PorA represents the major cell wall channel of the Gram-positive bacterium Corynebacterium glutamicum

Untargeted metabolomics coupled with genomics in the study of sucrose and xylose metabolism in Pectobacterium betavasculorum

Introduction

Pectobacterium betavasculorum is a member of the Pectobacerium genus that inhabits a variety of niches and is found in all climates. Bacteria from the Pectobacterium genus can cause soft rot disease on various plants due to the secretion of plant cell wall degrading enzymes (PCWDEs). The species P. betavasculorum is responsible for the vascular necrosis of sugar beet and soft rot of many vegetables. It also infects sunflowers and artichokes. The main sugar present in sugar beet is sucrose while xylose is one of the main sugars in artichoke and sunflower.

Methods

In our work, we applied metabolomic studies coupled with genomics to investigate the metabolism of P. betavasculorum in the presence of xylose and sucrose as the only carbon source. The ability of the strains to use various sugars as the only carbon source were confirmed by the polypyridyl complex of Ru(II) method in 96-well plates.

Results

Our studies provided information on the metabolic pathways active during the degradation of those substrates. It was observed that different metabolic pathways are upregulated in the presence of xylose in comparison to sucrose.

Discussion

The presence of xylose enhances extracellular metabolism of sugars and glycerol as well as stimulates EPS and IPS synthesis. In contrast, in the presence of sucrose the intensive extracellular metabolism of amines and amino acids is promoted.

A Target Engagement Assay to Assess Uptake, Potency, and Retention of Antibiotics in Living Bacteria

New antibiotics are urgently needed to counter the emergence of antimicrobial-resistant pathogenic bacteria. A major challenge in antibiotic drug discovery is to turn potent biochemical inhibitors of essential bacterial components into effective antimicrobials. This difficulty is underpinned by a lack of methods to investigate the physicochemical properties needed for candidate antibiotics to permeate the bacterial cell envelope and avoid clearance by the action of bacterial efflux pumps. To address these issues, here we used a target engagement assay to measure the equilibrium and kinetic binding parameters of antibiotics targeting dihydrofolate reductase (DHFR) in live bacteria. We also used this assay to identify novel DHFR ligands having antimicrobial activity. We validated this approach using the Gram-negative bacteria Escherichia coli and the emerging human pathogen Mycobacterium abscessus. We expect the use of target engagement assays in bacteria to expedite the discovery and progression of novel, cell-permeable antibiotics with on-target activity.

Modulation of host cellular responses by gram-negative bacterial porins

The outer membrane of a gram-negative bacteria encapsulates the plasma membrane thereby protecting it from the harsh external environment. This membrane acts as a sieving barrier due to the presence of special membrane-spanning proteins called "porins." These porins are β-barrel channel proteins that allow the passive transport of hydrophilic molecules and are impermeable to large and charged molecules. Many porins form trimers in the outer membrane. They are abundantly present on the bacterial surface and therefore play various significant roles in the host-bacteria interactions. These include the roles of porins in the adhesion and virulence mechanisms necessary for the pathogenesis, along with providing resistance to the bacteria against the antimicrobial substances. They also act as the receptors for phage and complement proteins and are involved in modulating the host cellular responses. In addition, the potential use of porins as adjuvants, vaccine candidates, therapeutic targets, and biomarkers is now being exploited. In this review, we focus briefly on the structure of the porins along with their important functions and roles in the host-bacteria interactions.

Factors Determining the Susceptibility of Bacteria to Antibacterial Photodynamic Inactivation

