Molecular analysis of expression of the lantibiotic pep5 immunity phenotype

Secondary Metabolites Governing Microbiome Interaction of Staphylococcal Pathogens and Commensals

Various Staphylococcus species colonize skin and upper airways of warm-blooded animals. They compete successfully with many other microorganisms under the hostile and nutrient-poor conditions of these habitats using mechanisms that we are only beginning to appreciate. Small-molecule mediators, whose biosynthesis requires complex enzymatic cascades, so-called secondary metabolites, have emerged as crucial components of staphylococcal microbiome interactions. Such mediators belong to a large variety of compound classes and several of them have attractive properties for future drug development. They include, for instance, bacteriocins such as lanthipeptides, thiopeptides, and fibupeptides that inhibit bacterial competitor species; signaling molecules such as thiolactone peptides that induce or inhibit sensory cascades in other bacteria; or metallophores such as staphyloferrins and staphylopine that scavenge scant transition metal ions. For some secondary metabolites such as the aureusimines, the exact function remains to be elucidated. How secondary metabolites shape the fitness of Staphylococcus species in the complex context of other microbial and host defense factors remains a challenging field of future research. A detailed understanding will help to harness staphylococcal secondary metabolites for excluding the pathogenic species Staphylococcus aureus from the nasal microbiomes of at-risk patients, and it will be instrumental for the development of advanced anti-infective interventions.

Self-immunity to antibacterial peptides by ABC transporters

Bacteria produce under certain stress conditions bacteriocins and microcins that display antibacterial activity against closely related species for survival. Bacteriocins and microcins exert their antibacterial activity by either disrupting the membrane or inhibiting essential intracellular processes of the bacterial target. To this end, they can lyse bacterial membranes and cause subsequent loss of their integrity or nutrients, or hijack membrane receptors for internalisation. Both bacteriocins and microcins are ribosomally synthesised and several are posttranslationally modified, whereas others are not. Such peptides are also toxic to the producer bacteria, which utilise immunity proteins or/and dedicated ATP-binding cassette (ABC) transporters to achieve self-immunity and peptide export. In this review, we discuss the structure and mechanism of self-protection that is conferred by these ABC transporters.

Two-component systems regulate ABC transporters in antimicrobial peptide production, immunity and resistance

Bacteria offer resistance to a broad range of antibiotics by activating their export channels of ATP-binding cassette transporters. These transporters perform a central role in vital processes of self-immunity, antibiotic transport and resistance. The majority of ATP-binding cassette transporters are capable of detecting the presence of antibiotics in an external vicinity and are tightly regulated by two-component systems. The presence of an extracellular loop and an adjacent location of both the transporter and two-component system offers serious assistance to induce a quick and specific response against antibiotics. Both systems have demonstrated their ability of sensing such agents, however, the exact mechanism is not yet fully established. This review highlighted the three key functions of antibiotic resistance, transport and self-immunity of ATP-binding cassette transporters and an adjacent two-component regulatory system.

Novel bioactive from Lactobacillus brevis DSM17250 to stimulate the growth of Staphylococcus epidermidis: a pilot study

Commensal skin microbiota plays an important role in both influencing the immune response of the skin and acting as a barrier against colonisation of potentially pathogenic microorganisms and overgrowth of opportunistic pathogens. Staphylococcus epidermidis is a key constituent of the normal microbiota on human skin. It balances the inflammatory response after skin injury and produces antimicrobial molecules that selectively inhibit skin pathogens. Here we describe Lactobacillus brevis DSM17250 that was identified among hundreds of Lactobacillus strains to exhibit an anti-inflammatory effect in human keratinocytes in vitro and specific stimulatory impact on the growth of S. epidermidis. The aqueous cell-free extract of L. brevis DSM17250 was used in an ointment formulation and tested in a randomized placebo-controlled double blinded human pilot study. Healthy volunteers with diagnosed dry skin were treated for four weeks. The study data shows that L. brevis DSM17250 extract induces re-colonisation of the skin by protective commensal microorganisms as judged from selective bacterial cultivation of surface-associated skin microorganism of the lower leg. Furthermore, the 4 week administration of the L. brevis DSM17250 extract significantly improved the transepidermal water loss value (TEWL), reduced the xerosis cutis symptoms and stinging. The data shows that daily application of L. brevis DSM17250 extract in a topical product significantly improves the microbial skin microbiota by promoting the growth of species which possess beneficial regulatory and protective properties such as S. epidermidis. Restoring the natural skin microbiota leads to significantly improved skin barrier function (as transepidermal water loss) and decrease of xeroderma (xerosis cutis) symptoms (as measured by dry skin area and severity index, DASI). We propose that improving and stabilizing the natural skin microbiota by specifically stimulating the growth of S. epidermidis is an important and novel concept to manage skin diseases associated with microbiota dysbiosis.

