A second ABC transporter is involved in oleandomycin resistance and its secretion by Streptomyces antibioticus

Analysis of the Properties of 44 ABC Transporter Genes from Biocontrol Agent Trichoderma asperellum ACCC30536 and Their Responses to Pathogenic Alternaria alternata Toxin Stress

ATP-binding cassette (ABC) transporters are involved in transporting multiple substrates, such as toxins, and may be important for the survival of Trichoderma when encountering biotic toxins. In this study, genome searching revealed that there are 44 ABC transporters encoded in the genome of Trichoderma asperellum. These ABC transporters were divided into six types based on three-dimensional (3D) structure prediction, of which four, represented by 39 ABCs, are involved in transport and the remaining two, represented by 5 ABCs, are involved in regulating translation. The characteristics of nucleotide-binding domain (NBD) are important in the identification of ABC proteins. Even though the 3D structures of the 79 NBDs in the 44 ABCs are similar, multiple sequence alignment showed they can be divided into three classes. In total, 794 motifs were found in the promoter regions of the 44 ABC genes, of which 541 were cis-regulators related to stress responses. To characterize how their ABCs respond when T. asperellum interact with fungi or plants, T. asperellum was cultivated in either minimal media (MM) control, C-hungry, N-hungry, or poplar medium (PdPap) to simulate normal conditions, competition with pathogens, interaction with pathogens, and interaction with plants, respectively. The results show that 17 of 39 transport ABCs are highly expressed in at least one condition, whereas four of the five translation-regulating ABCs are highly expressed in at least one condition. Of these 21 highly expressed ABCs, 6 were chosen for RT-qPCR expression under the toxin stress of phytopathogen Alternaria alternata, and the results show ABC01, ABC04, ABC05, and ABC31 were highly expressed and may be involved in pathogen interaction and detoxifying toxins from A. alternata.

Efflux Transporters' Engineering and Their Application in Microbial Production of Heterologous Metabolites

Metabolic engineering of microbial hosts for the production of heterologous metabolites and biochemicals is an enabling technology to generate meaningful quantities of desired products that may be otherwise difficult to produce by traditional means. Heterologous metabolite production can be restricted by the accumulation of toxic products within the cell. Efflux transport proteins (transporters) provide a potential solution to facilitate the export of these products, mitigate toxic effects, and enhance production. Recent investigations using knockout lines, heterologous expression, and expression profiling of transporters have revealed candidates that can enhance the export of heterologous metabolites from microbial cell systems. Transporter engineering efforts have revealed that some exhibit flexible substrate specificity and may have broader application potentials. In this Review, the major superfamilies of efflux transporters, their mechanistic modes of action, selection of appropriate efflux transporters for desired compounds, and potential transporter engineering strategies are described for potential applications in enhancing engineered microbial metabolite production. Future studies in substrate recognition, heterologous expression, and combinatorial engineering of efflux transporters will assist efforts to enhance heterologous metabolite production in microbial hosts.

Classification of acetic acid bacteria and their acid resistant mechanism

Acetic acid bacteria (AAB) are obligate aerobic Gram-negative bacteria that are commonly used in vinegar fermentation because of their strong capacity for ethanol oxidation and acetic acid synthesis as well as their acid resistance. However, low biomass and low production rate due to acid stress are still major challenges that must be overcome in industrial processes. Although acid resistance in AAB is important to the production of high acidity vinegar, the acid resistance mechanisms of AAB have yet to be fully elucidated. In this study, we discuss the classification of AAB species and their metabolic processes and review potential acid resistance factors and acid resistance mechanisms in various strains. In addition, we analyze the quorum sensing systems of Komagataeibacter and Gluconacetobacter to provide new ideas for investigation of acid resistance mechanisms in AAB in the form of signaling pathways. The results presented herein will serve as an important reference for selective breeding of high acid resistance AAB and optimization of acetic acid fermentation processes.