Photodynamic inactivation of microorganisms (aPDI) is an excellent method to destroy antibiotic-resistant microbial isolates. The use of an exogenous photosensitizer or irradiation of microbial cells already equipped with endogenous photosensitizers makes aPDI a convenient tool for treating the infections whenever technical light delivery is possible. Currently, aPDI research carried out on a vast repertoire of depending on the photosensitizer used, the target microorganism, and the light delivery system shows efficacy mostly on in vitro models. The search for mechanisms underlying different responses to photodynamic inactivation of microorganisms is an essential issue in aPDI because one niche (e.g., infection site in a human body) may have bacterial subpopulations that will exhibit different susceptibility. Rapidly growing bacteria are probably more susceptible to aPDI than persister cells. Some subpopulations can produce more antioxidant enzymes or have better performance due to efficient efflux pumps. The ultimate goal was and still is to identify and characterize molecular features that drive the efficacy of antimicrobial photodynamic inactivation. To this end, we examined several genetic and biochemical characteristics, including the presence of individual genetic elements, protein activity, cell membrane content and its physical properties, the localization of the photosensitizer, with the result that some of them are important and others do not appear to play a crucial role in the process of aPDI. In the review, we would like to provide an overview of the factors studied so far in our group and others that contributed to the aPDI process at the cellular level. We want to challenge the question, is there a general pattern of molecular characterization of aPDI effectiveness? Or is it more likely that a photosensitizer-specific pattern of molecular characteristics of aPDI efficacy will occur?

First access to a mycolic acid-based bioorthogonal reporter for the study of the mycomembrane and mycoloyltransferases in Corynebacteria

In this study we describe the first synthesis of an alkyne-based trehalose monomycolate probe closely mimicking the complex pattern of mycolic acids and its utility for the study of mycomembrane and mycoloyltransferases in Corynebacteria.

Identification and characterization of smallest pore-forming protein in the cell wall of pathogenic Corynebacterium urealyticum DSM 7109

Background

Corynebacterium urealyticum, a pathogenic, multidrug resistant member of the mycolata, is known as causative agent of urinary tract infections although it is a bacterium of the skin flora. This pathogenic bacterium shares with the mycolata the property of having an unusual cell envelope composition and architecture, typical for the genus Corynebacterium. The cell wall of members of the mycolata contains channel-forming proteins for the uptake of solutes.

Results

In this study, we provide novel information on the identification and characterization of a pore-forming protein in the cell wall of C. urealyticum DSM 7109. Detergent extracts of whole C. urealyticum cultures formed in lipid bilayer membranes slightly cation-selective pores with a single-channel conductance of 1.75 nS in 1 M KCl. Experiments with different salts and non-electrolytes suggested that the cell wall pore of C. urealyticum is wide and water-filled and has a diameter of about 1.8 nm. Molecular modelling and dynamics has been performed to obtain a model of the pore. For the search of the gene coding for the cell wall pore of C. urealyticum we looked in the known genome of C. urealyticum for a similar chromosomal localization of the porin gene to known porH and porA genes of other Corynebacterium strains. Three genes are located between the genes coding for GroEL2 and polyphosphate kinase (PKK2). Two of the genes (cur_1714 and cur_1715) were expressed in different constructs in C. glutamicum ΔporAΔporH and in porin-deficient BL21 DE3 Omp8 E. coli strains. The results suggested that the gene cur_1714 codes alone for the cell wall channel. The cell wall porin of C. urealyticum termed PorACur was purified to homogeneity using different biochemical methods and had an apparent molecular mass of about 4 kDa on tricine-containing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

Conclusions

Biophysical characterization of the purified protein (PorACur) suggested indeed that cur_1714 is the gene coding for the pore-forming protein in C. urealyticum because the protein formed in lipid bilayer experiments the same pores as the detergent extract of whole cells. The study is the first report of a cell wall channel in the pathogenic C. urealyticum.

Identification and characterization of a cell wall porin from Gordonia jacobaea

Gordonia jacobaea is a bacterium belonging to the mycolata group characterized by its ability to produce carotenoids. Mycolic acids in the cell wall contribute to reducing the permeability of their envelopes requiring the presence of channel-forming proteins to allow the exchange of hydrophilic molecules with the surrounding medium. Identification and purification of the channel-forming proteins was accomplished by SDS-PAGE, Mass spectrometry and Mass peptide fingerprinting and the channel-forming activity was studied by reconstitution in lipid bilayers. Here, we describe for the first time the presence of a cell-wall protein from G. jacobaea with channel-forming activity. Our results suggest that this protein bears a low similarity to other hypothetical proteins from the genus Gordonia of uncharacterized functions. The channel has an average single-channel conductance of 800 pS in 1 M KCl, is moderately anion-selective, and does not show any voltage dependence for voltages between +100 and -100 mV. The channel characteristics suggest that this protein could be of relevance in the import and export of negatively charged molecules across the cell wall. This could contribute to design treatments for mycobacterial infections, as well as being of interest in biotechnology applications.