Bacteriocins and their position in the next wave of conventional antibiotics

Micro-organisms are capable of producing a range of defence mechanisms, including antibiotics, bacteriocins, lytic agents, protein exotoxins, etc. Such mechanisms have been identified in nearly 99% of studied bacteria. The multiplicity and diversity of bacteriocins and the resultant effects of their interactions with targeted bacteria on microbial ecology has been thoroughly studied and remains an area of investigation attracting many researchers. However, the incorporation of bacteriocins into drug delivery systems used in conjunction with, or as potential alternatives to, conventional antibiotics is only a recent, although rapidly expanding, field. The extensive array of bacteriocins positions them as one of the most promising options in the next wave of antibiotics. The goal of this review was to explore bacteriocins as novel antimicrobials, alone and in combination with established antibiotics, and thus position them as a potential tool for addressing the current antibiotic crisis.

Lantibiotic resistance

The dramatic rise in the incidence of antibiotic resistance demands that new therapeutic options will have to be developed. One potentially interesting class of antimicrobials are the modified bacteriocins termed lantibiotics, which are bacterially produced, posttranslationally modified, lanthionine/methyllanthionine-containing peptides. It is interesting that low levels of resistance have been reported for lantibiotics compared with commercial antibiotics. Given that there are very few examples of naturally occurring lantibiotic resistance, attempts have been made to deliberately induce resistance phenotypes in order to investigate this phenomenon. Mechanisms that hinder the action of lantibiotics are often innate systems that react to the presence of any cationic peptides/proteins or ones which result from cell well damage, rather than being lantibiotic specific. Such resistance mechanisms often arise due to altered gene regulation following detection of antimicrobials/cell wall damage by sensory proteins at the membrane. This facilitates alterations to the cell wall or changes in the composition of the membrane. Other general forms of resistance include the formation of spores or biofilms, which are a common mechanistic response to many classes of antimicrobials. In rare cases, bacteria have been shown to possess specific antilantibiotic mechanisms. These are often species specific and include the nisin lytic protein nisinase and the phenomenon of immune mimicry.

Lantibiotic Reductase LtnJ Substrate Selectivity Assessed with a Collection of Nisin Derivatives as Substrates

Lantibiotics are potent antimicrobial peptides characterized by the presence of dehydrated amino acids, dehydroalanine and dehydrobutyrine, and (methyl)lanthionine rings. In addition to these posttranslational modifications, some lantibiotics exhibit additional modifications that usually confer increased biological activity or stability on the peptide. LtnJ is a reductase responsible for the introduction of D-alanine in the lantibiotic lacticin 3147. The conversion of L-serine into D-alanine requires dehydroalanine as the substrate, which is produced in vivo by the dehydration of serine by a lantibiotic dehydratase, i.e., LanB or LanM. In this work, we probe the substrate specificity of LtnJ using a system that combines the nisin modification machinery (dehydratase, cyclase, and transporter) and the stereospecific reductase LtnJ in Lactococcus lactis. We also describe an improvement in the production yield of this system by inserting a putative attenuator from the nisin biosynthesis gene cluster in front of the ltnJ gene. In order to clarify the sequence selectivity of LtnJ, peptides composed of truncated nisin and different mutated C-terminal tails were designed and coexpressed with LtnJ and the nisin biosynthetic machinery. In these tails, serine was flanked by diverse amino acids to determine the influence of the surrounding residues in the reaction. LtnJ successfully hydrogenated peptides when hydrophobic residues (Leu, Ile, Phe, and Ala) were flanking the intermediate dehydroalanine, while those in which dehydroalanine was flanked by one or two polar residues (Ser, Thr, Glu, Lys, and Asn) or Gly were either less prone to be modified by LtnJ or not modified at all. Moreover, our results showed that dehydrobutyrine cannot serve as a substrate for LtnJ.