Comparison of Antibiotic Resistance Mechanisms in Antibiotic-Producing and Pathogenic Bacteria

Antibiotic resistance poses a tremendous threat to human health. To overcome this problem, it is essential to know the mechanism of antibiotic resistance in antibiotic-producing and pathogenic bacteria. This paper deals with this problem from four points of view. First, the antibiotic resistance genes in producers are discussed related to their biosynthesis. Most resistance genes are present within the biosynthetic gene clusters, but some genes such as paromomycin acetyltransferases are located far outside the gene cluster. Second, when the antibiotic resistance genes in pathogens are compared with those in the producers, resistance mechanisms have dependency on antibiotic classes, and, in addition, new types of resistance mechanisms such as Eis aminoglycoside acetyltransferase and self-sacrifice proteins in enediyne antibiotics emerge in pathogens. Third, the relationships of the resistance genes between producers and pathogens are reevaluated at their amino acid sequence as well as nucleotide sequence levels. Pathogenic bacteria possess other resistance mechanisms than those in antibiotic producers. In addition, resistance mechanisms are little different between early stage of antibiotic use and the present time, e.g., β-lactam resistance in Staphylococcus aureus. Lastly, guanine + cytosine (GC) barrier in gene transfer to pathogenic bacteria is considered. Now, the resistance genes constitute resistome composed of complicated mixture from divergent environments.

Antibiotic Resistance ABC-F Proteins: Bringing Target Protection into the Limelight

Members of the ATP-binding cassette (ABC)-F protein subfamily collectively mediate resistance to a broader range of clinically important antibiotic classes than any other group of resistance proteins and are widespread in pathogenic bacteria. Following over 25 years' of controversy regarding the mechanism by which these proteins work, it has recently been established that they provide antibiotic resistance through the previously recognized but underappreciated phenomenon of target protection; they bind to the ribosome to effect the release of ribosome-targeted antibiotics, thereby rescuing the translation apparatus from antibiotic-mediated inhibition. Here we review the ABC-F resistance proteins with an emphasis on their mechanism of action, first exploring the history of the debate about how these proteins work and outlining our current state of knowledge and then considering key questions to be addressed in understanding the molecular detail of their function.

Multiple transporters are involved in natamycin efflux in Streptomyces chattanoogensis L10

Antibiotic-producing microorganisms have evolved several self-resistance mechanisms to prevent auto-toxicity. Overexpression of specific transporters to improve the efflux of toxic antibiotics has been found one of the most important and intrinsic resistance strategies used by many Streptomyces strains. In this work, two ATP-binding cassette (ABC) transporter-encoding genes located in the natamycin biosynthetic gene cluster, scnA and scnB, were identified as the primary exporter genes for natamycin efflux in Streptomyces chattanoogensis L10. Two other transporters located outside the cluster, a major facilitator superfamily transporter Mfs1 and an ABC transporter NepI/II were found to play a complementary role in natamycin efflux. ScnA/ScnB and Mfs1 also participate in exporting the immediate precursor of natamycin, 4,5-de-epoxynatamycin, which is more toxic to S. chattanoogensis L10 than natamycin. As the major complementary exporter for natamycin efflux, Mfs1 is up-regulated in response to intracellular accumulation of natamycin and 4,5-de-epoxynatamycin, suggesting a key role in the stress response for self-resistance. This article discusses a novel antibiotic-related efflux and response system in Streptomyces, as well as a self-resistance mechanism in antibiotic-producing strains.

ABC-F Proteins Mediate Antibiotic Resistance through Ribosomal Protection

Members of the ABC-F subfamily of ATP-binding cassette proteins mediate resistance to a broad array of clinically important antibiotic classes that target the ribosome of Gram-positive pathogens. The mechanism by which these proteins act has been a subject of long-standing controversy, with two competing hypotheses each having gained considerable support: antibiotic efflux versus ribosomal protection. Here, we report on studies employing a combination of bacteriological and biochemical techniques to unravel the mechanism of resistance of these proteins, and provide several lines of evidence that together offer clear support to the ribosomal protection hypothesis. Of particular note, we show that addition of purified ABC-F proteins to an in vitro translation assay prompts dose-dependent rescue of translation, and demonstrate that such proteins are capable of displacing antibiotic from the ribosome in vitro. To our knowledge, these experiments constitute the first direct evidence that ABC-F proteins mediate antibiotic resistance through ribosomal protection.

Antimicrobial resistance ranks among the greatest threats currently facing human health. Elucidation of the mechanisms by which microorganisms resist the effect of antibiotics is central to understanding the biology of this phenomenon and has the potential to inform the development of new drugs capable of blocking or circumventing resistance. Members of the ABC-F family, which include lsa(A), msr(A), optr(A), and vga(A), collectively yield resistance to a broader range of clinically significant antibiotic classes than any other family of resistance determinants, although their mechanism of action has been controversial since their discovery 25 years ago. Here we present the first direct evidence that proteins of the ABC-F family act to protect the bacterial ribosome from antibiotic-mediated inhibition.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.

