Transport of 5-aminolevulinic acid by the dipeptide permease in Salmonella typhimurium

A High-Throughput Method for Identifying Novel Genes That Influence Metabolic Pathways Reveals New Iron and Heme Regulation in Pseudomonas aeruginosa

The ability to simultaneously and more directly correlate genes with metabolite levels on a global level would provide novel information for many biological platforms yet has thus far been challenging. Here, we describe a method to help address this problem, which we dub “Met-Seq” (metabolite-coupled Tn sequencing).

Heme is an essential metabolite for most life on earth. Bacterial pathogens almost universally require iron to infect a host, often acquiring this nutrient in the form of heme. The Gram-negative pathogen Pseudomonas aeruginosa is no exception, where heme acquisition and metabolism are known to be crucial for both chronic and acute infections. To unveil unknown genes and pathways that could play a role with heme metabolic flux in this pathogen, we devised an omic-based approach we dubbed “Met-Seq,” for metabolite-coupled transposon sequencing. Met-Seq couples a biosensor with fluorescence-activated cell sorting (FACS) and massively parallel sequencing, allowing for direct identification of genes associated with metabolic changes. In this work, we first construct and validate a heme biosensor for use with P. aeruginosa and exploit Met-Seq to identify 188 genes that potentially influence intracellular heme levels. Identified genes largely consisted of metabolic pathways not previously associated with heme, including many secreted virulence effectors, as well as 11 predicted small RNAs (sRNAs) and riboswitches whose functions are not currently understood. We verify that five Met-Seq hits affect intracellular heme levels; a predicted extracytoplasmic function (ECF) factor, a phospholipid acquisition system, heme biosynthesis regulator Dnr, and two predicted antibiotic monooxygenase (ABM) domains of unknown function (PA0709 and PA3390). Finally, we demonstrate that PA0709 and PA3390 are novel heme-binding proteins. Our data suggest that Met-Seq could be extrapolated to other biological systems and metabolites for which there is an available biosensor, and provides a new template for further exploration of iron/heme regulation and metabolism in P. aeruginosa and other pathogens.

IMPORTANCE The ability to simultaneously and more directly correlate genes with metabolite levels on a global level would provide novel information for many biological platforms yet has thus far been challenging. Here, we describe a method to help address this problem, which we dub “Met-Seq” (metabolite-coupled Tn sequencing). Met-Seq uses the powerful combination of fluorescent biosensors, fluorescence-activated cell sorting (FACS), and next-generation sequencing (NGS) to rapidly identify genes that influence the levels of specific intracellular metabolites. For proof of concept, we create and test a heme biosensor and then exploit Met-Seq to identify novel genes involved in the regulation of heme in the pathogen Pseudomonas aeruginosa. Met-Seq-generated data were largely comprised of genes which have not previously been reported to influence heme levels in this pathogen, two of which we verify as novel heme-binding proteins. As heme is a required metabolite for host infection in P. aeruginosa and most other pathogens, our studies provide a new list of targets for potential antimicrobial therapies and shed additional light on the balance between infection, heme uptake, and heme biosynthesis.

Envelope Stress and Regulation of the Salmonella Pathogenicity Island 1 Type III Secretion System

Salmonella enterica serovar Typhimurium uses a type three secretion system (T3SS) encoded on the Salmonella pathogenicity island 1 (SPI1) to invade intestinal epithelial cells and induce inflammatory diarrhea. The SPI1 T3SS is regulated by numerous environmental and physiological signals, integrated to either activate or repress invasion. Transcription of hilA, encoding the transcriptional activator of the SPI1 structural genes, is activated by three AraC-like regulators, HilD, HilC, and RtsA, that act in a complex feed-forward loop. Deletion of bamB, encoding a component of the β-barrel assembly machinery, causes a dramatic repression of SPI1, but the mechanism was unknown. Here, we show that partially defective β-barrel assembly activates the RcsCDB regulon, leading to decreased hilA transcription. This regulation is independent of RpoE activation. Though Rcs has been previously shown to repress SPI1 when disulfide bond formation is impaired, we show that activation of Rcs in a bamB background is dependent on the sensor protein RcsF, whereas disulfide bond status is sensed independently. Rcs decreases transcription of the flagellar regulon, including fliZ, the product of which indirectly activates HilD protein activity. Rcs also represses hilD, hilC, and rtsA promoters by an unknown mechanism. Both dsbA and bamB mutants have motility defects, though this is simply regulatory in a bamB background; motility is restored in the absence of Rcs. Effector secretion assays show that repression of SPI1 in a bamB background is also regulatory; if expressed, the SPI1 T3SS is functional in a bamB background. This emphasizes the sensitivity of SPI1 regulation to overall envelope homeostasis.IMPORTANCE Salmonella causes worldwide foodborne illness, leading to massive disease burden and an estimated 600,000 deaths per year. Salmonella infects orally and invades intestinal epithelial cells using a type 3 secretion system that directly injects effector proteins into host cells. This first step in invasion is tightly regulated by a variety of inputs. In this work, we demonstrate that Salmonella senses the functionality of outer membrane assembly in determining regulation of invasion machinery, and we show that Salmonella uses distinct mechanisms to detect specific perturbations in envelope assembly.

Enhanced sterilization and healing of cutaneous pseudomonas infection using 5-aminolevulinic acid as a photosensitizer with 410-nm LED light

BACKGROUND

Pseudomonas aeruginosa (PA) frequently develops antibiotic-resistant characteristics, which is clinically problematic. The main reason behind the rise of antibiotic-resistant PA is the extensive use of antibiotics. Therefore, a novel technique is needed to treat PA infections. Photodynamic therapy (PDT) is thought to have the potential to be a non-antibiotic treatment for infections. 5-Aminolevulinic acid (ALA), which works as a photosensitizer after being metabolized into protoporphyrin IX (PpIX) in the heme synthetic pathway, is used for PDT. Thus far, the in vivo effectiveness of PDT using ALA against PA is unknown.

OBJECTIVE

In this study, we investigated PDT using ALA both in vitro and in vivo.

METHODS AND RESULTS

Although PDT with ALA alone did not show a bactericidal effect on PA, PDT with both ALA and EDTA-2Na had a bactericidal effect in vitro. In in vivo experiments, wounds healed faster in PA-infected mice treated with PDT using both EDTA-2Na and ALA compared to non-PDT.

CONCLUSION

These results suggest that PDT with EDTA-2Na and ALA is a potential novel treatment option for PA-infected wounds.