ApnI, a transmembrane protein responsible for subtilomycin immunity, unveils a novel model for lantibiotic immunity

Subtilomycin was detected from the plant endophytic strain Bacillus subtilis BSn5 and was first reported from B. subtilis strain MMA7. In this study, a gene cluster that has been proposed to be related to subtilomycin biosynthesis was isolated from the BSn5 genome and was experimentally validated by gene inactivation and heterologous expression. Comparison of the subtilomycin gene cluster with other verified related lantibiotic gene clusters revealed a particular organization of the genes apnI and apnT downstream of apnAPBC, which may be involved in subtilomycin immunity. Through analysis of expression of the apnI and/or apnT genes in the subtilomycin-sensitive strain CU1065 and inactivation of apnI and apnT in the producer strain BSn5, we showed that the single gene apnI, encoding a putative transmembrane protein, was responsible for subtilomycin immunity. To our knowledge, evidence for lantibiotic immunity that is solely dependent on a transmembrane protein is quite rare. Further bioinformatic analysis revealed the abundant presence of ApnI-like proteins that may be responsible for lantibiotic immunity in Bacillus and Paenibacillus. We cloned the paeI gene, encoding one such ApnI-like protein, into CU1065 and showed that it confers resistance to paenibacillin. However, no cross-resistance was detected between ApnI and PaeI, even though subtilomycin and paenibacillin share similar structures, suggesting that the protection provided by ApnI/ApnI-like proteins involves a specific-sequence recognition mechanism. Peptide release/binding assays indicated that the recombinant B. subtilis expressing apnI interacted with subtilomycin. Thus, ApnI represents a novel model for lantibiotic immunity that appears to be common.

Heterologous expression of thuricin CD immunity genes in Listeria monocytogenes

Bacteriocins are ribosomally synthesized peptides that can have a narrow or broad spectrum of antimicrobial activity. Bacteriocin producers typically possess dedicated immunity systems that often consist of an ATP-binding cassette (ABC) transporter system and/or a dedicated immunity protein. Here we investigated the genes responsible for immunity to thuricin CD, a narrow-spectrum two-peptide sactibiotic produced by Bacillus thuringiensis DPC6431. Heterologous expression of putative thuricin CD immunity determinants allowed us to identify and investigate the relative importance of the individual genes and gene products that contribute to thuricin CD immunity. We established that TrnF and TrnG are the individual components of an ABC transporter system that provides immunity to thuricin CD. We also identified a hitherto overlooked open reading frame located upstream of trnF predicted to encode a 79-amino-acid transmembrane protein. We designated this newly discovered gene trnI and established that TrnI alone can provide protection against thuricin CD.

Insights into Lantibiotic Immunity Provided by Bioengineering of LtnI

The lantibiotic lacticin 3147 has been the focus of much research due to its broad spectrum of activity against many microbial targets, including drug-resistant pathogens. In order to protect itself, a lacticin 3147 producer must possess a cognate immunity mechanism. Lacticin 3147 immunity is provided by an ABC transporter, LtnFE, and a dedicated immunity protein, LtnI, both of which are capable of independently providing a degree of protection. In the study described here, we carried out an in-depth investigation of LtnI structure-function relationships through the creation of a series of fusion proteins and LtnI determinants that have been the subject of random and site-directed mutagenesis. We establish that LtnI is a transmembrane protein that contains a number of individual residues and regions, such as those between amino acids 20 and 27 and amino acids 76 and 83, which are essential for LtnI function. Finally, as a consequence of the screening of a bank of 28,000 strains producing different LtnI derivatives, we identified one variant (LtnI I81V) that provides enhanced protection. To our knowledge, this is the first report of a lantibiotic immunity protein with enhanced functionality.

Lantibiotics: diverse activities and unique modes of action

Lantibiotics are one of the most promising alternative candidates for future antibiotics that maintain their antibacterial efficacy through many mechanisms. Of these mechanisms, some modes of activity have recently been reported, providing opportunities to show these peptides as potential candidates for forthcoming applications. Many findings providing new insight into the detailed molecular activities of numerous lantibiotics are constantly being uncovered. The combination of antibiotic mechanisms in one lantibiotic molecule shows its diverse antimicrobial usefulness as a future generation of antibiotic. Since lantibiotics do not have any known candidate resistance mechanisms, the discovered distinct modes of activity may revolutionize the design of anti-infective drugs through the knowledge provided by these super molecules. In this review, we discuss the rising assortment of lantibiotics, with special emphasis on their structure-function relationships, addressing the unique activities involved in their individual modes of action.

