transition-state analog inhibitor of OTC, delta-PALO, the novel processing product did bind and was affinity-purified, along with mature OTC, on a PALO-affinity column. The OTC expressed in insect cells was located in the same compartment of the mitochondrion as in rat liver. The incomplete processing occurred in vitro in both noninfected and infected insect cells. The high level of expression of iOTC using the baculoviral expression system provides a means of overproducing an obligatory intermediate in the mitochondrial import process.

Complementation of the Aspergillus nidulans arg B1 mutation by ornithine transcarbamylase cDNA from rat liver

An Aspergillus nidulans strain which is deficient in ornithine transcarbamylase due to the arg B1 mutation was transformed with a plasmid containing the ornithine transcarbamylase cDNA from rat liver under the control of the amd S promoter. Stable transformants were obtained by selection on arginine free medium indicating complementation of the arg B mutation. Proof of expression of the rat enzyme in transformants was obtained by immunoprecipitation of all ornithine transcarbamylase activity from cell extracts with antibodies specific for the rat enzyme. The presence of catalytically active rat ornithine transcarbamylase in the transformants indicated that it is capable of being imported into mitochondria in A. nidulans, proteolytically processed and assembled into its homotrimeric form. In vitro uptake experiments using isolated A. nidulans mitochondria demonstrate that processing of the precursor of rat ornithine transcarbamylase occurs in two temporally separated steps as it does in rat liver mitochondria suggesting evolutionary conservation of the processing machinery. Up to 560 ng of active rat enzyme was produced per gm wet weight mycelia. Use of beta-D-alanine, an inducer of amd S, as sole N-source resulted in increased levels of active rat ornithine transcarbamylase relative to uninduced cultures.

Arginine restriction induced by delta-N-(phosphonacetyl)-L-ornithine signals increased expression of HIS3, TRP5, CPA1, and CPA2 in Saccharomyces cerevisiae

delta-N-(Phosphonacetyl)-L-ornithine (PALO), a transition state analog inhibitor of ornithine transcarbamylase, induced arginine limitation in vivo in Saccharomyces cerevisiae. Arginine restriction caused increased expression of HIS3 and TRP5, measured by the beta-galactosidase activity in strains carrying chromosomally integrated fusions of the promoter regions of each gene with the lacZ gene of Escherichia coli. The increase in beta-galactosidase activity induced by PALO was reversed by the addition of arginine and was dependent on GCN4 protein. These results indicate that PALO, like 3-amino-1,2,4-triazole DL-5-methyltryptophan, can be used to study the effect of limitation of a single amino acid, arginine, on the expression of genes under the general amino acid control regulatory system. Arginine deprivation imposed by PALO also caused increased expression of CPA1 and CPA2, coding respectively for the small and large subunits of arginine-specific carbamyl-phosphate synthetase. The observed increase was GCN4 dependent and was genetically separable from arginine-specific repression of CPA1 mRNA translation. The 5'-flanking regions of CPA1 (reported previously) and CPA2 determined in this study each contained at least two copies of the sequence TGACTC, shown to bind GCN4 protein. The beta-galactosidase activities expressed from CPA1- and CPA2-lacZ fusions integrated into the nuclear DNA of gcn4 mutant strains were five to six times less than in the wild type, when both strains were grown under depressed conditions. The gcn4 mutation reduced basal expression of both CPA1 and CPA2. The addition of arginine to strains containing the CPA1-lacZ fusion further reduced beta-galactosidase activity of the gcn4 mutant, indicating independent regulation of the CPA1 gene by the general amino acid control and by arginine-specific repression. In strains overproducing GCN4 protein, the translational control completely overrode transcriptional activation of CPA1 by general amino acid control.

Arginine restriction induced by delta-N-(phosphonacetyl)-L-ornithine signals increased expression of HIS3, TRP5, CPA1, and CPA2 in Saccharomyces cerevisiae