Detailed mutational analysis of Vga(A) interdomain linker: implication for antibiotic resistance specificity and mechanism

Detailed mutational analysis examines the roles of individual residues of the Vga(A) linker in determining the antibiotic resistance phenotype. It defines a narrowed region of residues 212 to 220 whose composition determines the resistance specificity to lincosamides, pleuromutilins, and/or streptogramins A. From the analogy with the recently described function of the homologous ABC-F protein EttA as a translational factor, we infer that the Vga(A) linker interacts with the ribosome and directly or indirectly affects the binding of the respective antibiotic.

Assessing the regioselectivity of OleD-catalyzed glycosylation with a diverse set of acceptors

To explore the acceptor regioselectivity of OleD-catalyzed glucosylation, the products of OleD-catalyzed reactions with six structurally diverse acceptors flavones- (daidzein), isoflavones (flavopiridol), stilbenes (resveratrol), indole alkaloids (10-hydroxycamptothecin), and steroids (2-methoxyestradiol)-were determined. This study highlights the first synthesis of flavopiridol and 2-methoxyestradiol glucosides and confirms the ability of OleD to glucosylate both aromatic and aliphatic nucleophiles. In all cases, molecular dynamics simulations were consistent with the determined product distribution and suggest the potential to develop a virtual screening model to identify additional OleD substrates.

ABC transporter genes from Streptomyces ghanaensis moenomycin biosynthetic gene cluster: roles in antibiotic production and export

Streptomyces ghanaensis ATCC14672 produces antibiotic moenomycin A (MmA), which possesses strong antibacterial activity. The genetic control of MmA biosynthesis has been recently elucidated; nevertheless, little is known about the roles of two pairs of genes, moeX5moeP5 and moeD5moeJ5, coding for ATP-dependent transporter systems. Here we report that both gene pairs form transcriptional units actively expressed during MmA production phase. Streptomyces ghanaensis mutants deficient in either (one) or both transporter systems are characterized by a decreased ability to produce moenomycins, and the ΔmoeP5moeX5 mutant exported less moenomycins. However, even the quadruple S. ghanaensis mutant (ΔmoeD5moeJ5 + ΔmoeX5moeP5) remains able to extrude significant amounts of moenomycin. Similar results were observed under conditions of heterologous expression of moe cluster. Transporter genes other than those located in moe cluster are likely to participate in moenomycin efflux.

The role of ATP-binding cassette transporters in bacterial pathogenicity

The ATP-binding cassette transporter superfamily is present in all three domains of life. This ubiquitous class of integral membrane proteins have diverse biological functions, but their fundamental role involves the unidirectional translocation of compounds across cellular membranes in an ATP coupled process. The importance of this class of proteins in eukaryotic systems is well established as typified by their association with genetic diseases and roles in the multi-drug resistance of cancer. In stark contrast, the ABC transporters of prokaryotes have not been exhaustively investigated due to the sheer number of different roles and organisms in which they function. In this review, we examine the breadth of functions associated with microbial ABC transporters in the context of their contribution to bacterial pathogenicity and virulence.

Avoidance of suicide in antibiotic-producing microbes

Many microbes synthesize potentially autotoxic antibiotics, mainly as secondary metabolites, against which they need to protect themselves. This is done in various ways, ranging from target-based strategies (i.e. modification of normal drug receptors or de novo synthesis of the latter in drug-resistant form) to the adoption of metabolic shielding and/or efflux strategies that prevent drug-target interactions. These self-defence mechanisms have been studied most intensively in antibiotic-producing prokaryotes, of which the most prolific are the actinomycetes. Only a few documented examples pertain to lower eukaryotes while higher organisms have hardly been addressed in this context. Thus, many plant alkaloids, variously described as herbivore repellents or nitrogen excretion devices, are truly antibiotics-even if toxic to humans. As just one example, bulbs of Narcissus spp. (including the King Alfred daffodil) accumulate narciclasine that binds to the larger subunit of the eukaryotic ribosome and inhibits peptide bond formation. However, ribosomes in the Amaryllidaceae have not been tested for possible resistance to narciclasine and other alkaloids. Clearly, the prevalence of suicide avoidance is likely to extend well beyond the remit of the present article.