Processing of as-48ABC RNA in AS-48 enterocin production by Enterococcus faecalis

Enterocin AS-48 production and immunity characters are encoded by 10 genes (as-48ABCC(1)DD(1)EFGH) of the pMB2 plasmid from the Enterococcus faecalis S-48 strain. Among these, as-48A, encoding the AS-48 peptide, and the as-48BC genes constitute a cluster required for AS-48 biogenesis and full immunity. In this study, the levels of expression of this cluster have been altered by insertion and site-directed mutagenesis as well as by expression coupled to trans complementation. Phenotypic studies of the mutants have indicated cotranscription of the three genes and revealed that the inactivation of as-48B prevents the production of AS-48, thus confirming its essentiality in AS-48 biogenesis. These studies have also supported the involvement of as-48C in enterocin immunity. In addition, they established that the intergenic region between the as-48A and as-48B genes is decisive for AS-48 expression, since a 3-bp substitution, which should disrupt a potential 47-nucleotide complex secondary structure, resulted in a hypoproducing phenotype. Transcriptional analyses of the E. faecalis wild-type and mutant strains supports the possibility that the as-48ABC genes are transcribed from the P(A) promoter located upstream of as-48A. Moreover, analysis and bioinformatic predictions of RNA folding indicate that as-48ABC mRNA is processed at the secondary structure located between as-48A and as-48B. Thus, synthesis of the AS-48 peptide appears to be controlled at the posttranscriptional level and is uncoupled from as-48BC translation. This mechanism of genetic regulation has not been previously described for the regulation of bacteriocin expression in enterococci.

Identification of a novel two-peptide lantibiotic, Haloduracin, produced by the alkaliphileBacillus haloduransC-125

Complete genome sequencing of the alkaliphilic bacterium Bacillus halodurans C-125 revealed the presence of several genes homologous to those involved in the production of lantibiotic peptides. Additional bioinformatic analysis identified a total of eleven genes, spanning a 15 kbp region, potentially involved in the production, modification, immunity and transport of a two-peptide lantibiotic. Having established that strain C-125 exhibited antimicrobial activity against a wide range of Gram-positive bacteria, it was demonstrated through peptide purification, MS and site-directed mutagenesis that this activity was indeed attributable to the production of a lantibiotic encoded by these genes. This antimicrobial has been designated haloduracin and represents the first occasion wherein production of two-peptide lantibiotic has been associated with a Bacillus sp. It is also the first example of a lantibiotic of any kind to be produced by an alkaliphilic species.

Sec-mediated transport of posttranslationally dehydrated peptides in Lactococcus lactis

Nisin is a lanthionine-containing antimicrobial peptide produced by Lactococcus lactis. Its (methyl)lanthionines are introduced by two posttranslational enzymatic steps involving the dehydratase NisB, which dehydrates serine and threonine residues, and the cyclase NisC, which couples these dehydrated residues to cysteines, yielding thioether-bridged amino acids called lanthionines. The prenisin is subsequently exported by the ABC transporter NisT and extracellularly processed by the peptidase NisP. L. lactis expressing the nisBTC genes can modify and secrete a wide range of nonlantibiotic peptides. Here we demonstrate that in the absence of NisT and NisC, the Sec pathway of L. lactis can be exploited for the secretion of dehydrated variants of therapeutic peptides. Furthermore, posttranslational modifications by NisB and NisC still occur even when the nisin leader is preceded by a Sec signal peptide or a Tat signal peptide 27 or 44 amino acids long, respectively. However, transport of fully modified prenisin via the Sec pathway is impaired. The extent of NisB-mediated dehydration could be improved by raising the intracellular concentration NisB or by modulating the export efficiency through altering the signal sequence. These data demonstrate that besides the traditional lantibiotic transporter NisT, the Sec pathway with an established broad substrate range can be utilized for the improved export of lantibiotic enzyme-modified (poly)peptides.