delta-N-(Phosphonacetyl)-L-ornithine (PALO), a transition state analog inhibitor of ornithine transcarbamylase, induced arginine limitation in vivo in Saccharomyces cerevisiae. Arginine restriction caused increased expression of HIS3 and TRP5, measured by the beta-galactosidase activity in strains carrying chromosomally integrated fusions of the promoter regions of each gene with the lacZ gene of Escherichia coli. The increase in beta-galactosidase activity induced by PALO was reversed by the addition of arginine and was dependent on GCN4 protein. These results indicate that PALO, like 3-amino-1,2,4-triazole DL-5-methyltryptophan, can be used to study the effect of limitation of a single amino acid, arginine, on the expression of genes under the general amino acid control regulatory system. Arginine deprivation imposed by PALO also caused increased expression of CPA1 and CPA2, coding respectively for the small and large subunits of arginine-specific carbamyl-phosphate synthetase. The observed increase was GCN4 dependent and was genetically separable from arginine-specific repression of CPA1 mRNA translation. The 5'-flanking regions of CPA1 (reported previously) and CPA2 determined in this study each contained at least two copies of the sequence TGACTC, shown to bind GCN4 protein. The beta-galactosidase activities expressed from CPA1- and CPA2-lacZ fusions integrated into the nuclear DNA of gcn4 mutant strains were five to six times less than in the wild type, when both strains were grown under depressed conditions. The gcn4 mutation reduced basal expression of both CPA1 and CPA2. The addition of arginine to strains containing the CPA1-lacZ fusion further reduced beta-galactosidase activity of the gcn4 mutant, indicating independent regulation of the CPA1 gene by the general amino acid control and by arginine-specific repression. In strains overproducing GCN4 protein, the translational control completely overrode transcriptional activation of CPA1 by general amino acid control.

Mitochondrial import and processing of mutant human ornithine transcarbamylase precursors in cultured cells

We have investigated mitochondrial import and processing of the precursor for human ornithine transcarbamylase (OTC; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) in HeLa cells stably transformed with cDNA sequences encoding OTC precursors carrying mutations in their leader peptides. The mutant precursors studied included two with amino acid substitutions in the 32-amino-acid leader peptide (glycine for arginine at position 23, designated gly23; glycines for arginines at positions 15, 23, and 26, designated gly15,23,26) and two with deletions (deletion of residues 8 to 22, designated d8-22; deletion of residues 17 to 32, designated N16). Specific immunoprecipitation with anti-OTC antiserum of extracts of L-[35S]methionine-labeled cells expressing these mutations yielded only precursor species; neither mature nor intermediate-size OTC subunits were observed. Fractionation of radiolabeled cells, however, revealed important differences among the various mutants: the gly23 precursor was associated with mitochondria and was not detected in the cytosol; the d8-22 and N16 precursors were found with both the mitochondrial fraction and the cytosol; only the gly15,23,26 precursor was detected exclusively in the cytosol. A large fraction of each of the mitochondrially associated OTC species was in a trypsin-protected compartment. In particular, the gly23 precursor behaved in trypsin protection and mitochondrial fractionation studies in a manner consistent with its translocation into the mitochondrial matrix. On the other hand, the lack of binding of the gly23 protein to a delta-N-phosphonoacetyl-L-ornithine affinity column, which specifically recognizes active OTC enzyme, indicated that, despite its intramitochondrial location, the mutant protein did not assemble into the normal, active trimer. Further, the gly23 mutant precursor was unstable within the mitochondria and was degraded with a t1/2 of less further than 4 h. Thus, we have shown that, in intact HeLa cells, cleavage of the OTC leader peptide is not required for translocation into mitochondria, but is required for assembly into active enzyme.

Purification and properties of ornithine carbamoyl transferase from liver of Squalus acanthias

Citrulline synthesis from ammonia by hepatic mitochondria in elasmobranchs involves intermediate formation of glutamine as the result of the presence of high levels of glutamine synthetase and a unique glutamine- and N-acetyl-glutamate-dependent carbamoyl phosphate synthetase, both of which have properties unique to the function of glutamine-dependent synthesis of urea, which is retained in the tissues of elasmobranchs at high concentrations for the purpose of osmoregulation [P.M. Anderson and C.A. Casey (1984) J. Biol. Chem. 259, 456-462; R.A. Shankar and P.M. Anderson (1985) Arch. Biochem. Biophys. 239, 248-259]. The objective of this study was to determine if ornithine carbamoyl transferase, which catalyzes the last step of mitochondrial citrulline synthesis and which has not been previously isolated from any species of fish, also has properties uniquely related to this function. Ornithine carbamoyl transferase was highly purified from isolated liver mitochondria of Squalus acanthias, a representative elasmobranch. The purified enzyme is a trimer with a subunit molecular weight of 38,000 and a native molecular weight of about 114,000. The effect of pH is significantly influenced by ornithine concentration; optimal activity is at pH 7.8 when ornithine is saturating. The apparent Km values for ornithine and carbamoyl phosphate at pH 7.8 are 0.71 and 0.05 mM, respectively. Ornithine displays considerable substrate inhibition above pH 7.8. The activity is not significantly affected by physiological concentrations of the osmolyte urea or trimethylamine-N-oxide or by a number of other metabolites. The results of kinetic studies are consistent with a steady-state ordered addition of substrates (carbamoyl phosphate binding first) and rapid equilibrium random release of products. Except for an unusually low specific activity, the properties of the purified elasmobranch enzyme are similar to the properties of ornithine carbamoyl transferase from mammalian ureotelic and other species and do not appear to be unique to its role in glutamine-dependent synthesis of urea for the purpose of osmoregulation.

