Transport of impermeant substances in E. coli by way of oligopeptide permease

Rational design of a bacterial import system for new-to-nature molecules

Integration of novel compounds into biological processes holds significant potential for modifying or expanding existing cellular functions. However, the cellular uptake of these compounds is often hindered by selectively permeable membranes. We present a novel bacterial transport system that has been rationally designed to address this challenge. Our approach utilizes a highly promiscuous sulfonate membrane transporter, which allows the passage of cargo molecules attached as amides to a sulfobutanoate transport vector molecule into the cytoplasm of the cell. These cargoes can then be unloaded from the sulfobutanoyl amides using an engineered variant of the enzyme γ-glutamyl transferase, which hydrolyzes the amide bond and releases the cargo molecule within the cell. Here, we provide evidence for the broad substrate specificity of both components of the system by evaluating a panel of structurally diverse sulfobutanoyl amides. Furthermore, we successfully implement the synthetic uptake system in vivo and showcase its functionality by importing an impermeant non-canonical amino acid.

Genetic Encoding of Phosphorylated Amino Acids into Proteins

Reversible phosphorylation is a fundamental mechanism for controlling protein function. Despite the critical roles phosphorylated proteins play in physiology and disease, our ability to study individual phospho-proteoforms has been hindered by a lack of versatile methods to efficiently generate homogeneous proteins with site-specific phosphoamino acids or with functional mimics that are resistant to phosphatases. Genetic code expansion (GCE) is emerging as a transformative approach to tackle this challenge, allowing direct incorporation of phosphoamino acids into proteins during translation in response to amber stop codons. This genetic programming of phospho-protein synthesis eliminates the reliance on kinase-based or chemical semisynthesis approaches, making it broadly applicable to diverse phospho-proteoforms. In this comprehensive review, we provide a brief introduction to GCE and trace the development of existing GCE technologies for installing phosphoserine, phosphothreonine, phosphotyrosine, and their mimics, discussing both their advantages as well as their limitations. While some of the technologies are still early in their development, others are already robust enough to greatly expand the range of biologically relevant questions that can be addressed. We highlight new discoveries enabled by these GCE approaches, provide practical considerations for the application of technologies by non-GCE experts, and also identify avenues ripe for further development.

Applications of genetic code expansion technology in eukaryotes

Unnatural amino acids (UAAs) have gained significant attention in protein engineering and drug development owing to their ability to introduce new chemical functionalities to proteins. In eukaryotes, genetic code expansion (GCE) enables the incorporation of UAAs and facilitates posttranscriptional modification (PTM), which is not feasible in prokaryotic systems. GCE is also a powerful tool for cell or animal imaging, the monitoring of protein interactions in target cells, drug development, and switch regulation. Therefore, there is keen interest in utilizing GCE in eukaryotic systems. This review provides an overview of the application of GCE in eukaryotic systems and discusses current challenges that need to be addressed.

Engineering Translation Components for Genetic Code Expansion

The expansion of the genetic code consisting of four bases and 20 amino acids into diverse building blocks has been an exciting topic in synthetic biology. Many biochemical components are involved in gene expression; therefore, adding a new component to the genetic code requires engineering many other components that interact with it. Genetic code expansion has advanced significantly for the last two decades with the engineering of several components involved in protein synthesis. These components include tRNA/aminoacyl-tRNA synthetase, new codons, ribosomes, and elongation factor Tu. In addition, biosynthesis and enhanced uptake of non-canonical amino acids have been attempted and have made meaningful progress. This review discusses the efforts to engineer these translation components, to improve the genetic code expansion technology.