The biology of lantibiotics from the lacticin 481 group is coming of age

Lantibiotics are antimicrobial peptides from the bacteriocin family, secreted by Gram-positive bacteria. These peptides differ from other bacteriocins by the presence of (methyl)lanthionine residues, which result from enzymatic modification of precursor peptides encoded by structural genes. Several groups of lantibiotics have been distinguished, the largest of which is the lacticin 481 group. This group consists of at least 16 members, including lacticin 481, streptococcin A-FF22, mutacin II, nukacin ISK-1, and salivaricins. We present the first review devoted to this lantibiotic group, knowledge of which has increased significantly within the last few years. After updating the group composition and defining the common properties of these lantibiotics, we highlight the most recent developments. The latter concern: transcriptional regulation of the lantibiotic genes; understanding the biosynthetic machinery, in particular the ability to perform in vitro prepeptide maturation; characterization of a novel type of immunity protein; and broad application possibilities. This group differs in many aspects from the best known lantibiotic group (nisin group), but shares properties with less-studied groups such as the mersacidin, cytolysin and lactocin S groups.

Overproduction of wild-type and bioengineered derivatives of the lantibiotic lacticin 3147

Lacticin 3147 is a broad-spectrum two-peptide lantibiotic whose genetic determinants are located on two divergent operons on the lactococcal plasmid pMRC01. Here we introduce each of 14 subclones, containing different combinations of lacticin 3147 genes, into MG1363 (pMRC01) and determine that a number of them can facilitate overproduction of the lantibiotic. Based on these studies it is apparent that while the provision of additional copies of genes encoding the biosynthetic/production machinery and the regulator LtnR is a requirement for high-level overproduction, the presence of additional copies of the structural genes (i.e., ltnA1A2) is not.

Localization and functional analysis of PepI, the immunity peptide of Pep5-producing Staphylococcus epidermidis strain 5

Pep5 is a cationic pore-forming lantibiotic produced by Staphylococcus epidermidis strain 5. The producer strain protects itself from the lethal action of its own bacteriocin through the 69-amino-acid immunity peptide PepI. The N-terminal segment of PepI contains a 20-amino-acid stretch of apolar residues, whereas the C terminus is very hydrophilic, with a net positive charge. We used green fluorescent protein (GFP)-PepI fusions to obtain information on its localization in vivo. PepI was found to occur outside the cytoplasm and to accumulate at the membrane-cell wall interface. The extracellular localization appeared essential for conferring immunity. We analyzed the functional role of the specific segments by constructing various mutant peptides, which were also fused to GFP. When the hydrophobic N-terminal segment of PepI was disrupted by introducing charged amino acids, the export of PepI was blocked and clones expressing such mutant peptides were Pep5 sensitive. When PepI was successively shortened at the C terminus, in contrast, its export properties remained unchanged whereas its ability to confer immunity was gradually reduced. The results show that the N-terminal part is required for the transport of PepI and that the C-terminal part is important for conferring the immunity phenotype. A concept based on target shielding is proposed for the PepI immunity mechanism.

Functional analysis of the gene cluster involved in production of the bacteriocin circularin A by Clostridium beijerinckii ATCC 25752

A region of 12 kb flanking the structural gene of the cyclic antibacterial peptide circularin A of Clostridium beijerinckii ATCC 25752 was sequenced, and the putative proteins involved in the production and secretion of circularin A were identified. The genes are tightly organized in overlapping open reading frames. Heterologous expression of circularin A in Enterococcus faecalis was achieved, and five genes were identified as minimally required for bacteriocin production and secretion. Two of the putative proteins, CirB and CirC, are predicted to contain membrane-spanning domains, while CirD contains a highly conserved ATP-binding domain. Together with CirB and CirC, this ATP-binding protein is involved in the production of circularin A. The fifth gene, cirE, confers immunity towards circularin A when expressed in either Lactococcus lactis or E. faecalis and is needed in order to allow the bacteria to produce bacteriocin. Additional resistance against circularin A is conferred by the activity of the putative transporter consisting of CirB and CirD.

Arming the enemy: the evolution of resistance to self-proteins

A remarkable range of novel antibiotics is attracting increasing interest as a major new weapon in the campaign against bacterial infection. They are based on the toxic peptides that provide the innate immune system of animals, and it is claimed that bacteria will be unable to evolve resistance to them because they attack the 'Achilles' heel' of bacterial membrane structure. Both experimental evidence and theoretical arguments suggest that this claim is doubtful. If so, the introduction of these substances into general use may provoke the evolution of resistance to our own defence proteins and thus compromise our natural defences against infection.