Accumulation of heme biosynthetic intermediates contributes to the antibacterial action of the metalloid tellurite

The metalloid tellurite is highly toxic to microorganisms. Several mechanisms of action have been proposed, including thiol depletion and generation of hydrogen peroxide and superoxide, but none of them can fully explain its toxicity. Here we use a combination of directed evolution and chemical and biochemical approaches to demonstrate that tellurite inhibits heme biosynthesis, leading to the accumulation of intermediates of this pathway and hydroxyl radical. Unexpectedly, the development of tellurite resistance is accompanied by increased susceptibility to hydrogen peroxide. Furthermore, we show that the heme precursor 5-aminolevulinic acid, which is used as an antimicrobial agent in photodynamic therapy, potentiates tellurite toxicity. Our results define a mechanism of tellurite toxicity and warrant further research on the potential use of the combination of tellurite and 5-aminolevulinic acid in antimicrobial therapy.

The mechanisms of action of the antibacterial metalloid tellurite are unclear. Here, the authors show that tellurite induces an accumulation of hydroxyl radical and intermediates of heme biosynthesis in E. coli, and that the heme precursor 5-aminolevulinic acid potentiates tellurite toxicity.

High-throughput screening of dipeptide utilization mediated by the ABC transporter DppBCDF and its substrate-binding proteins DppA1-A5 in Pseudomonas aeruginosa

In this study, we show that the dppBCDF operon of Pseudomonas aeruginosa PA14 encodes an ABC transporter responsible for the utilization of di/tripeptides. The substrate specificity of ABC transporters is determined by its associated substrate-binding proteins (SBPs). Whereas in E. coli only one protein, DppA, determines the specificity of the transporter, five orthologous SBPs, DppA1-A5 are present in P. aeruginosa. Multiple SBPs might broaden the substrate specificity by increasing the transporter capacity. We utilized the Biolog phenotype MicroArray technology to investigate utilization of di/tripeptides in mutants lacking either the transport machinery or all of the five SBPs. This high-throughput method enabled us to screen hundreds of dipeptides with various side-chains, and subsequently, to determine the substrate profile of the dipeptide permease. The substrate spectrum of the SBPs was elucidated by complementation of a penta mutant, deficient of all five SBPs, with plasmids carrying individual SBPs. It became apparent that some dipeptides were utilized with different affinity for each SBP. We found that DppA2 shows the highest flexibility on substrate recognition and that DppA2 and DppA4 have a higher tendency to utilize tripeptides. DppA5 was not able to complement the penta mutant under our screening conditions. Phaseolotoxin, a toxic tripeptide inhibiting the enzyme ornithine carbamoyltransferase, is also transported into P. aeruginosa via the DppBCDF permease. The SBP DppA1, and with much greater extend DppA3, are responsible for delivering the toxin to the permease. Our results provide a first overview of the substrate pattern of the ABC dipeptide transport machinery in P. aeruginosa.

An insight on bacterial cellular targets of photodynamic inactivation

The emergence of microbial resistance is becoming a global problem in clinical and environmental areas. As such, the development of drugs with novel modes of action will be vital to meet the threats created by the rise in microbial resistance. Microbial photodynamic inactivation is receiving considerable attention for its potentialities as a new antimicrobial treatment. This review addresses the interactions between photosensitizers and bacterial cells (binding site and cellular localization), the ultrastructural, morphological and functional changes observed at initial stages and during the course of photodynamic inactivation, the oxidative alterations in specific molecular targets, and a possible development of resistance.

Identification and characterization of outer membrane vesicle-associated proteins in Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium is a primary cause of enteric diseases and has acquired a variety of virulence factors during its evolution into a pathogen. Secreted virulence factors interact with commensal flora and host cells and enable Salmonella to survive and thrive in hostile environments. Outer membrane vesicles (OMVs) released from many Gram-negative bacteria function as a mechanism for the secretion of complex mixtures, including virulence factors. We performed a proteomic analysis of OMVs that were isolated under standard laboratory and acidic minimal medium conditions and identified 14 OMV-associated proteins that were observed in the OMV fraction isolated only under the acidic minimal medium conditions, which reproduced the nutrient-deficient intracellular milieu. The inferred roles of these 14 proteins were diverse, including transporter, enzyme, and transcriptional regulator. The absence of these proteins influenced Salmonella survival inside murine macrophages. Eleven of these proteins were predicted to possess secretion signal sequences at their N termini, and three (HupA, GlnH, and PhoN) of the proteins were found to be translocated into the cytoplasm of host cells. The comparative proteomic profiling of OMVs performed in this study revealed different protein compositions in the OMVs isolated under the two different conditions, which indicates that the OMV cargo depends on the growth conditions and provides a deeper insight into how Salmonella utilizes OMVs to adapt to environmental changes.

Photodynamic therapy based on 5-aminolevulinic acid and its use as an antimicrobial agent

Exogenous 5-aminolevulinic acid (ALA) is taken up directly by bacteria, yeasts, fungi, and some parasites, which then induces the accumulation of protoporphyrin IX (PPIX). Subsequent light irradiation of PPIX leads to the inactivation of these organisms via photodamage to their cellular structures. ALA uptake and light irradiation of PPIX produced by host cells leads to the inactivation of other parasites, along with some viruses, via the induction of an immune response. ALA-mediated PPIX production by host cells and light irradiation result in the inactivation of other viruses via either the induction of a host cell response or direct photodynamic attack on viral particles. This ALA-mediated production of light-activated PPIX has been extensively used as a form of photodynamic therapy (PDT) and has shown varying levels of efficacy in treating conditions that are associated with microbial infection, ranging from acne and verrucae to leishmaniasis and onychomycosis. However, for the treatment of some of these conditions by ALA-based PDT, the role of an antimicrobial effect has been disputed and in general, the mechanisms by which the technique inactivates microbes are not well understood. In this study, we review current understanding of the antimicrobial mechanisms used by ALA-based PDT and its role in the treatment of microbial infections along with its potential medical and nonmedical applications.