Mutant Variants of the Substrate-Binding Protein DppA from Escherichia coli Enhance Growth on Nonstandard γ-Glutamyl Amide-Containing Peptides

The import of nonnatural molecules is a recurring problem in fundamental and applied aspects of microbiology. The dipeptide permease (Dpp) of Escherichia coli is an ABC-type multicomponent transporter system located in the cytoplasmic membrane, which is capable of transporting a wide range of di- and tripeptides with structurally and chemically diverse amino acid side chains into the cell. Given this low degree of specificity, Dpp was previously used as an entry gate to deliver natural and nonnatural cargo molecules into the cell by attaching them to amino acid side chains of peptides, in particular, the γ-carboxyl group of glutamate residues. However, the binding affinity of the substrate-binding protein dipeptide permease A (DppA), which is responsible for the initial binding of peptides in the periplasmic space, is significantly higher for peptides consisting of standard amino acids than for peptides containing side-chain modifications. Here, we used adaptive laboratory evolution to identify strains that utilize dipeptides containing γ-substituted glutamate residues more efficiently and linked this phenotype to different mutations in DppA. In vitro characterization of these mutants by thermal denaturation midpoint shift assays and isothermal titration calorimetry revealed significantly higher binding affinities of these variants toward peptides containing γ-glutamyl amides, presumably resulting in improved uptake and therefore faster growth in media supplemented with these nonstandard peptides.IMPORTANCE Fundamental and synthetic biology frequently suffer from insufficient delivery of unnatural building blocks or substrates for metabolic pathways into bacterial cells. The use of peptide-based transport vectors represents an established strategy to enable the uptake of such molecules as a cargo. We expand the scope of peptide-based uptake and characterize in detail the obtained DppA mutant variants. Furthermore, we highlight the potential of adaptive laboratory evolution to identify beneficial insertion mutations that are unlikely to be identified with existing directed evolution strategies.

(Some) current concepts in antibacterial drug discovery

The rise of multidrug resistance in bacteria rendering pathogens unresponsive to many clinical drugs is widely acknowledged and considered a critical global healthcare issue. There is broad consensus that novel antibacterial chemotherapeutic options are extremely urgently needed. However, the development pipeline of new antibacterial drug lead structures is poorly filled and not commensurate with the scale of the problem since the pharmaceutical industry has shown reduced interest in antibiotic development in the past decades due to high economic risks and low profit expectations. Therefore, academic research institutions have a special responsibility in finding novel treatment options for the future. In this mini review, we want to provide a broad overview of the different approaches and concepts that are currently pursued in this research field.

Genetically encoding phosphotyrosine and its nonhydrolyzable analog in bacteria

Tyrosine phosphorylation is a common protein posttranslational modification, which plays a critical role in signal transduction and the regulation of many cellular processes. Using a pro-peptide strategy to increase cellular uptake of O-phosphotyrosine (pTyr) and its nonhydrolyzable analog 4-phosphomethyl-L-phenylalanine (Pmp), we identified an orthogonal aminoacyl-tRNA synthetase/tRNA pair that allows the site-specific incorporation of both pTyr and Pmp into recombinant proteins in response to the amber stop codon in Escherichia coli in good yields. The X-ray crystal structure of the synthetase reveals a reconfigured substrate binding site formed by non-conservative mutations and substantial local structural perturbations. We demonstrate the utility of this method by introducing Pmp into a putative phosphorylation site whose corresponding kinase is unknown and determined the affinities of the individual variants for the substrate 3BP2. In summary, this work provides a useful recombinant tool to dissect the biological functions of tyrosine phosphorylation at specific sites in the proteome.

Portage of various compounds into bacteria by attachment to glycine residues in peptides

Synthetic di- and oligopeptides are described that contain nucleophilic moieties attached to the alpha carbon of a glycine residue. These peptides are accepted by the peptide transport systems of Escherichia coli (and other microorganisms) and are capable of being hydrolyzed by intracellular peptidases. After liberation of its amino group the alpha-substituted glycine is chemically unstable (although it is stable in peptide form) and decomposes, releasing the nucleophilic moiety. Thus, the combined result of peptide transport and peptidase action is the intracellular release of the nucleophile. Peptides containing glycine residues alpha-substituted with thiophenol, aniline, or phenol are used as models for this type of peptide-assisted entry and their metabolism by E. coli is described. Peptides of this type have broad applicability to the study of microbial physiology and the development of an additional class of antimicrobial agents.