Click-chemistry approach to study mycoloylated proteins: Evidence for PorB and PorC porins mycoloylation in Corynebacterium glutamicum

Protein mycoloylation is a recently identified, new form of protein acylation. This post-translational modification consists in the covalent attachment of mycolic acids residues to serine. Mycolic acids are long chain, α-branched, β-hydroxylated fatty acids that are exclusively found in the cell envelope of Corynebacteriales, a bacterial order that includes important genera such as Mycobacterium, Nocardia or Corynebacterium. So far, only 3 mycoloylated proteins have been identified: PorA, PorH and ProtX from C. glutamicum. Whereas the identity and function of ProtX is unknown, PorH and PorA associate to form a membrane channel, the activity of which is dependent upon PorA mycoloylation. However, the exact role of mycoloylation and the generality of this phenomenon are still unknown. In particular, the identity of other mycoloylated proteins, if any, needs to be determined together with establishing whether such modification occurs in Corynebacteriales genera other than Corynebacterium. Here, we tested whether a metabolic labeling and click-chemistry approach could be used to detect mycoloylated proteins. Using a fatty acid alkyne analogue, we could indeed label PorA, PorH and ProtX and determine ProtX mycoloylation site. Importantly, we also show that two other porins from C. glutamicum, PorB and PorC are mycoloylated.

Corynebacterium jeikeium jk0268 constitutes for the 40 amino acid long PorACj, which forms a homooligomeric and anion-selective cell wall channel

Corynebacterium jeikeium, a resident of human skin, is often associated with multidrug resistant nosocomial infections in immunodepressed patients. C. jeikeium K411 belongs to mycolic acid-containing actinomycetes, the mycolata and contains a channel-forming protein as judged from reconstitution experiments with artificial lipid bilayer experiments. The channel-forming protein was present in detergent treated cell walls and in extracts of whole cells using organic solvents. A gene coding for a 40 amino acid long polypeptide possibly responsible for the pore-forming activity was identified in the known genome of C. jeikeium by its similar chromosomal localization to known porH and porA genes of other Corynebacterium strains. The gene jk0268 was expressed in a porin deficient Corynebacterium glutamicum strain. For purification temporarily histidine-tailed or with a GST-tag at the N-terminus, the homogeneous protein caused channel-forming activity with an average conductance of 1.25 nS in 1M KCl identical to the channels formed by the detergent extracts. Zero-current membrane potential measurements of the voltage dependent channel implied selectivity for anions. This preference is according to single-channel analysis caused by some excess of cationic charges located in the channel lumen formed by oligomeric alpha-helical wheels. The channel has a suggested diameter of 1.4 nm as judged from the permeability of different sized hydrated anions using the Renkin correction factor. Surprisingly, the genome of C. jeikeium contained only one gene coding for a cell wall channel of the PorA/PorH type found in other Corynebacterium species. The possible evolutionary relationship between the heterooligomeric channels formed by certain Corynebacterium strains and the homooligomeric pore of C. jeikeium is discussed.

Identification and characterization of the channel-forming protein in the cell wall of Corynebacterium amycolatum

The mycolic-acid layer of certain gram-positive bacteria, the mycolata, represents an additional permeability barrier for the permeation of small water-soluble solutes. Consequently, it was shown in recent years that the mycolic acid layer of individual bacteria of the group mycolata contains pores, called porins, for the passage of hydrophilic solutes. Corynebacterium amycolatum, a pathogenic Corynebacterium species, belongs to the Corynebacteriaceae family but it lacks corynomycolic acids in its cell wall. Despite the absence of corynomycolic acids the cell wall of C. amycolatum contains a cation-selective cell wall channel, which may be responsible for the limited permeability of the cell wall of C. amycolatum. Based on partial sequencing of the protein responsible for channel formation derived from C. amycolatum ATCC 49368 we were able to identify the gene coram0001_1986 within the known genome sequence of C. amycolatum SK46 that codes for the cell wall channel. The corresponding gene of C. amycolatum ATCC 49368 was cloned into the plasmid pXHis for its expression in Corynebacterium glutamicum ∆porA∆porH. Biophysical characterization of the purified protein (PorAcoram) suggested that coram0001_1986 is indeed the gene coding for the pore-forming protein PorAcoram in C. amycolatum ATCC 49368. The protein belongs to the DUF (Domains of Unknown Function) 3068 superfamily of proteins, mainly found in bacteria from the family Corynebacteriaceae. The nearest relative to PorAcoram within this family is an ORF which codes for PorAcres, which was also recognized in reconstitution experiments as a channel-forming protein in Corynebacterium resistens.