Transport of the photodynamic therapy agent 5-aminolevulinic acid by distinct H+-coupled nutrient carriers coexpressed in the small intestine

5-Aminolevulinic acid (ALA) is a prodrug used in photodynamic therapy, fluorescent diagnosis, and fluorescent-guided resection because it leads to accumulation of the photosensitizer protoporphyrin IX (PpIX) in tumor tissues. ALA has good oral bioavailability, but high oral doses are required to obtain selective PpIX accumulation in colonic tumors because accumulation is also observed in normal gut mucosa. Structural similarities between ALA and GABA led us to test the hypothesis that the H(+)-coupled amino acid transporter PAT1 (SLC36A1) will contribute to luminal ALA uptake. Radiolabel uptake and electrophysiological measurements identified PAT1-mediated H(+)-coupled ALA symport after heterologous expression in Xenopus oocytes. The selectivity of the nontransported inhibitors 5-hydroxytryptophan and 4-aminomethylbenzoic acid for, respectively, PAT1 and the H(+)-coupled di/tripeptide transporter PepT1 (SLC15A1) were examined. 5-Hydroxytryptophan selectively inhibited PAT1-mediated amino acid uptake across the brush-border membrane of the human intestinal (Caco-2) epithelium whereas 4-aminomethylbenzoic acid selectively inhibited PepT1-mediated dipeptide uptake. The inhibitory effects of 5-hydroxytryptophan and 4-aminomethylbenzoic acid were additive, demonstrating that both PAT1 and PepT1 contribute to intestinal transport of ALA. This is the first demonstration of overlap in substrate specificity between these distinct transporters for amino acids and dipeptides. PAT1 and PepT1 expression was monitored by reverse transcriptase-polymerase chain reaction using paired samples of normal and cancer tissue from human colon. mRNA for both transporters was detected. PepT1 mRNA was increased 2.3-fold in cancer tissues. Thus, increased PepT1 expression in colonic cancer could contribute to the increased PpIX accumulation observed. Selective inhibition of PAT1 could enhance PpIX loading in tumor tissue relative to that in normal tissue.

A comparative proteomic analysis of Gluconacetobacter diazotrophicus PAL5 at exponential and stationary phases of cultures in the presence of high and low levels of inorganic nitrogen compound

A proteomic view of G. diazotrophicus PAL5 at the exponential (E) and stationary phases (S) of cultures in the presence of low (L) and high levels (H) of combined nitrogen is presented. The proteomes analyzed on 2D-gels showed 131 proteins (42E+32S+29H+28L) differentially expressed by G. diazotrophicus, from which 46 were identified by combining mass spectrometry and bioinformatics tools. Proteins related to cofactor, energy and DNA metabolisms and cytoplasmic pH homeostasis were differentially expressed in E growth phase, under L and H conditions, in line with the high metabolic rate of the cells and the low pH of the media. Proteins most abundant in S-phase cells were stress associated and transporters plus transferases in agreement with the general phenomenon that binding protein-dependent systems are induced under nutrient limitation as part of hunger response. Cells grown in L condition produced nitrogen-fixation accessory proteins with roles in biosynthesis and stabilization of the nitrogenase complex plus proteins for protection of the nitrogenases from O(2)-induced inactivation. Proteins of the cell wall biogenesis apparatus were also expressed under nitrogen limitation and might function in the reshaping of the nitrogen-fixing G. diazotrophicus cells previously described. Genes whose protein products were detected in our analysis were mapped onto the chromosome and, based on the tendency of functionally related bacterial genes to cluster, we identified genes of particular pathways that could be organized in operons and are co-regulated. These results showed the great potential of proteomics to describe events in G. diazotrophicus cells by looking at proteins expressed under distinct growth conditions.

RosE represses Std fimbrial expression in Salmonella enterica serotype Typhimurium

The Salmonella enterica serotype Typhimurium (S. typhimurium) genome contains a large repertoire of putative fimbrial operons that remain poorly characterized because they are not expressed in vitro. In this study, insertions that induced expression of the putative stdABCD fimbrial operon were identified from a random bank of transposon mutants by screening with immuno-magnetic particles for ligand expression (SIMPLE). Transposon insertions upstream of csgC and lrhA or within dam, setB and STM4463 (renamed rosE) resulted in expression of StdA and its assembly into fimbrial filaments on the cell surface. RosE is a novel negative regulator of Std fimbrial expression as indicated by its repression of a std::lacZ reporter construct and by binding of the purified protein to a DNA region upstream of the stdA start codon. Expression of Std fimbriae in the rosE mutant resulted in increased attachment of S. typhimurium to human colonic epithelial cell lines (T-84 and CaCo-2). A rosE mutant exhibited a reduced ability to compete with virulent S. typhimurium for colonization of murine organs, while no defect was observed when both competing strains carried a stdAB deletion. These data suggest that a tight control of Std fimbrial expression mediated by RosE is required during host pathogen interaction.

Derivatives of 5-Aminolevulinic Acid for Photodynamic Therapy

Photodynamic therapy (PDT) is a clinical treatment that combines the effects of visible light irradiation with subsequent biochemical events that arise from the presence of a photosensitising drug (possessing no dark toxicity) to cause destruction of selected cells. Today, the most common agent used in dermatological PDT is 5-aminolevulinic acid (ALA). As a result of its hydrophilic character, ALA penetrates skin lesions poorly when applied topically. Its systemic bioavailability is limited and it is known to cause significant side effects when given orally or intravenously. Numerous chemical derivatives of ALA have been synthesised with the aims of either improving topical penetration or enhancing systemic bioavailability, while reducing side effects. In vitro cell culture experiments with ALA derivatives have yielded promising results. However, if ALA derivatives are to demonstrate meaningful clinical benefits, a rational approach to topical formulation design is required, along with a systematic study aimed at uncovering the true potential of ALA derivatives in photodynamic therapy. With respect to systemic ALA delivery, more study is required in the developing area of ALA-containing dendrons and dendrimers.

Derivatives of 5-aminolevulinic Acid for photodynamic therapy

Photodynamic therapy (PDT) is a clinical treatment that combines the effects of visible light irradiation with subsequent biochemical events that arise from the presence of a photosensitising drug (possessing no dark toxicity) to cause destruction of selected cells. Today, the most common agent used in dermatological PDT is 5-aminolevulinic acid (ALA). As a result of its hydrophilic character, ALA penetrates skin lesions poorly when applied topically. Its systemic bioavailability is limited and it is known to cause significant side effects when given orally or intravenously. Numerous chemical derivatives of ALA have been synthesised with the aims of either improving topical penetration or enhancing systemic bioavailability, while reducing side effects. In vitro cell culture experiments with ALA derivatives have yielded promising results. However, if ALA derivatives are to demonstrate meaningful clinical benefits, a rational approach to topical formulation design is required, along with a systematic study aimed at uncovering the true potential of ALA derivatives in photodynamic therapy. With respect to systemic ALA delivery, more study is required in the developing area of ALA-containing dendrons and dendrimers.