Transport and hydrolysis of peptides by microorganisms

The structural specificities of the dipeptide and oligopeptide permeases of E. coli are briefly reviewed and related to the requirements found for other microorganisms. New, quick, sensitive methods for studying peptide transport are described, based on the following: (i) peptide-dependent incorporation of free radioactive amino acid into newly synthesized protein by a double amino acid auxotroph, (ii) colorimetric assay of peptide-dependent enzyme synthesis by an amino acid auxotroph, (iii) dansyl fingerprint technique. These approaches provide information on peptide binding affinity to a permease and rates of peptide uptake and amino acid efflux. Among current and future research areas considered are: the influence of the pKb of the N-terminal amino group on transport, generality of peptide transport in microorganisms, energy coupling and regulation, involvement of binding proteins, and the 'smugglin' concept. Peptide hydrolysis, and nutritional ultilization of peptides, by microorganisms are briefly discussed.

Synthesis of a pyridoxine-peptide based delivery system for nucleotides

As a first step in the development of a nucleotide delivery system making use of oligopeptide permease, we have synthesized pyridoxine-peptide-nucleotide conjugates. The nucleotides are bonded on serine residues. The peptides terminate with a pyroglutamate residue. In the first example, the pyridoxine moiety is connected at the end of the peptides, while, in the second example, the pyridoxine moiety is bonded at an aspartic acid residue in a middle position of the peptide. Nucleotides are introduced as phosphoramidites. The synthetic strategy involves a series of protection, deprotection, and coupling reactions, and integrates peptide, nucleotide, and pyridoxine chemistry. The final deprotection step was carried out in basic conditions using 10% K2CO3 in MeOH.

Biotinylation facilitates the uptake of large peptides by Escherichia coli and other gram-negative bacteria

Gram-negative bacteria such as Escherichia coli can normally only take up small peptides less than 650 Da, or five to six amino acids, in size. We have found that biotinylated peptides up to 31 amino acids in length can be taken up by E. coli and that uptake is dependent on the biotin transporter. Uptake could be competitively inhibited by free biotin or avidin and blocked by the protonophore carbonyl m-chlorophenylhydrazone and was abolished in E. coli mutants that lacked the biotin transporter. Biotinylated peptides could be used to supplement the growth of a biotin auxotroph, and the transported peptides were shown to be localized to the cytoplasm in cell fractionation experiments. The uptake of biotinylated peptides was also demonstrated for two other gram-negative bacteria, Salmonella enterica serovar Typhimurium and Pseudomonas aeruginosa. This finding may make it possible to create new peptide antibiotics that can be used against gram-negative pathogens. Researchers have used various moieties to cause the illicit transport of compounds in bacteria, and this study demonstrates the illicit transport of the largest known compound to date.

Synergistic effect of CMP/KDO synthase inhibitors with antimicrobial agents on inhibition of production and release of vero toxin by enterohaemorrhagic Escherichia coli O157:H7

Synergistic effect of CMP/KDO synthase inhibitors in LPS biosynthesis of Gram-negative bacteria with kanamycin (KM) and fosfomycin (FOM) on the production and release of Vero toxins (VTs) by Escherichia coli O157 was evaluated in vitro. While CMP/KDO synthase inhibitors, KM and FOM showed no inhibitory effect on the production/release of VTs by themselves alone, both KM and FOM showed the remarkable inhibition of VT2 release through synergistic collaboration with CMP:KDO synthase inhibitor.

Peptide-mediated delivery of green fluorescent protein into yeasts and bacteria

Stringent microbial cell barriers limit the application of many substances in research and therapeutics. Carrier peptides that penetrate or translocate across cell membranes may help overcome this problem. To assess peptide-mediated delivery into two yeast and three bacterial species, a range of cell penetrating and signal peptide sequences were fused to green fluorescent protein (GFP), expressed in Escherichia coli, partially purified and incubated with growing cells. Fluorescence microscopy indicated several peptides that mediated delivery. In particular, VLTNENPFSDP efficiently delivered GFP into Candida albicans and Staphylococcus aureus, while YKKSNNPFSD was most efficient for Bacillus subtilis and CFFKDEL for Escherichia coli. Carrier peptides may improve delivery of certain large molecular mass molecules into microorganisms for research and therapeutic applications.