Cell envelope of corynebacteria: structure and influence on pathogenicity

To date the genus Corynebacterium comprises 88 species. More than half of these are connected to human and animal infections, with the most prominent member of the pathogenic species being Corynebacterium diphtheriae, which is also the type species of the genus. Corynebacterium species are characterized by a complex cell wall architecture: the plasma membrane of these bacteria is followed by a peptidoglycan layer, which itself is covalently linked to a polymer of arabinogalactan. Bound to this, an outer layer of mycolic acids is found which is functionally equivalent to the outer membrane of Gram-negative bacteria. As final layer, free polysaccharides, glycolipids, and proteins are found. The composition of the different substructures of the corynebacterial cell envelope and their influence on pathogenicity are discussed in this paper.

Biochemical disclosure of the mycolate outer membrane of Corynebacterium glutamicum

Corynebacterineae is a specific suborder of Gram-positive bacteria that includes Mycobacterium tuberculosis and Corynebacterium glutamicum. The cell wall of these bacteria is composed of a heteropolymer of peptidoglycan (PG) linked to arabinogalactan (AG), which in turn is covalently associated with an atypical outer membrane, here called mycomembrane (M). The latter structure has been visualized by cryo-electron microscopy of vitreous sections, but its biochemical composition is still poorly defined, thereby hampering the elucidation of its physiological function. In this report, we show for the first time that the mycomembrane-linked heteropolymer of PG and AG (M-AG-PG) of C. glutamicum can be physically separated from the inner membrane on a flotation density gradient. Analysis of purified M-AG-PG showed that the lipids that composed the mycomembrane consisted almost exclusively of mycolic acid derivatives, with only a tiny amount, if any, of phospholipids and lipomannans, which were found with the characteristic lipoarabinomannans in the plasma membrane. Proteins associated with or inserted in the mycomembrane were extracted from M-AG-PG with lauryl-dimethylamine-oxide (LDAO), loaded on an SDS-PAGE gel, and analyzed by tandem mass spectrometry or by Western blotting. Sixty-eight different proteins were identified, 19 of which were also found in mycomembrane fragments released by the terminal-arabinosyl-transferase-defective ΔAftB strain. Almost all of them are predicted to contain a signal sequence and to adopt the characteristic β-barrel structure of Gram-negative outer membrane proteins. These presumed mycomembrane proteins include the already-known pore-forming proteins (PorA and PorB), 5 mycoloyltransferases (cMytA, cMytB, cMytC, cMytD, and cMytF), several lipoproteins, and unknown proteins typified by a putative C-terminal hydrophobic anchor.

Quantitative lipid composition of cell envelopes of Corynebacterium glutamicum elucidated through reverse micelle extraction