Biosynthesis of Hemes

This review is concerned specifically with the structures and biosynthesis of hemes in E. coli and serovar Typhimurium. However, inasmuch as all tetrapyrroles share a common biosynthetic pathway, much of the material covered here is applicable to tetrapyrrole biosynthesis in other organisms. Conversely, much of the available information about tetrapyrrole biosynthesis has been gained from studies of other organisms, such as plants, algae, cyanobacteria, and anoxygenic phototrophs, which synthesize large quantities of these compounds. This information is applicable to E. coli and serovar Typhimurium. Hemes play important roles as enzyme prosthetic groups in mineral nutrition, redox metabolism, and gas-and redox-modulated signal transduction. The biosynthetic steps from the earliest universal precursor, 5-aminolevulinic acid (ALA), to protoporphyrin IX-based hemes constitute the major, common portion of the pathway, and other steps leading to specific groups of products can be considered branches off the main axis. Porphobilinogen (PBG) synthase (PBGS; also known as ALA dehydratase) catalyzes the asymmetric condensation of two ALA molecules to form PBG, with the release of two molecules of H2O. Protoporphyrinogen IX oxidase (PPX) catalyzes the removal of six electrons from the tetrapyrrole macrocycle to form protoporphyrin IX in the last biosynthetic step that is common to hemes and chlorophylls. Several lines of evidence converge to support a regulatory model in which the cellular level of available or free protoheme controls the rate of heme synthesis at the level of the first step unique to heme synthesis, the formation of GSA by the action of GTR.

The sorbitol phosphotransferase system is responsible for transport of 2-C-methyl-D-erythritol into Salmonella enterica serovar typhimurium

2-C-methyl-D-erythritol 4-phosphate is the first committed intermediate in the biosynthesis of the isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate. Supplementation of the growth medium with 2-C-methyl-D-erythritol has been shown to complement disruptions in the Escherichia coli gene for 1-deoxy-D-xylulose 5-phosphate synthase, the enzyme that synthesizes the immediate precursor of 2-C-methyl-D-erythritol 4-phosphate. In order to be utilized in isoprenoid biosynthesis, 2-C-methyl-D-erythritol must be phosphorylated. We describe the construction of Salmonella enterica serovar Typhimurium strain RMC26, in which the essential gene encoding 1-deoxy-D-xylulose 5-phosphate synthase has been disrupted by insertion of a synthetic mevalonate operon consisting of the yeast ERG8, ERG12, and ERG19 genes, responsible for converting mevalonate to isopentenyl diphosphate under the control of an arabinose-inducible promoter. Random mutagenesis of RMC26 produced defects in the sorbitol phosphotransferase system that prevented the transport of 2-C-methyl-D-erythritol into the cell. RMC26 and mutant strains of RMC26 unable to grow on 2-C-methyl-D-erythritol were incubated in buffer containing mevalonate and deuterium-labeled 2-C-methyl-D-erythritol. Ubiquinone-8 was isolated from these cells and analyzed for deuterium content. Efficient incorporation of deuterium was observed for RMC26. However, there was no evidence of deuterium incorporation into the isoprenoid side chain of ubiquinone Q8 in the RMC26 mutants.

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

dpp genes of Rhizobium leguminosarum specify uptake of delta-aminolevulinic acid

An operon with homology to the dppABCDF genes required to transport dipeptides in bacteria was identified in the N2-fixing symbiont, Rhizobium leguminosarum. As in other bacteria, dpp mutants were severely affected in the import of delta-aminolevulinic acid (ALA), a heme precursor. ALA uptake was antagonized by adding dipeptides, indicating that these two classes of molecule share the same transporter. Mutations in dppABCDF did not affect symbiotic N2 fixation on peas, suggesting that the ALA needed for heme synthesis is not supplied by the plant or that another uptake system functions in the bacteroids. The dppABCDF operon of R. leguminosarum resembles that in other bacteria, with a gap between dppA and dppB containing inverted repeats that may stabilize mRNA and may explain why transcription of dppA alone was higher than that of dppBCDF. The dppABCDF promoter was mapped and is most likely recognized by sigma70.

Evidence for direct interaction between enzyme I(Ntr) and aspartokinase to regulate bacterial oligopeptide transport

Bradyrhizobium japonicum transports oligopeptides and the heme precursor delta-aminolevulinic acid (ALA) by a common mechanism. Two Tn5-induced mutants disrupted in the lysC and ptsP genes were identified based on the inability to use prolyl-glycyl-glycine as a proline source and were defective in [(14)C]ALA uptake activity. lysC and ptsP were shown to be proximal genes in the B. japonicum genome. However, RNase protection and in trans complementation analysis showed that lysC and ptsP are transcribed separately, and that both genes are involved in oligopeptide transport. Aspartokinase, encoded by lysC, catalyzes the phosphorylation of aspartate for synthesis of three amino acids, but the lysC strain is not an amino acid auxotroph. The ptsP gene encodes Enzyme I(Ntr) (EI(Ntr)), a paralogue of Enzyme I of the phosphoenolpyruvate:sugar phosphotransferase (PTS) system. In vitro pull-down experiments indicated that purified recombinant aspartokinase and EI(Ntr) interact directly with each other. Expression of ptsP in trans from a multicopy plasmid complemented the lysC mutant, suggesting that aspartokinase normally affects Enzyme I(Ntr) in a manner that can be compensated for by increasing the copy number of the ptsP gene. ATP was not a phosphoryl donor to purified EI(Ntr), but it was phosphorylated by ATP in the presence of cell extracts. This phosphorylation was inhibited in the presence of aspartokinase. The findings demonstrate a role for a PTS protein in the transport of a non-sugar solute and suggest an unusual regulatory function for aspartokinase in regulating the phosphorylation state of EI(Ntr).

OmpC is the receptor for Gifsy-1 and Gifsy-2 bacteriophages of Salmonella

Mutations in the Salmonella enterica serovar Typhimurium ompC gene conferred resistance to Gifsy-1 and Gifsy-2 bacteriophages. Selection for complementing plasmids yielded clones of ompC. Introduction of an ompC clone into Escherichia coli conferred the ability to adsorb Gifsy phage. These data show that OmpC is the receptor for Gifsy-1 and Gifsy-2 phages.