N3-oxoacyl derivatives of L-2,3-diaminopropanoic acid and their peptides; novel inhibitors of glucosamine-6-phosphate synthase

Novel inhibitors 1-4 of glucosamine-6-phosphate synthase from Candida albicans have been designed based on acylation of the N3 amino group of L-2,3-diaminopropanoic acid with the corresponding ketoacids. These inhibitors have been shown to alkylate the fungal enzyme in a time-dependent manner. Compound 3 containing trans-beta-benzoyl acrylic acid as an acyl residue was found to be the most potent inhibitor in the series. Dipeptides composed of the active inhibitors and norvaline demonstrated potent antifungal activity against selected strains of Candida spp. and Saccharomyces cerevisiae. Their activity was reversed upon addition of N-acetylglucosamine to the medium.

Specificity and interactions of the protein OppA: partitioning solvent binding effects using mass spectrometry

Mass spectrometry (MS) was used to characterise the binding of the 58 kDa protein OppA to 11 peptides with diverse properties. Peptides with two, three and five amino acid residues were added to OppA, and the mass spectra showed that the highest-affinity complexes are formed between OppA and tripeptide ligands. Lower-affinity complexes were observed for OppA and dipeptide ligands, and no complex formation was detected with pentapeptides or a tripeptide in which the N-terminal amino group was acetylated. Tripeptides containing a single d amino acid residue were found not to bind to native OppA. Evidence from the peak width and the, charge in the spectra of the complexes suggests that the bound peptides are encapsulated by the protein in a solvent-filled cavity in the gas phase of the mass spectrometer. Analysis of the proportions of peptide-bound and free proteins under low-energy MS conditions shows a good correlation with solution-phase K(d) measurements where available. Increasing the internal energy of the gas-phase complex led to dissociation of the complex. The ease of dissociation is interpreted in terms of the intrinsic stability of the complex in the absence of the stabilising effects of bulk solvent. The results from this study demonstrate insensitivity to the hydrophobic and ionic properties, of the side-chains of the peptides, in contrast to the investigation of other protein ligand systems by MS. Moreover, these findings are in accord with the physiological role of this protein in allowing into the cell di- and tripeptides containing naturally occurring amino acids, regardless of their sequence, while barring access to potentially harmful peptide mimics.

Isolation and characterization of a Saccharomyces cerevisiae peptide transport gene

We have cloned and characterized a Saccharomyces cerevisiae peptide transport gene (PTR2) isolated from a genomic DNA library by directly selecting for functional complementation of a peptide transport-deficient mutant. Deletion and frameshift mutageneses were used to localize the complementing activity to a 3.1-kbp region on the transforming plasmid. DNA sequencing of the complementing region identified an open reading frame spanning 1,803 bp. The deduced amino acid sequence predicts a hydrophobic peptide consisting of 601 amino acids, having a molecular mass of 68.1 kDa, composed in part of 12 hydrophobic segments, and sharing significant similarities with a nitrate transport protein encoded by the CHL1 gene of Arabidopsis thaliana. Northern (RNA) hybridization experiments demonstrated a single transcript that was 1.8 kb in length and that was transiently induced by the addition of L-leucine to the growth medium. The PTR2 gene was localized to the right arm of chromosome XI by contour-clamped homogeneous electric field gel chromosome blotting and by hybridization to known chromosome XI lambda phage clones of S. cerevisiae DNA. PTR2 was tightly linked to the UBI2 gene, with the coding sequences being separated by a 466-bp region and oriented so that the genes were transcribed convergently. A chromosomal disruption of the PTR2 gene in a haploid strain was not lethal under standard growth conditions. The cloning of PTR2 represents the first example of the molecular genetic characterization of a eucaryotic peptide transport gene.