Mitochondrial import and processing of mutant human ornithine transcarbamylase precursors in cultured cells

We have investigated mitochondrial import and processing of the precursor for human ornithine transcarbamylase (OTC; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) in HeLa cells stably transformed with cDNA sequences encoding OTC precursors carrying mutations in their leader peptides. The mutant precursors studied included two with amino acid substitutions in the 32-amino-acid leader peptide (glycine for arginine at position 23, designated gly23; glycines for arginines at positions 15, 23, and 26, designated gly15,23,26) and two with deletions (deletion of residues 8 to 22, designated d8-22; deletion of residues 17 to 32, designated N16). Specific immunoprecipitation with anti-OTC antiserum of extracts of L-[35S]methionine-labeled cells expressing these mutations yielded only precursor species; neither mature nor intermediate-size OTC subunits were observed. Fractionation of radiolabeled cells, however, revealed important differences among the various mutants: the gly23 precursor was associated with mitochondria and was not detected in the cytosol; the d8-22 and N16 precursors were found with both the mitochondrial fraction and the cytosol; only the gly15,23,26 precursor was detected exclusively in the cytosol. A large fraction of each of the mitochondrially associated OTC species was in a trypsin-protected compartment. In particular, the gly23 precursor behaved in trypsin protection and mitochondrial fractionation studies in a manner consistent with its translocation into the mitochondrial matrix. On the other hand, the lack of binding of the gly23 protein to a delta-N-phosphonoacetyl-L-ornithine affinity column, which specifically recognizes active OTC enzyme, indicated that, despite its intramitochondrial location, the mutant protein did not assemble into the normal, active trimer. Further, the gly23 mutant precursor was unstable within the mitochondria and was degraded with a t1/2 of less further than 4 h. Thus, we have shown that, in intact HeLa cells, cleavage of the OTC leader peptide is not required for translocation into mitochondria, but is required for assembly into active enzyme.

Meiotic expression of human ornithine transcarbamylase in the testes of transgenic mice

In an attempt to use mouse metallothionein-I (mMT-I) regulatory sequences to direct expression of human ornithine transcarbamylase in the liver of transgenic animals, fusion genes joining either 1.6 kilobases or 185 base pairs of the mMT-I regulatory region to the human ornithine transcarbamylase protein-coding sequence were used to produce transgenic mice. In mice carrying the fusion gene with 1.6 kilobases of the mMT-I 5'-flanking sequences, transgene expression was observed in a wide range of tissues, but, unexpectedly, expression in liver was never observed. Surprisingly, in mice carrying the fusion gene regulated by only 185 base pairs of the mMT-I 5'-flanking sequences, the transgene was expressed exclusively in male germ cells during the tetraploid, pachytene stage of meiosis.

Meiotic expression of human ornithine transcarbamylase in the testes of transgenic mice

In an attempt to use mouse metallothionein-I (mMT-I) regulatory sequences to direct expression of human ornithine transcarbamylase in the liver of transgenic animals, fusion genes joining either 1.6 kilobases or 185 base pairs of the mMT-I regulatory region to the human ornithine transcarbamylase protein-coding sequence were used to produce transgenic mice. In mice carrying the fusion gene with 1.6 kilobases of the mMT-I 5'-flanking sequences, transgene expression was observed in a wide range of tissues, but, unexpectedly, expression in liver was never observed. Surprisingly, in mice carrying the fusion gene regulated by only 185 base pairs of the mMT-I 5'-flanking sequences, the transgene was expressed exclusively in male germ cells during the tetraploid, pachytene stage of meiosis.