Characterization of grvA, an antivirulence gene on the gifsy-2 phage in Salmonella enterica serovar typhimurium

The lambdoid phage Gifsy-2 contributes significantly to Salmonella enterica serovar Typhimurium virulence. The phage carries the periplasmic superoxide dismutase gene, sodCI, and other unidentified virulence factors. We have characterized the gene grvA, a single open reading frame inserted in the opposite orientation in the tail operon of the Gifsy-2 phage. Contrary to what is observed with classic virulence genes, grvA null mutants were more virulent than wild type as measured by intraperitoneal competition assays in mice. We have termed this effect antivirulence. Wild-type grvA in single copy complemented this phenotype. However, grvA(+) on a multicopy plasmid also conferred the antivirulence phenotype. Neither a grvA null mutation nor the grvA(+) plasmid conferred a growth advantage or disadvantage in laboratory media. The antivirulence phenotype conferred by the grvA null mutation and the grvA(+) plasmid required wild-type sodCI but was independent of other virulence factors encoded on Gifsy-2. These results suggest that in a wild-type situation, GrvA decreases the pathogenicity of serovar Typhimurium in the host, most likely by affecting resistance to toxic oxygen species. These virulence phenotypes were independent of functional Gifsy-2 phage production. Our data suggest that the contribution of Gifsy-2 is a complicated sum of both positive virulence factors such as sodCI and antivirulence factors such as grvA.

Characterization of a group of anaerobically induced, fnr-dependent genes of Salmonella typhimurium

We have previously reported the isolation of a group of anaerobically regulated, fnr-dependent lac fusions in Salmonella typhimurium and have grouped these oxd genes into classes based on map position. In order to identify these genes, we have replaced the original Mud-lac fusion in a member of each oxd class with the much smaller Mud-cam element, cloned the fusion, and determined DNA sequence sufficient to define the oxd gene. Several of the fusions correspond to previously known genes from S. typhimurium or Escherichia coli: oxd-4 = cbiA and oxd-11 = cbiK, oxd-5 = hybB, oxd-7 = dcuB, oxd-8 = moaB, oxd-12 = dmsA, and oxd-14 = napB (aeg-46. 5). Two other fusions correspond to previously unknown loci: oxd-2 encodes an acetate/propionate kinase, and oxd-6 encodes a putative ABC transporter present in S. typhimurium but not in E. coli.

Substrate specificity of the periplasmic dipeptide-binding protein from Escherichia coli: experimental basis for the design of peptide prodrugs

Pure dipeptide-binding protein (DppA) from Escherichia coli was studied in a filter binding assay to determine its binding specificity. A substrate:DppA stoichiometry of 1:1 was found with both [14C]AlaAla and Ala[14C]Phe. Surprisingly, substrate binding did not vary over the pH range pH 3-9.5. Different dipeptides yielded liganded protein with various pI values, implying that DppA can undergo subtly different conformational changes to accommodate different substrates. Using [125I]Tyr-peptides as substrates in competition assays, the relative binding affinities for a range of dipeptides were found to parallel their overall transport rates into E. coli through the dipeptide permease (Dpp), showing that DppA alone controls the specificity of Dpp. With a series of substituted glycyl peptides, binding affinity was progressively enhanced by alkylation (with methyl to butyl) of the N-terminal alpha-amino group. Thus, results from this approach provide an essential experimental basis, which complements the information from the crystal structure of DppA, for the design of peptidomimetic antibacterials targeted for transport through Dpp.

RpoS synthesis is growth rate regulated in Salmonella typhimurium, but its turnover is not dependent on acetyl phosphate synthesis or PTS function

The RpoS sigma factor of enteric bacteria is either required for or augments the expression of a number of genes that are induced during nutrient limitation, growth into stationary phase, or in response to stresses, including high osmolarity. RpoS is regulated at multiple levels, including posttranscriptional control of its synthesis, protein turnover, and mechanisms that affect its activity directly. Here, the control of RpoS stability was investigated in Salmonella typhimurium by the isolation of a number of mutants specifically defective in RpoS turnover. These included 20 mutants defective in mviA, the ortholog of Escherichia coli rssB/sprE, and 13 mutants defective in either clpP or clpX which encode the protease active on RpoS. An hns mutant was also defective in RpoS turnover, thus confirming that S. typhimurium and E. coli have identical genetic requirements for this process. Some current models predict the existence of a kinase to phosphorylate the response regulator MviA, but no mutants affecting a kinase were recovered. An mviA mutant carrying the D58N substitution altering the predicted phosphorylation site is substantially defective, suggesting that phosphorylation of MviA on D58 is important for its function. No evidence was obtained to support models in which acetyl phosphate or the PTS system contributes to MviA phosphorylation. However, we did find a significant (fivefold) elevation of RpoS during exponential growth on acetate as the carbon and energy source. This behavior is due to growth rate-dependent regulation which increases RpoS synthesis at slower growth rates. Growth rate regulation operates at the level of RpoS synthesis and is mainly posttranscriptional but, surprisingly, is independent of hfq function.

Delta-aminolevulinic acid transport by intestinal and renal peptide transporters and its physiological and clinical implications

Delta-aminolevulinic acid (ALA) is the precursor of porphyrin synthesis and has been recently used in vitro and in clinical studies as an endogenous photosensitizer for photodynamic therapy in the treatment of various tumors. For this purpose, ALA is given topically, systemically, or orally. When administered by the oral route, it shows excellent intestinal absorption. ALA is also efficiently reabsorbed in the renal proximal tubule after glomerular filtration. However, the pathways and mechanisms for its transmembrane transport into epithelial cells of intestine and kidney are unknown. Here we demonstrate that ALA uses the intestinal and renal apical peptide transporters for entering into epithelial cells. Kinetics and characteristics of ALA transport were determined in Xenopus laevis ooyctes and Pichia pastoris yeast cells expressing either the cloned intestinal peptide transporter PEPT1 or the renal form PEPT2. By using radiolabeled ALA and electrophysiological techniques in these heterologous expression systems, we established that: (a) PEPT1 and PEPT2 translocate 3H-ALA by saturable and pH-dependent transport mechanisms, (b) that ALA and di-/tripeptides, but not GABA or related amino acids, compete at the same substrate-binding site of the carriers, and (c) that ALA transport is electrogenic in nature as a consequence of H+/ALA cotransport. Reverse transcriptase-PCR analysis performed with specific primers for PEPT1 and PEPT2 in rabbit tissues demonstrates that, in particular, the PEPT2 mRNA is expressed in a variety of other tissues including lung, brain, and mammary gland, which have been shown to accumulate ALA. This suggests that these tissues could take up the porphyrin precusor via expressed peptide transporters, providing the endogenous photosensitizers for efficient photodynamic therapy.