Isolation and characterization of a Saccharomyces cerevisiae peptide transport gene

We have cloned and characterized a Saccharomyces cerevisiae peptide transport gene (PTR2) isolated from a genomic DNA library by directly selecting for functional complementation of a peptide transport-deficient mutant. Deletion and frameshift mutageneses were used to localize the complementing activity to a 3.1-kbp region on the transforming plasmid. DNA sequencing of the complementing region identified an open reading frame spanning 1,803 bp. The deduced amino acid sequence predicts a hydrophobic peptide consisting of 601 amino acids, having a molecular mass of 68.1 kDa, composed in part of 12 hydrophobic segments, and sharing significant similarities with a nitrate transport protein encoded by the CHL1 gene of Arabidopsis thaliana. Northern (RNA) hybridization experiments demonstrated a single transcript that was 1.8 kb in length and that was transiently induced by the addition of L-leucine to the growth medium. The PTR2 gene was localized to the right arm of chromosome XI by contour-clamped homogeneous electric field gel chromosome blotting and by hybridization to known chromosome XI lambda phage clones of S. cerevisiae DNA. PTR2 was tightly linked to the UBI2 gene, with the coding sequences being separated by a 466-bp region and oriented so that the genes were transcribed convergently. A chromosomal disruption of the PTR2 gene in a haploid strain was not lethal under standard growth conditions. The cloning of PTR2 represents the first example of the molecular genetic characterization of a eucaryotic peptide transport gene.

Peptide transport and chemotaxis in Escherichia coli and Salmonella typhimurium: characterization of the dipeptide permease (Dpp) and the dipeptide-binding protein

The dipeptide permease (Dpp) is one of three genetically distinct peptide-transport systems in enteric bacteria. Dpp also plays a role in chemotaxis towards peptides. We have devised three selections for dpp mutations based on resistance to toxic peptides (bacilysin, valine-containing peptides, and bialaphos). All dpp mutations mapped to a single chromosomal locus between 77 and 78 min in Salmonella typhimurium and at 79.2 min in Escherichia coli. Expression of dpp was constitutive in both species but the absolute level of expression varied widely between strains. At least in part this difference in expression levels is determined by cis-acting sequences. The dpp locus of E. coli was cloned. The first gene in the operon, dppA, encodes a periplasmic dipeptide-binding protein (DBP) required for dipeptide transport and chemotaxis. Downstream of dppA are other genes required for transport but not for chemotaxis. The dipeptide-binding protein was found to share 26.5% sequence identity with the periplasmic oligopeptide-binding protein OppA.

Mechanism of action of anticandidal dipeptides containing inhibitors of glucosamine-6-phosphate synthase

The mechanism of anticandidal action of novel synthetic dipeptides containing N3-(4-methoxyfumaroyl)-L-2,3-diaminopropanoic acid (FMDP) residues was shown to be consistent with the "warhead delivery" concept. FMDP dipeptides were shown to be transported into Candida albicans cells by the di-tripeptide permease and subsequently hydrolyzed by intracellular peptidases, especially aminopeptidase. The anticandidal activity of the particular FMDP dipeptide was influenced by the rate of its transport and, to a lower extent, by the intracellular cleavage rate. A high transport rate accompanied by a high cleavage rate resulted in the high anticandidal activity of L-norvalyl-FMDP. The strong growth-inhibitory effect of this compound was the consequence of inhibition of the enzyme glucosamine-6-phosphate synthase by the released FMDP. The action of L-norvalyl-FMDP on exponentially growing C. albicans cells resulted in a sharp decrease of incorporation of 14C label from [14C]glucose into chitin, mannoprotein, and glucan. This effect, as well as the growth-inhibitory effect, was fully reversed by exogenous N-acetyl-D-glucosamine. Glucosamine-6-phosphate synthase was proved to be the only essential target for FMDP dipeptides. Scanning electron microscopy of C. albicans cells treated with L-norvalyl-FMDP revealed highly distorted, wrinkled, and collapsed forms. Cells formed long, bulbous chains, and partial lysis occurred.