Function of Lactococcus lactis nisin immunity genes nisI and nisFEG after coordinated expression in the surrogate host Bacillus subtilis

Nisin-producing Lactococcus lactis strains show a high degree of resistance to the action of nisin, which is based upon expression of the self-protection (immunity) genes nisI, nisF, nisE, and nisG. Different combinations of nisin immunity genes were integrated into the chromosome of a nisin-sensitive Bacillus subtilis host strain under the control of an inducible promoter. For the recipient strain, the highest level of acquired nisin tolerance was achieved after coordinated expression of all four nisin immunity genes. But either the lipoprotein NisI or the ABC transporter-homologous system NisFEG, respectively, were also able to protect the Bacillus host cells. The acquired immunity was specific to nisin and provided no tolerance to subtilin, a closely related lantibiotic. Quantitative in vivo peptide release assays demonstrated that NisFEG diminished the quantity of cell-associated nisin, providing evidence that one role of NisFEG is to transport nisin from the membrane into the extracellular space. NisI solubilized from B. subtilis membrane vesicles and recombinant hexahistidine-tagged NisI from Escherichia coli interacted specifically with nisin and not with subtilin. This suggests a function of NisI as a nisin-intercepting protein.

Role of tfxE, but not tfxG, in trifolitoxin resistance

Eight genes, tfxABCDEFG and tfuA, confer production of trifolitoxin (TFX), a ribosomally synthesized, posttranslationally modified peptide antibiotic, in TFX-sensitive alpha-proteobacteria. An in-frame deletion in tfxE significantly reduced a strain's resistance to TFX in comparison to that of an otherwise identical construct containing wild-type tfxE. The deletion of tfxG had no effect on TFX resistance. Nevertheless, RNase protection assays showed that tfxE and tfxG are transcribed, showing that the tfxDEFG mRNA was produced on the same transcript. Examination of the role of tfxG in TFX production showed that the tfxG mutant expressed slightly less TFX activity and produced only one TFX isomer while four are produced by the wild-type strain. Thus, tfxE plays an important role in TFX resistance while tfxG is important in optimal TFX production through the production of TFX isomers.

Staphylococcal resistance to antimicrobial peptides of mammalian and bacterial origin

Antimicrobial host defense peptides, such as defensins, protegrins, and platelet microbicidal proteins are deployed by mammalian skin, epithelia, phagocytes, and platelets in response to Staphylococcus aureus infection. In addition, staphylococcal products with similar structures and activities, called bacteriocins, inhibit competing microorganisms. Staphylococci have developed resistance mechanisms, which are either highly specific for certain host defense peptides or bacteriocins or which broadly protect against a range of cationic antimicrobial peptides. Experimental infection models can be used to study the molecular mechanisms of antimicrobial peptides, the peptide resistance strategies of S. aureus, and the therapeutic potential of peptides in staphylococcal diseases.

Lantibiotics: structure, biosynthesis and mode of action

The lantibiotics are a group of ribosomally synthesised, post-translationally modified peptides containing unusual amino acids, such as dehydrated and lanthionine residues. This group of bacteriocins has attracted much attention in recent years due to the success of the well characterised lantibiotic, nisin, as a food preservative. Numerous other lantibiotics have since been identified and can be divided into two groups on the basis of their structures, designated type-A and type-B. To date, many of these lantibiotics have undergone extensive characterisation resulting in an advanced understanding of them at both the structural and mechanistic level. This review outlines some of the more recent developments in the biochemistry, genetics and mechanism of action of these peptides.

Dual role of GdmH in producer immunity and secretion of the Staphylococcal lantibiotics gallidermin and epidermin

The biosynthetic gene clusters of the staphylococcal lantibiotics epidermin and gallidermin are distinguished by the presence of the unique genes epiH and gdmH, respectively. They encode accessory factors for the ATP-binding cassette transporters that mediate secretion of the antimicrobial peptides. Here, we show that gdmH also contributes to immunity to gallidermin but not to nisin. gdmH alone affected susceptibility to gallidermin only moderately, but it led to a multiplication of the immunity level mediated by the FEG immunity genes when cloned together with the gdmT gene, suggesting a synergistic activity of the H and FEG systems. gdmH-related genes were identified in the genomes of several bacteria, indicating an involvement in further cellular functions.