The group A streptococcal dipeptide permease (Dpp) is involved in the uptake of essential amino acids and affects the expression of cysteine protease

The majority of characterized bacterial dipeptide permeases (Dpp) are membrane-associated complexes of five proteins belonging to the ABC-transporter family. They have been found to be involved in the uptake of essential amino acids, haem production, chemotaxis and sporulation. A 5.8 kb genomic DNA fragment of the serotype M49 group A streptococcal (GAS) strain CS101 was sequenced and found to contain five putative GAS Dpp genes (dppA to dppE). Deduced amino acid sequences exhibited 17-54% similarity to corresponding ABC-transporter sequences. The operon organization of the five genes was confirmed by transcriptional analysis, and a shorter, more abundant, dppA-only transcript was detected similar to that found in the GAS oligopeptide permease (Opp) system. Insertional inactivation was used to create serotype M2 and M49 strains that did not express the dppD and dppEATPase genes or nearly the entire operon. In feeding experiments with di- to hexapeptides, the wild-type strain grew with each peptide tested. The dpp mutants were unable to grow on dipeptides, whereas hexapeptides did not sustain the growth of opp mutants. Expression of the dpp operon was induced approximately fourfold in late exponential growth phase. In addition, a striking increase in the dppA to dppA-E ratio from 5:1 to more than 20:1 occurred during late exponential growth phase in complex medium. Growth in chemically defined medium (CDM) supplemented with various dipeptides specifically induced the expression of dpp and reduced both the dppA to dppA-E and oppA to oppA-F mRNA ratios. Expression of the virulence factor SpeB (major cysteine protease) was reduced eightfold in dpp mutants, whereas dpp expression was decreased about fourfold in a Mga virulence regulator mutant. Taken together, these data indicate a correlation between levels of intracellular essential amino acids and the regulation of virulence factor expression.

H-dependent formation of 5-aminolaevulinic acid-induced protoporphyrin IX in fibrosarcoma cells

pH-Dependent variations in the fluorescence intensity of 5-aminolaevulinic acid-induced 5-aminolaevulinic acid protoporphyrin IX (PP IX) were compared with the cell viability following light irradiation. The fluorescence intensity was determined by flow cytometry and the cell activity was investigated by a colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay 24 h after photodynamic treatment. The results obtained by fluorescence measurements clearly showed that pH values of the incubation medium containing (5-ALA) below and above pH 7.4 led to a significant decrease in the fluorescence intensity. The viability of cells incubated with 0.6 mM 5-ALA in a medium at pH 6.0 was unaffected on exposure to light at lambda = 635 nm up to 15 J cm-2. However, cells incubated at pH 7.4 (with the other treatment parameters the same) were nearly completely destroyed. In addition, depletion of intracellular PPIX was faster in physiological medium than in acid medium.

Avirulence of LT2 strains of Salmonella typhimurium results from a defective rpoS gene

In order to identify the genetic basis for the attenuation of Salmonella typhimurium LT2 strains, experiments were performed to identify a gene(s) which restores virulence to an avirulent LT2 strain. These and further experiments confirmed that an rpoS mutation is the sole determinant of the attenuation of S. typhimurium LT2.

Regulation of heme biosynthesis in Salmonella typhimurium: activity of glutamyl-tRNA reductase (HemA) is greatly elevated during heme limitation by a mechanism which increases abundance of the protein

In Salmonella typhimurium and Escherichia coli, the hemA gene encodes the enzyme glutamyl-tRNA reductase, which catalyzes the first committed step in heme biosynthesis. We report that when heme limitation is imposed on cultures of S. typhimurium, glutamyl-tRNA reductase (HemA) enzyme activity is increased 10- to 25-fold. Heme limitation was achieved by a complete starvation for heme in hemB, hemE, and hemH mutants or during exponential growth of a hemL mutant in the absence of heme supplementation. Equivalent results were obtained by both methods. To determine the basis for this induction, we developed a panel of monoclonal antibodies reactive with HemA, which can detect the small amount of protein present in a wild-type strain. Western blot (immunoblot) analysis with these antibodies reveals that the increase in HemA enzyme activity during heme limitation is mediated by an increase in the abundance of the HemA protein. Increased HemA protein levels were also observed in heme-limited cells of a hemL mutant in two different E. coli backgrounds, suggesting that the observed regulation is conserved between E. coli and S. typhimurium. In S. typhimurium, the increase in HemA enzyme and protein levels was accompanied by a minimal (less than twofold) increase in the expression of hemA-lac operon fusions; thus HemA regulation is mediated either at a posttranscriptional step or through modulation of protein stability.

Identification of the lrp gene in Bradyrhizobium japonicum and its role in regulation of delta-aminolevulinic acid uptake

The heme precursor delta-aminolevulinic acid (ALA) is taken up by the dipeptide permease (Dpp) system in Escherichia coli. In this study, we identified a Bradyrhizobium japonicum genomic library clone that complemented both ALA and dipeptide uptake activities in E. coli dpp mutants. The complementing B. japonicum DNA encoded a product with 58% identity to the E. coli global transcriptional regulator Lrp (leucine-responsive regulatory protein), implying the presence of Dpp-independent ALA uptake activity in those cells. Data support the conclusion that the Lrp homolog induced the oligopeptide permease system in the complemented cells by interfering with the repressor activity of the endogenous Lrp, thus conferring oligopeptide and ALA uptake activities. ALA uptake by B. japonicum was effectively inhibited by a tripeptide and, to a lesser extent, by a dipeptide, and a mutant strain that expressed the lrp homolog from a constitutive promoter was deficient in ALA uptake activity. The data show that Lrp negatively affects ALA uptake in E. coli and B. japonicum. Furthermore, the product of the isolated B. japonicum gene is both a functional and structural homolog of E. coli Lrp, and thus the regulator is not restricted to enteric bacteria.