Effect of deletions within the leader peptide of pre-ornithine transcarbamylase on mitochondrial import

The uptake of the cytoplasmically synthesized mammalian enzyme, ornithine transcarbamylase, into mitochondria is directed by an N-terminal peptide of 32 amino acids. We have investigated some of the structural requirements for the import of the enzyme from rat liver into isolated mitochondria and into mitochondria of COS cells transfected with cDNA encoding the precursor form of ornithine transcarbamylase. Deletion of 21 amino acids from the N terminus of the leader peptide blocked the import of the precursor; deletion of 5 amino acids at positions 15-19 from the N terminus of the leader peptide had no deleterious effect on the import of the enzyme, nor on the processing and assembly of subunits in mitochondria. The region deleted contained three of eight basic residues in the leader peptide suggesting that specific structural elements containing basic residues, rather than the general basic nature of the leader, may be involved in mitochondrial import.

Comparison of ornithine transcarbamylase from rat liver and intestine. Evidence for differential regulation of enzyme levels

Ornithine transcarbamylase (OTCase) was purified from the small intestine of rat and the properties of the gut enzyme were compared with those of the enzyme from liver. The enzymes from both sources bound to the transition-state analog inhibitor, delta-N-(phosphonoacetyl)-L-ornithine, immobilized on Sepharose and eluted with carbamyl phosphate as a homogeneous preparation. The specific activities of the pure enzymes were 966 mumol min-1 mg-1 and 928 mumol min-1 mg-1 from liver and gut respectively, and the molecular mass, based on electrophoretic mobility, was 38 000 Da. The isoelectric point of the enzymes from both sources was 7.3. The enzymes from both sources cross-react to the same extent with antibodies against the liver enzyme on Western transfers and the size of the mRNA was identical on Northern transfers probed with a cDNA for the liver enzyme. Although OTCase is apparently the same gene product in both liver and gut, the enzyme levels respond differently to alterations in the protein content of the diet. OTCase in liver increased from 0.76 mumol min-1 microgram-1 DNA on 15% casein to 1.3 mumol min-1 microgram-1 DNA on 60% casein (P less than 0.01) whereas in small intestine the level decreased from 8.8 nmol min-1 microgram DNA on 15% casein to 5.7 nmol min-1 microgram-1 DNA on 60% casein (P less than 0.05). When expressed on a fresh-weight basis, the enzyme activity in liver shows the characteristic increase with increasing protein, whereas the activity in gut does not. The connection between these differences in gene expression and the different physiological roles of OTCase in liver and gut is discussed.

The inhibition of ornithine transcarbamoylase from Escherichia coli W by phaseolotoxin

The mechanism of inhibition of ornithine transcarbamoylase by the bacterial toxin phaseolotoxin [N-delta-(phosphosulphamyl)ornithylalanylhomoarginine] was investigated. Ornithine transcarbamoylase was purified by affinity chromatography from Escherichia coli W argR- by using N-delta-(phosphonoacetyl)ornithine as the ligand. Under steady-state conditions phaseolotoxin inhibition was reversible and exhibited mixed kinetics with respect to carbamoyl phosphate. The apparent Ki and apparent K'i were 0.2 microM and 10 microM respectively. Inhibition with respect to ornithine was noncompetitive, with an apparent Ki of 0.9 microM. These data are consistent with competitive binding of phaseolotoxin to the carbamoyl phosphate-binding site of the enzyme. The toxin also appears to be able to bind to the enzyme-carbamoyl phosphate complex, although, since K'i is 50 times greater than Ki, this event is kinetically much less significant. In the presence of phaseolotoxin ornithine transcarbamoylase exhibited a transient phase of activity before a steady state. This is consistent with low rates of association and dissociation for the toxin with enzyme and the enzyme-toxin complex. Rate constants of 2.5 X 10(4)M-1 X s-1 and 5 X 10(-3)s-1 were estimated for the association and dissociation constants respectively.

Phaseolotoxin transport in Escherichia coli and Salmonella typhimurium via the oligopeptide permease

Phaseolotoxin [(N delta-phosphosulfamyl)ornithylalanylhomoarginine], a phytotoxic tripeptide produced by Pseudomonas syringae pv. phaseolicola that inhibits ornithine carbamoyltransferase, is transported into Escherichia coli and Salmonella typhimurium via the oligopeptide transport system (Opp). Mutants defective in oligopeptide permease (Opp-) were resistant to phaseolotoxin. Spontaneous phaseolotoxin-resistant mutants (Toxr) lacked the Opp function as evidenced by their cross-resistance to triornithine and failure to utilize glycylhistidylglycine as a source of histidine. Growth inhibition by phaseolotoxin was prevented by peptides known to be transported via the Opp system and by treatment of the toxin with L-aminopeptidase. In both E. coli and S. typhimurium, Toxr mutations were cotransducible with trp, suggesting that the opp locus occupies similar positions in genetic maps of these bacteria.