Cells of the Corynebacterium-Nocardia-Mycobacterium group of bacteria are surrounded by an outer membrane (OM) containing mycolic acids that are covalently linked to the underlying arabinogalactan-peptidoglycan complex. This OM presumably acts as a permeability barrier that imparts high levels of intrinsic drug resistance to some members of this group, such as Mycobacterium tuberculosis, and its component lipids have been studied intensively in a qualitative manner over the years. However, the quantitative lipid composition of this membrane has remained obscure, mainly because of difficulties in isolating it without contamination from the inner cytoplasmic membrane. Here we use the extraction, with reverse surfactant micelles, of intact cells of Corynebacterium glutamicum and show that this method extracts the free OM lipids quantitatively with no contamination from lipids of the cytoplasmic membrane, such as phosphatidylglycerol. Although only small amounts of corynomycolate were esterified to arabinogalactan, a large amount of cardiolipin was present in a nonextractable form, tightly associated, possibly covalently, with the peptidoglycan-arabinogalactan complex. Furthermore, we show that the OM contains just enough lipid hydrocarbons to produce a bilayer covering the cell surface, with its inner leaflet composed mainly of the aforementioned nonextractable cardiolipin and its outer leaflet composed of trehalose dimycolates, phosphatidylinositol mannosides, and highly apolar lipids, similar to the Minnikin model of 1982. The reverse micelle extraction method is also useful for extracting proteins associated with the OM, such as porins.

Reconstitution experiments and gene deletions reveal the existence of two-component major cell wall channels in the genus Corynebacterium

Two small polypeptides, PorA and PorH, are known to form cell wall channels in Corynebacterium glutamicum and in Corynebacterium efficiens. The genes coding for both polypeptides are localized in close proximity to one another between the genes coding for GroEl2 and a polyphosphate kinase (PKK2). In this study, we investigated the relationship of PorA and PorH to one another. The results suggested that the major cell wall channels of Corynebacterium glutamicum, Corynebacterium efficiens, and Corynebacterium diphtheriae need the obligatory presence of two distinct polypeptides, one of class PorA and one of class PorH, to form an active cell wall channel. Identification of genes coding for homologous proteins in the chromosome of Corynebacterium callunae suggested a similar result for this strain. Contrary to our previous reports on channel-forming proteins in these strains, a heterooligomeric structure composed of PorA and PorH is needed in all of them to form the major cell wall channel. This was concluded from complementation experiments using a porH- and porA-deficient C. glutamicum strain. The stringent necessity of proteins of either class to recover the wild-type channels was demonstrated by black lipid bilayer experiments using detergent or organic solvent extracts of the complemented porH- and porA-deficient C. glutamicum strain. The channel-forming capability of recombinant expressed, affinity-purified PorA and PorH proteins of C. glutamicum revealed that the channels consisted solely of these two components. This agreed with results obtained from a transcript coding for both channel-forming components identified in C. glutamicum by Northern blot analysis and reverse transcription-PCR analysis. The transcription start point of the genes was determined by the rapid amplification of cDNA ends approach, allowing the prediction of the -35 and -10 regions of the promoter. The results demonstrate that the cell wall channels within the genus Corynebacterium may be formed by two-component oligomers.

Physiological adaptation of Corynebacterium glutamicum to benzoate as alternative carbon source - a membrane proteome-centric view

The ability of microorganisms to assimilate aromatic substances as alternative carbon sources is the basis of biodegradation of natural as well as industrial aromatic compounds. In this study, Corynebacterium glutamicum was grown on benzoate as sole carbon and energy source. To extend the scarce knowledge about physiological adaptation processes occurring in this cell compartment, the membrane proteome was investigated under quantitative and qualitative aspects by applying shotgun proteomics to reach a comprehensive survey. Membrane proteins were relatively quantified using an internal standard metabolically labeled with (15)N. Altogether, 40 proteins were found to change their abundance during growth on benzoate in comparison to glucose. A global adaptation was observed in the membrane of benzoate-grown cells, characterized by increased abundance of proteins of the respiratory chain, by a starvation response, and by changes in sulfur metabolism involving the regulator McbR. Additional to the relative quantification, stable isotope-labeled synthetic peptides were used for the absolute quantification of the two benzoate transporters of C. glutamicum, BenK and BenE. It was found that both transporters were expressed during growth on benzoate, suggesting that both contribute substantially to benzoate uptake.