Efficient translation of the RpoS sigma factor in Salmonella typhimurium requires host factor I, an RNA-binding protein encoded by the hfq gene

The RpoS transcription factor (also called sigma Sor sigma 38) is required for the expression of a number of stationary-phase and osmotically inducible genes in Escherichia coli. RpoS is also a virulence factor for several pathogenic bacteria, including Salmonella typhimurium. The activity of RpoS is regulated in response to several different signals, at the transcriptional and translational levels as well as by proteolysis. Here we report that host factor I (HF-I), the product of the hfq gene, is required for efficient expression of rpoS in S. typhimurium. HF-I is a small, heat-stable, site-specific RNA-binding protein originally characterized for its role in replication of the RNA bacteriophage Q beta of E. coli. Its role in the uninfected bacterial cell has previously been unknown. Assays of Beta-galactosidase in strains with rpoS-lac fusions, Western blot (immunoblot) analysis, and pulse-labeling and immunoprecipitation of both fusion proteins and native RpoS show that an S. typhimurium hfq mutant has a four- to sevenfold reduction in expression of rpoS that is attributable primarily to a defect in translation. These results add a new level of complexity to the regulation of RpoS activity.

Evidence that SbcB and RecF pathway functions contribute to RecBCD-dependent transductional recombination

A role for the RecF, RecJ, and SbcB proteins in the RecBCD-dependent recombination pathway is suggested on the basis of the effect of null recF, recJ, and sbcB mutations in Salmonella typhimurium on a "short-homology" P22 transduction assay. The assay requires recombination within short (approximately 3-kb) sequences that flank the selected marker and lie at the ends of the transduced fragment. Since these ends are subject to exonucleolytic degradation, the assay may demand rapid recombination by requiring that the exchange be completed before the essential recombining sequences are degraded. In this assay, recF, recJ, and sbcB null mutations, tested individually, cause a small decrease in recombinant recovery but all pairwise combinations of these mutations cause a 10- to 30-fold reduction. In a recD mutant recipient, which shows increased recombination, these pairwise mutation combinations cause a 100-fold reduction in recombinant recovery. In a standard transduction assay (about 20 kb of flanking sequence), recF, recJ, and sbcB mutations have a very small effect on recombinant frequency. We suggest that these three proteins promote a rate-limiting step in the RecBC-dependent recombination process. The above results were obtained with a lysogenic recipient strain which represses expression of superinfecting phage genomes and minimizes the contribution of phage recombination functions. When a nonlysogenic recipient strain is used, coinfecting phage genomes express functions that alter the genetic requirements for recombination in the short-homology assay.

Pharmacokinetics of 5-aminolevulinic-acid-induced porphyrins in tumour-bearing mice

Photodynamic therapy and photodynamic diagnosis help to support efficient treatment of superficial and early-stage cancer. During the last few years, 5-aminolevulinic acid (5-ALA), a precursor of haemoglobin in the haem biosynthetic pathway, was used to stimulate endogenous porphyrin production. In the following the time dependence of 5-ALA-induced porphyrin concentration will be investigated on several tissues in an in-vivo tumour model. 5-ALA was administered intravenously at a concentration of 50 mg-1 body weight. According to a certain time schedule the animals were sacrificed and 12 different organs as well as the tumour were removed. During excitation with the violet light of a Kr+ laser, porphyrin fluorescence spectra in the range 550-750 nm could be detected on the tissue samples. The intensity of the emission spectra at lambda = 635 +/- 2 nm was taken as a measure of the porphyrin concentration. All tissues showed porphyrin fluorescence. Brightest fluorescence was found on the tumour. A maximum contrast of the fluorescence intensity between the tumour and the non-malignant organs of up to 30 was observed at 4-6 h post-injection. The kinetics of the porphyrin concentration depend on the organ. Simple mathematical models will be derived and discussed.

The Rhizobial hemA Gene Is Required for Symbiosis in Species with Deficient [delta]-Aminolevulinic Acid Uptake Activity

Most rhizobial hemA mutants induce root nodules on their respective legume hosts that lack nitrogen fixation activity and leghemoglobin expression. However, a Bradyrhizobium japonicum hemA mutant elicits effective nodules on soybean, and we proposed previously that synthesis and uptake of the heme precursor [delta]-aminolevulinic acid (ALA) by the plant and bacterial symbiont, respectively, allow mutant rescue (I. Sangwan, M.R. O'Brian [1991] Science 251: 1220-1222). In the present work, the B. japonicum hemA mutant MLG1 elicited normal nodules on three hosts, including cowpea, a plant that is not effectively nodulated by a hemA mutant of Rhizobium sp. These data indicate that B. japonicum rather than soybean possesses the unique trait that allows normal nodule development by a hemA mutant. Cowpea expressed glutamate-dependent ALA formation activity in nodules induced by B. japonicum strains I110 or MLG1 and by Rhizobium sp. ANU240. Exogenous ALA was taken up by B. japonicum bacteroids isolated from soybean or cowpea nodules, and the kinetics of uptake were biphasic. By comparison, Rhizobium sp. ANU240 had very low ALA uptake activity. In addition, ALA uptake was observed in cultured cells of B. japonicum but not in cultured cells of three other rhizobial species tested. We suggest that the differential success of legume-rhizobial hemA symbioses is due to an ALA uptake activity in B. japonicum that is deficient in other rhizobia, thereby further validating the ALA rescue hypothesis.

Genetic map of Salmonella typhimurium, edition VIII

We present edition VIII of the genetic map of Salmonella typhimurium LT2. We list a total of 1,159 genes, 1,080 of which have been located on the circular chromosome and 29 of which are on pSLT, the 90-kb plasmid usually found in LT2 lines. The remaining 50 genes are not yet mapped. The coordinate system used in this edition is neither minutes of transfer time in conjugation crosses nor units representing "phage lengths" of DNA of the transducing phage P22, as used in earlier editions, but centisomes and kilobases based on physical analysis of the lengths of DNA segments between genes. Some of these lengths have been determined by digestion of DNA by rare-cutting endonucleases and separation of fragments by pulsed-field gel electrophoresis. Other lengths have been determined by analysis of DNA sequences in GenBank. We have constructed StySeq1, which incorporates all Salmonella DNA sequence data known to us. StySeq1 comprises over 548 kb of nonredundant chromosomal genomic sequences, representing 11.4% of the chromosome, which is estimated to be just over 4,800 kb in length. Most of these sequences were assigned locations on the chromosome, in some cases by analogy with mapped Escherichia coli sequences.