Fungitoxicity of m-fluorophenylalanine-containing peptides towards Pythium ultimum

The tripeptide L-m-fluorophenylalanyl-L-alanyl-L-alanine was much more fungitoxic towards Pythium ultimum than the dipeptide L-m-fluorophenylalanyl-L-alanine or m-fluorophenylalanine. The fungitoxicity of the tripeptide was reduced by L-alanyl peptides and phenylalanine, but not by other amino acids. In contrast, the fungitoxicity of m-fluorophenylalanine was unaffected by peptides, and was antagonized by several amino acids. These results suggest the effective delivery of m-fluorophenylalanine into the cell by a tripeptide carrier.

Antibacterial agents specifically inhibiting lipopolysaccharide synthesis

The spread of antibiotic resistance in Gram-negative bacteria has sustained a continuing search for new agents with antibacterial activity against this important class of bacterial pathogen. Because the biosynthesis of lipopolysaccharide (LPS) is unique to Gram-negative bacteria and required by them for growth and virulence, attempts have been made to discover or design antibacterial agents acting at this site; however, no such agents have so far been developed. We now present definitive experimental data documenting design of the first member of the class of antibacterial compounds which specifically inhibit LPS synthesis. The target enzyme is 3-deoxy-D-manno-octulosonate cytidylytransferase (CMP-KDO synthetase), a cytoplasmic enzyme which activates 3-deoxy-D-manno-octulosonate (KDO) for incorporation into LPS. A specific inhibitor of CMP-KDO synthetase, alpha-C-(1,5-anhydro-7-amino-2,7-dideoxy-D-manno-heptopyranosyl)-carboxy late was designed using results of our studies of the purified enzyme. LPS synthesis ceased and lipid A precursor accumulated, causing growth stasis and perturbation of outer membrane structure and function, following delivery of the inhibitor to the intracellular target by a peptide carrier. Antibacterial action required an intact oligopeptide permease system and specific intracellular aminopeptidase activity to release inhibitor from the peptide prodrug.

Molecular characterization of the oligopeptide permease of Salmonella typhimurium

The oligopeptide permease (Opp) of Salmonella typhimurium is a periplasmic binding protein-dependent transport system and handles any peptides containing from two to five amino acid residues. Opp plays an important nutritional role and is also required for the recycling of cell wall peptides. We have determined the nucleotide sequence of the opp operon. In addition to the four opp genes identified previously by genetic means (oppABCD) a fifth gene, oppF, is shown to be cotranscribed as part of the opp operon. Using reverse genetics, we show that oppF also encodes an essential component of the Opp transport system. The five proteins, OppABCDF, are shown to be the only proteins required for Opp function. Regulation of opp expression and of the differential expression of genes within the operon is investigated. We have devised a simple means of constructing lacZ gene fusions to any S. typhimurium chromosomal gene in vivo, using derivatives of bacteriophage Mu. Using this procedure, opp-lacZ gene fusions were selected. The resultant Opp-LacZ hybrid proteins were used to show that OppB, OppC and OppD are membrane-associated proteins. A detailed comparison of the Opp components with those of other binding protein-dependent transport systems provides insight into the mechanisms and evolution of these transport systems.

Binding specificity of the periplasmic oligopeptide-binding protein from Escherichia coli

The structural properties required for the binding of peptide substrates to the Escherichia coli periplasmic protein involved in oligopeptide transport were surveyed by measuring the ability of different peptides to compete for binding in an equilibrium dialysis assay with the tripeptide Ala-Phe-[3H]Gly. The protein specifically bound oligopeptides and failed to bind amino acids or dipeptides. Acetylation of the peptide amino terminus of (Ala)3 severely impaired binding, whereas esterification of the carboxyl terminus significantly reduced but did not completely eliminate binding. Peptides composed of L-amino acids competed more effectively than did peptides containing D-residues or glycine. Experiments with a series of alanyl peptide homologs demonstrated a decrease in competitive ability with increasing chain length beyond tripeptide. Competition studies with tripeptide homologs indicated that a wide variety of amino acyl side chains were tolerated by the periplasmic protein, but side-chain composition did affect binding. Fluorescence emission data suggested that this periplasmic protein possesses more than one substrate-binding site capable of distinguishing peptides on the basis of amino acyl side chains.