Uptake pathways of anionic and cationic photosensitizers into bacteria

The effect of divalent cations (calcium and magnesium) and a permeabilizing agent (EDTA) on the uptake of a cationic photosensitizer (PS), methylene blue (MB), and two anionic PSs, rose bengal (RB) and indocyanine green (ICG), by Gram-positive Enterococcus faecalis and Gram-negative Actinobacillus actinomycetemcomitans was examined. The possible roles of multidrug efflux pumps and protein transporters in photosensitizer uptake were assessed in E. faecalis cells by studies using an efflux pump inhibitor (verapamil) and trypsin treatment respectively. Divalent cations enhanced the uptake and photodynamic inactivation potential of both RB and ICG in E. faecalis and A. actinomycetemcomitans, while they decreased the uptake and bacterial killing by MB. Verapamil increased the uptake of RB (possibly due to efflux pump inhibition), whereas trypsin treatment resulted in significant decrease in RB and ICG uptake. The results suggested that the uptake of anionic PSs by bacterial cells may be mediated through a combination of electrostatic charge interaction and by protein transporters, while the uptake of cationic PSs, as previously reported, is mediated by electrostatic interactions and self promoted uptake pathways.

Corynebacterium diphtheriae: identification and characterization of a channel-forming protein in the cell wall

The cell wall fraction of the gram-positive, nontoxic Corynebacterium diphtheriae strain C8r(-) Tox- (=ATCC 11913) contained a channel-forming protein, as judged from reconstitution experiments with artificial lipid bilayer experiments. The channel-forming protein was present in detergent-treated cell walls and in extracts of whole cells obtained using organic solvents. The protein had an apparent molecular mass of about 66 kDa as determined on Tricine-containing sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and consisted of subunits having a molecular mass of about 5 kDa. Single-channel experiments with the purified protein suggested that the protein formed channels with a single-channel conductance of 2.25 nS in 1 M KCl. Further single-channel analysis suggested that the cell wall channel is wide and water filled because it has only slight selectivity for cations over anions and its conductance followed the mobility sequence of cations and anions in the aqueous phase. Antibodies raised against PorA, the subunit of the cell wall channel of Corynebacterium glutamicum, detected both monomers and oligomers of the isolated protein, suggesting that there are highly conserved epitopes in the cell wall channels of C. diphtheriae and PorA. Localization of the protein on the cell surface was confirmed by an enzyme-linked immunosorbent assay. The prospective homology of PorA with the cell wall channel of C. diphtheriae was used to identify the cell wall channel gene, cdporA, in the known genome of C. diphtheriae. The gene and its flanking regions were cloned and sequenced. CdporA is a protein that is 43 amino acids long and does not have a leader sequence. cdporA was expressed in a C. glutamicum strain that lacked the major outer membrane channels PorA and PorH. Organic solvent extracts of the transformed cells formed in lipid bilayer membranes the same channels as the purified CdporA protein of C. diphtheriae formed, suggesting that the expressed protein is able to complement the PorA and PorH deficiency of the C. glutamicum strain. The study is the first report of a cell wall channel in a pathogenic Corynebacterium strain.

The key role of the mycolic acid content in the functionality of the cell wall permeability barrier in Corynebacterineae

Recently, it has been shown that trehalose and mycolic acids are essential for the growth of Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium smegmatis, and important but not indispensable to the survival of Corynebacterium glutamicum. Therefore, to investigate the function of mycolic acids in both the permeability of the cell wall to small nutrients and antibiotics, and the excretion of amino acids by C. glutamicum, a trehalose-deficient mutant of the L-lysine producer ATCC 21527, designated LP Delta treS Delta otsA Delta treY, was constructed. By using different carbon sources in either the presence or the absence of external trehalose, a set of endogenously trehalose-free LP Delta treS Delta otsA Delta treY cells that exhibited various mycolate contents was generated. The results showed that the structure of the arabinogalactan of these different cell types of LP Delta treS Delta otsA Delta treY was not affected when the mycolic acid layer was either missing or impaired. Nevertheless, cells were more susceptible to antibiotics, and the permeability of their cell walls to glycerol was increased. Interestingly, a concomitant increase in the excretion of both L-lysine and L-glutamate was also observed, indicating that the mycolic acid content of the permeability barrier (and not only the peptidoglycan and/or the arabinogalactan) is implicated in the glutamate excretion process.