Transcriptional control of the nuo operon which encodes the energy-conserving NADH dehydrogenase of Salmonella typhimurium

The 14 nuo genes encode the subunits of the type I (energy-conserving) NADH dehydrogenase, a key component of the respiratory chain. Salmonella typhimurium, like Escherichia coli, has two enzymes that can oxidize NADH and transfer electrons to ubiquinone, but only the type I enzyme translocates protons across the membrane to generate a proton motive force. Cells with the type I enzyme are energetically more efficient; the role of the type II enzyme (encoded by ndh) is not established, but it may function like a relief valve to allow more rapid NADH recycling. Here, we have investigated transcription of the nuo gene cluster, primarily in S. typhimurium. Studies with polar insertion mutants demonstrate that these genes are arranged as a single, large operon that is expressed from a complex promoter region upstream of nuoA. The DNA sequence of the promoter region was determined, and primer extension analysis of nuo transcripts was used to map four major RNA 5' ends to this region. A set of lac operon fusions to various DNA segments from the nuo promoter region was also constructed. Analysis of these fusions confirmed the presence of at least two nuo promoters. Mutations in the global regulatory genes arcA, oxrA (fnr), crp, cya, and katF were tested for effects on expression of the nuo operon. However, none of the mutations tested had a large effect on expression of type I NADH dehydrogenase.

MudSacI, a transposon with strong selectable and counterselectable markers: use for rapid mapping of chromosomal mutations in Salmonella typhimurium

The transposable bacteriophage Mu and its mini-Mu derivatives are useful tools for the genetic analysis of many bacteria. A variety of antibiotic-resistant Mu derivatives have been constructed, allowing direct selection for cells which contain the transposon. However, in many cases a counterselection against the transposon would greatly facilitate further genetic analysis. In this paper we report the construction of MudSacI, a mini-Mu derived transposon containing the sacB (secretory levansucrase) gene of Bacillus subtilis, which confers sucrose sensitivity upon gram-negative bacteria. We describe the use of this transposon as a tool for rapid genetic mapping of chromosomal genes in Salmonella typhimurium. Simple modifications of this approach should facilitate rapid mapping in many other bacteria as well.

A di- and tripeptide transport system can supply Listeria monocytogenes Scott A with amino acids essential for growth

Listeria monocytogenes takes up di- and tripeptides via a proton motive force-dependent carrier protein. This peptide transport system resembles the recently cloned and sequenced secondary di- and tripeptide transport system of Lactococcus lactis (A. Hagting, E. R. S. Kunji, K. J. Leenhouts, B. Poolman, and W. N. Konings, J. Biol. Chem. 269:11391-11399, 1994). The peptide permease of L. monocytogenes has a broad substrate specificity and allows transport of the nonpeptide substrate 5-aminolevulinic acid, the toxic di- and tripeptide analogs, alanyl-beta-chloroalanine and alanyl-alanyl-beta-chloroalanine, and various di- and tripeptides. No extracellular peptide hydrolysis was detected, indicating that peptides are hydrolyzed after being transported into the cell. Indeed, peptidase activities in response to various synthetic substrates were detected in cell extracts obtained from L. monocytogenes cells grown in brain heart infusion broth or defined medium. The di- and tripeptide permease can supply L. monocytogenes with essential amino acids for growth and might contribute to growth of this pathogen in various foods where peptides are supplied by proteolytic activity of other microorganisms present in these foods. Possible roles of this di- and tripeptide transport system in the osmoregulation and virulence of L. monocytogenes are discussed.

The dipeptide permease of Escherichia coli closely resembles other bacterial transport systems and shows growth-phase-dependent expression

The dipeptide permease (Dpp) of Escherichia coli transports peptides consisting of two or three L-amino acids. The periplasmic dipeptide-binding protein (DBP), encoded by the dppA gene, also serves as a chemoreceptor. We sequenced the dpp locus, which comprises an operon of five genes, dppABCDE. Its organization is the same as the oligopeptide permease (opp) operon of Salmonella typhimurium and the spo0K operon of Bacillus subtilis. The dpp genes are also closely related to the hbpA gene, which encodes a haem-binding lipoprotein, and four other genes in an unlinked operon of unknown function in Haemophilus influenzae. Each Dpp protein has an Opp, Spo0K and H. influenzae homologue. Transcription of the dpp operon initiates 165 bases upstream of the predicted dppA start codon. The start site for transcription is preceded by potential -35 and -10 regions of a sigma 70 promoter. During exponential growth in Luria-Bertani (LB) broth, the level of dpp mRNA increases in two steps, one between A590 0.2 and 0.4 and one between A590 0.7 and 1.0. The 310 nucleotides between dppA and dppB include a RIP (repetitive IHF-binding palindromic) element, whose deletion from a multi-copy plasmid causes fivefold and 10-fold reductions in the levels of upstream and downstream dpp mRNA, respectively.

Cloning, DNA sequence, and complementation analysis of the Salmonella typhimurium hemN gene encoding a putative oxygen-independent coproporphyrinogen III oxidase

Coproporphyrinogen oxidation is a last step in heme biosynthesis. The biochemically characterized eukaryotic coproporphyrinogen III oxidases have an obligate requirement for molecular oxygen, and a similar enzyme is encoded by the hemF gene in Salmonella typhimurium. Anaerobic heme synthesis requires an oxygen-independent coproporphyrinogen oxidase, which is probably encoded by the hemN gene in S. typhimurium. The hemN gene has been cloned from an insertion mutant. The nucleotide sequence was obtained and used for PCR amplification of the wild-type gene. A single open reading frame was identified as the hemN gene on the basis of its interruption by the insertion mutation and plasmid complementation studies of hemF hemN double mutants. The predicted HemN protein has 38% amino acid sequence identity to a putative anaerobic Rhodobacter sphaeroides coproporphyrinogen oxidase. The hemN RNA 5' end and the inferred transcription initiation site were mapped by primer extension. The 52.8-kDa HemN protein is expressed from the second ATG codon of the hemN open reading frame. An open reading frame with an unknown function directly upstream of hemN has a striking amino acid sequence, including 11 acidic residues in a row.

Mutants defective in the energy-conserving NADH dehydrogenase of Salmonella typhimurium identified by a decrease in energy-dependent proteolysis after carbon starvation

NADH dehydrogenase is the first component of the respiratory chain. It transfers electrons from NADH to ubiquinone and concomitantly establishes a proton motive force across the membrane. Salmonella typhimurium mutants defective in this enzyme were isolated in a screen for strains with increased expression of beta-galactosidase from a hemA-lacZ protein fusion. This unexpected phenotype results from stabilization of the hybrid protein during carbon starvation and is apparently due to an energy requirement for proteolytic attack. Sequence analysis of DNA fragments cloned from an insertion mutant indicates that S. typhimurium has a large cluster of genes encoding the energy-conserving NADH dehydrogenase, similar to one recently described in Paracoccus denitrificans. These findings establish the potential for genetic analysis of a complex enzyme whose function, especially in proton efflux, is poorly understood.