Relative rates of transport of peptidyl drugs by Candida albicans

A variety of peptide drugs are known to be active against Candida albicans; however, little is known about the transport of such agents into the target organism. To provide further information concerning transport of this type, we studied the uptake of two classes of small linear peptides: polyoxins which act intact within the cell and the m-fluorophenylalanyl (m-F-Phe) peptides which require peptidase cleavage to release m-F-Phe. Competition studies with a specific dipeptide detector (alanyl-alpha-thiophenylglycine) enabled us to determine Ki values of 2.6 microM for nikkomycin Z and 350 microM for polyoxin D. Rates of uptake of the peptidyl-nucleosides are approximately 30 times lower than those of the m-F-Phe peptides (apparent maximal velocities: nikkomycin Z, 62 pmol min-1 mg (dry weight) of cells-1; M-F-Phe alanine 1.3 nmol min-1 mg (dry weight) of cells-1). For both the m-F-Phe peptides and the peptidyl-nucleosides, the affinity of the drug for the transport system is an important determinant of its whole-cell activity.

Genetic characterization and molecular cloning of the tripeptide permease (tpp) genes of Salmonella typhimurium

Of the three bacterial peptide transport systems only one, the oligopeptide permease, has been characterized in any detail. We have now isolated Salmonella typhimurium mutants deficient in a second transport system, the tripeptide permease (Tpp), using the toxic peptide alafosfalin. Alafosfalin resistance mutations map at three loci, the gene encoding peptidase A (pepA) and two transport-defective loci, tppA and tppB. Locus tppA has been mapped to 74 min on the S. typhimurium chromosome, cotransducible with aroB, and is a positive regulator of tppB. Locus tppB maps at 27 min in the cotransduction gap between purB and pyrF. We cloned tppB, the structural locus for the tripeptide permease. Two simple methods are described for mapping the location of cloned DNA fragments on the chromosome of S. typhimurium.

Anaerobic and leucine-dependent expression of a peptide transport gene in Salmonella typhimurium

Using Mu d1-mediated lac operon fusions, we studied the transcriptional regulation of the genes encoding two peptide transport systems, the oligopeptide permease and the tripeptide permease. The four opp genes were found to be constitutively expressed, whereas the genes encoding the tripeptide permease are under a complex set of regulatory controls. Two loci, tppA and tppB, are required for tripeptide permease function. Locus tppA is shown to be a positive regulator of tppB expression. In addition, tppB expression is specifically induced by exogeneous leucine or by anaerobiosis. Anaerobic induction of tppB is independent of the fnr gene product which is required for the anaerobic expression of several respiratory enzymes. Thus, there must be at least two distinct pathways for the anaerobic regulation of gene expression.

Peptidase activity in the inner membrane of Pseudomonas aeruginosa

The location of peptidase activity within the cell envelope structure of Pseudomonas aeruginosa has been studied. Inner and outer membrane fractions were separated on the basis of buoyant density using two consecutive sucrose steps gradients and identified on the basis of known components. The inner membrane was shown to contain peptidase activity while the outer membrane contained none. These data support the hypothesis that P. aeruginosa transports intact peptides.

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.

[Membrane permeability and antibiotic resistance in bacteria]

The resistance of mainly gram-negative bacteria against antibiotics is discussed with respect to new discoveries on the permeability barrier of the outer membrane. Proteins originally characterized as virus receptors have been found to be involved in the translocation of certain substrates across the membrane. Antibiotics can reach their target by the same "pores".

Basis for the design of anticandidal agents from studies of peptide utilization in Canadida albicans

The growth of Candida albicans WD 18-4, a methionine and lysine double auxotroph, on a variety of methionine- and lysine-containing peptides was determined. This yeast does not excrete extracellular peptidases. Thus, the growth response to peptides containing the required amino acid is a measure of peptide transport. A variety of methionine-containing peptides such as Met-Met, Met-Met-Met, and Met-Met-Met-Met-Met are transported. Acylation of the N-terminus of transported peptides does not affect their transport, but derivitization of the C-terminus prevents peptide uptake. In contrast, all lysine-containing peptides tested, except Lys-Gly, were not growth substrates. The inability of a peptide to substitute for the requisite amino acid was not due to the absence of cellular peptidases or to toxicity of the nonutilized peptides. Several potentially toxic amino acids were carried into Candida as a component of transported peptides. This establishes the peptide transport system as a possible tool for the design of antibiotics for Candida albicans.