Production of l-Lysine from starch by Corynebacterium glutamicum displaying α-amylase on its cell surface

We engineered a Corynebacterium glutamicum strain displaying alpha-amylase from Streptococcus bovis 148 (AmyA) on its cell surface to produce amino acids directly from starch. We used PgsA from Bacillus subtilis as an anchor protein, and the N-terminus of alpha-amylase was fused to the PgsA. The genes of the fusion protein were integrated into the homoserine dehydrogenase gene locus on the chromosome by homologous recombination. L-Lysine fermentation was carried out using C. glutamicum displaying AmyA in the growth medium containing 50 g/l soluble starch as the sole carbon source. We performed L-lysine fermentation at various temperatures (30-40 degrees C) and pHs (6.0-7.0), as the optimal temperatures and pHs of AmyA and C. glutamicum differ significantly. The highest L-lysine yield was recorded at 30 degrees C and pH 7.0. The amount of soluble starch was reduced to 18.29 g/l, and 6.04 g/l L-lysine was produced in 24 h. The L-lysine yield obtained using soluble starch as the sole carbon source was higher than that using glucose as the sole carbon source after 24 h when the same amount of substrates was added. The results shown in the current study demonstrate that C. glutamicum displaying alpha-amylase has a potential to directly convert soluble starch to amino acids.

PorH, a new channel-forming protein present in the cell wall of Corynebacterium efficiens and Corynebacterium callunae

Corynebacterium callunaeandCorynebacterium efficiensare close relatives of the glutamate-producing mycolata speciesCorynebacterium glutamicum. The properties of the pore-forming proteins, extracted by organic solvents, were studied. The cell extracts contained channel-forming proteins that formed ion-permeable channels with a single-channel conductance of about 2 to 3 nS in 1 M KCl in a lipid bilayer assay. The corresponding proteins from both corynebacteria were purified to homogeneity and were named PorHC.calland PorHC.eff. Electrophysiological studies of the channels suggested that they are wide and water-filled. Channels formed by PorHC.callare cation-selective, whereas PorHC.effforms slightly anion-selective channels. Both proteins were partially sequenced. A multiple sequence alignment search within the known chromosome ofC. efficiensdemonstrated that it contains a gene that fits the partial amino acid sequence of PorHC.eff. PorHC.callshows high homology to PorHC.eff. PorHC.effis encoded in the bacterial chromosome by a gene that is localized within the vicinity of theporAgene ofC. efficiens. PorHC.effhas no signal sequence at the N terminus, which means that it is not exported by the Sec-secretion pathway. The structure of PorH in the cell wall of the corynebacteria is discussed.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Identification of an anion-specific channel in the cell wall of the Gram-positive bacterium Corynebacterium glutamicum

A cation-selective channel (porin), designated PorA, facilitates the passage of hydrophilic solutes across the cell wall of the mycolic acid-containing actinomycete Corynebacterium glutamicum. Biochemical and electrophysiological investigations of the cell wall of the mutant strain revealed the presence of an alternative channel-forming protein. This porin was purified to homogeneity and studied in lipid bilayer membranes. It forms small anion-selective channels with a diameter of about 1.4 nm and an average single-channel conductance of about 700 pS in 1 M KCl. The PorBCglut channel could be blocked by citrate in a dose-dependent manner. This result was in agreement with growth experiments in citrate as sole carbon source where growth in citrate was impaired as compared with growth in other carbon sources. The PorBCglut protein was partially sequenced and based on the resulting amino acid sequence of the corresponding gene, which was designated as porB, was identified as an unannotated 381 bp long open reading frame (ORF) in the published genome sequence of C. glutamicum ATCC13032. PorBCglut contains 126 amino acids with an N-terminal extension of 27 amino acids. One hundred and thirty-eight base pairs downstream of porB, we found an ORF that codes for a protein with about 30% identity to PorBCglut, which was named PorCCglut. The arrangement of porB and porC on the chromosome suggested that both genes belong to the same cluster. RT-PCR from overlapping regions between genes from wild-type C. glutamicum ATCC 13032 and its ATCC 13032DeltaporA mutant demonstrated that this is the case and that porB and porC are cotranscribed. The gene products PorBCglut and PorCCglut represent obviously other permeability pathways for the transport of hydrophilic compounds through the cell wall of C. glutamicum.