Regulation of immunity to the two-component lantibiotic, lacticin 3147, by the transcriptional repressor LtnR

Lacticin 3147 is a membrane-active, two-component lantibiotic produced by Lactococcus lactis ssp. lactis DPC3147. In this study, the promoters of the lacticin 3147 gene cluster were mapped to the intergenic region between ltnR and ltnA1 (the genes encoding the regulatory protein LtnR and the first structural gene, LtnA1), and Northern analyses revealed that the biosynthetic and immunity genes are divergently transcribed in two operons, ltnA1A2M1TM2D and ltnRIFE respectively. Although the promoter controlling biosynthesis (Pbac) appears to be constitutive, characterization of a downstream beta-galactosidase (beta-gal) fusion beyond an intragenic stem-loop structure in ltnM1 confirmed that this putative transcriptional attenuator allows limited readthrough to the downstream biosynthetic genes, thus maintaining the correct stoichiometry between structural peptides and biosynthetic machinery. The promoter of the ltnRIFE operon (Pimm) was shown to be regulated by the transcriptional repressor LtnR. A mutant with a truncated ltnR gene exhibited a hyperimmune phenotype, whereas overexpression of ltnR resulted in cells with increased sensitivity to lacticin 3147. Gel mobility shift analysis indicated that LtnR binds to the Pimm promoter region, and fusion of this promoter to the beta-gal gene of pAK80 revealed that expression from Pimm is significantly reduced in the presence of LtnR. Thus, we have demonstrated that lacticin 3147 uses a regulatory mechanism not previously identified in lantibiotic systems.

Biosynthesis of the lantibiotic mersacidin: organization of a type B lantibiotic gene cluster

The biosynthetic gene cluster (12.3 kb) of mersacidin, a lanthionine-containing antimicrobial peptide, is located on the chromosome of the producer, Bacillus sp. strain HIL Y-85,54728 in a region that corresponds to 348 degrees on the chromosome of Bacillus subtilis 168. It consists of 10 open reading frames and contains, in addition to the previously described mersacidin structural gene mrsA (G. Bierbaum, H. Brötz, K.-P. Koller, and H.-G. Sahl, FEMS Microbiol. Lett. 127:121-126, 1995), two genes, mrsM and mrsD, coding for enzymes involved in posttranslational modification of the prepeptide; one gene, mrsT, coding for a transporter with an associated protease domain; and three genes, mrsF, mrsG, and mrsE, encoding a group B ABC transporter that could be involved in producer self-protection. Additionally, three regulatory genes are part of the gene cluster, i.e., mrsR2 and mrsK2, which encode a two-component regulatory system which seems to be necessary for the transcription of the mrsFGE operon, and mrsR1, which encodes a protein with similarity to response regulators. Transcription of mrsA sets in at early stationary phase (between 8 and 16 h of culture).

Identification and overexpression of ltnl, a novel gene which confers immunity to the two-component lantibiotic lacticin 3147

Production and immunity of the two-component lantibiotic lacticin 3147 is encoded by the 60-2 kb lactococcal plasmid pMRC01. A 12.6 kb region of this plasmid, containing ten genes in two divergently arranged gene clusters, has been subcloned in Lactococcus lactis subsp. cremoris MG1363 and has been shown to confer both lacticin 3147 production and immunity. Further subcloning revealed that the smaller of the two clusters (ItnRIFE) confers immunity. Although the ItnF and E genes are homologous to ABC transporters which confer immunity to other lantibiotics, deletion analysis indicates that they do not play a role in the immunity exhibited by this subclone in L. lactis subsp. cremoris MG1363. Also, a deletion in ItnR (which resembles a family of transcriptional repressors) had no effect on immunity. The remaining gene, Itnl, encodes a 116 amino acid protein with a predicted membrane location which bears no homology to other bacteriocin immunity proteins. Confirmation of its role in immunity was obtained when it was observed that disruption of Itnl resulted in a complete loss of immunity. When Itnl was cloned into the expression vector pMG36e, the resulting construct conferred levels of immunity comparable to pMRC01. This confirmed that under the control of a strong promoter, the Itnl gene product alone is sufficient to confer lacticin immunity. In addition, heterologous expression of Itnl was observed in Enterococcus faecalis OG1X. On cloning Itnl behind a nisin-inducible promoter, it was observed that the level of immunity was dependent on nisin concentration. Using this construct, the authors have demonstrated a potential role for Itnl as food-grade selectable marker. Thus, Ltnl appears to represent a new class of lantibiotic immunity proteins.