On the transport of tripeptide antibiotics in bacteria

The two tripeptide antibiotics L-2-amino-4-methylphosphinobutyryl-alanyl-alanyl-alanine (L-phosphinothricyl-alanyl-alanine) and L-(N5-phosphono)methionine-S-sulfoximinyl-alanyl-alanine, both inhibitors of the glutamine synthetase, are transported into the cell of Escherichia coli K 12 via the oligopeptide transport system. The uptake by this system is proved first of all by cross-resistance with tri-L-ornithine using oligopeptide-transport-deficient mutants, and secondly by antagonism tests demonstrating competitive reversal of the action of the antibiotic by several peptides which have been shown to be transported via the oligopeptide transport system, e.g. tri-L-alanine, tetra-L-alanine, tri-L-lysine, tri-L-serine, tri-glycine, glycyl-glycyl-L-alanine and the synthetic tripeptide L-azadenyl-aminohexanoyl-alanyl-alanine. On the other hand, there is no effect on the action of the antibiotic in antagonism tests with compounds which use different transport systems, such as L-alanyl-alanine, L-lysyl-lysine, glutathione and the synthetic amino acid azaadenylaminohexanoic acid, i.e. 2-amino-6-(7-amino-3H-v-triazolo-[4,5-d]-pyrimidin-3-yl)hexanoic acid. Another inhibitor of the glutamine synthetase, L-methionine-S-dioxide (methioninesulfone) could be converted into a tripeptide form by linkage to L-alanyl-alanine analogously to the tripeptide antibiotics described above. Whereas the free L-methionine-S-dioxide seems to be transported via the methionine transport system, the tripeptide form is transported via the oligopeptide transport system. Thus, this glutamine synthetase inhibitor can be taken up by the cell via two different transport mechanisms. Our results indicate that this could provide a synergistic effect. The syntheses of the new tripeptides L-azaadenylaminohexanoyl-alanyl-alanine and L-methionine-S-dioxidyl-alanyl-alanine were performed by dicyclohexylcarbodiimide couplings of the unusual N-protected L-alpha-amino acids azaadenylaminohexanoic acid and L-methionine-S-dioxide to L-alanyl-alanine-tert-butyl ester followed by common deprotection steps. Tri-L-ornithine was synthesized without carboxyl protection via two successive couplings of hydroxybenzotriazol esters of Nalpha-butoxycarbonyl-Ndelta-benzyloxycarbonyl-L-ornithine.

Specialized peptide transport system in Escherichia coli

Trileucine is utilized as a source of leucine for growth of strains of Escherichia coli K-12 that are deficient in the oligopeptide transport system (Opp). Trithreonine is toxic to E. coli K-12. Opp- mutants of E. coli K-12 retain complete sensitivity to this tripeptide. Moreover, E. coli W, which is resistant to trithreonine, can utlize this tripeptide as a threonine source and this capability is fully maintained in E. coli W (Opp-). A spontaneous trithreonine-resistant mutant of E. coli K-12 (Opp-) has been isolated that has an impaired growth response to trileucine and is resistant to trithreonine. Trileucine competes with the uptake of trithreonine as measured by its ability to relieve trithreonine toxicity in E. coli K-12. It is concluded that trileucine as well as trithreonine are transported into E. coli K-12 or W by a common uptake system that is distinct from the Opp system. Trimethionine can act as a competitor of trileucine or trithreonine-supported growth and as an antagonist of trithreonine toxicity in Opp- mutants. It is concluded that trimethionine is recognized by the trileucine-trithreonine transport system. Trithreonine, trimethionine, and trileucine are also transported by the Opp system, as they all relieve triornithine toxicity towards E. coli W and compete with tetralysine utilization as lysine source for growth of a lysine auxotroph of this strain.