Characterization of an ATP-binding cassette from Clostridium perfringens with homology to an ABC transporter from Clostridium hathewayi

A ciprofloxacin-resistant mutant of Clostridium perfringens, strain VPI-C, which had stable mutations in the topoisomerase genes, accumulated less norfloxacin and ethidium bromide than the wild type, strain VPI. Efflux pump inhibitors both increased the accumulation of ethidium bromide by cells of the mutant and enhanced their sensitivity to this toxic dye. Cloning a gene, which codes for a putative ABC transporter protein (NP_562422) of 527 amino acids, from the mutant strain VPI-C into the wild-type strain VPI not only reduced the accumulation of ethidium bromide by the recombinant strain but also reduced its sensitivity to norfloxacin and ciprofloxacin. Efflux pump inhibitors decreased the rate at which ethidium bromide was removed from the cells of the recombinant strain. It appears that the putative ABC transporter protein (NP_562422) may contribute to extrusion of drugs from C. perfringens.

Self-resistance mechanism in Streptomyces peucetius: overexpression of drrA, drrB and drrC for doxorubicin enhancement

The resistance genes drrABC from Streptomyces peucetius ATCC 27952 were cloned into the pIBR25 expression vector under a strong ermE* promoter to enhance doxorubicin (DXR) production. The recombinant expression plasmids, pDrrAB25, pDrrC25 and pDrrABC25, were constructed to overexpress drrAB, drrC and drrABC, respectively, in S. peucetius ATCC 27952. The recombinant strains produced more DXR than the parental strain: a 2.2-fold increase with pDrrAB25, a 5.1-fold increase with pDrrC25, and a 2.4-fold increase with pDrrABC25. We also studied the relative ratios of doxorubicin, daunorubicin and epsilon-rhodomycinone produced in these recombinant strains.

Improving production of bioactive secondary metabolites in actinomycetes by metabolic engineering

Production of secondary metabolites is a process influenced by several physico-chemical factors including nutrient supply, oxygenation, temperature and pH. These factors have been traditionally controlled and optimized in industrial fermentations in order to enhance metabolite production. In addition, traditional mutagenesis programs have been used by the pharmaceutical industry for strain and production yield improvement. In the last years, the development of recombinant DNA technology has provided new tools for approaching yields improvement by means of genetic manipulation of biosynthetic pathways. These efforts are usually focused in redirecting precursor metabolic fluxes, deregulation of biosynthetic pathways and overexpression of specific enzymes involved in metabolic bottlenecks. In addition, efforts have been made for the heterologous expression of biosynthetic gene clusters in other organisms, looking not only for an increase of production levels but also to speed the process by using rapidly growing and easy to manipulate organisms compared to the producing organism. In this review, we will focus on these genetic approaches as applied to bioactive secondary metabolites produced by actinomycetes.

Structure, function, and evolution of bacterial ATP-binding cassette systems

SUMMARY

ATP-binding cassette (ABC) systems are universally distributed among living organisms and function in many different aspects of bacterial physiology. ABC transporters are best known for their role in the import of essential nutrients and the export of toxic molecules, but they can also mediate the transport of many other physiological substrates. In a classical transport reaction, two highly conserved ATP-binding domains or subunits couple the binding/hydrolysis of ATP to the translocation of particular substrates across the membrane, through interactions with membrane-spanning domains of the transporter. Variations on this basic theme involve soluble ABC ATP-binding proteins that couple ATP hydrolysis to nontransport processes, such as DNA repair and gene expression regulation. Insights into the structure, function, and mechanism of action of bacterial ABC proteins are reported, based on phylogenetic comparisons as well as classic biochemical and genetic approaches. The availability of an increasing number of high-resolution structures has provided a valuable framework for interpretation of recent studies, and realistic models have been proposed to explain how these fascinating molecular machines use complex dynamic processes to fulfill their numerous biological functions. These advances are also important for elucidating the mechanism of action of eukaryotic ABC proteins, because functional defects in many of them are responsible for severe human inherited diseases.

Detection and characterization of an ABC transporter in Clostridium hathewayi

An ABC transporter gene from Clostridium hathewayi is characterized. It has duplicated ATPase domains in addition to a transmembrane protein. Its deduced amino acid sequence has conserved functional domains with ATPase components of the multidrug efflux pump genes of several bacteria. Cloning this transporter gene into C. perfringens and E. coli resulted in decreased sensitivities of these bacteria to fluoroquinolones. It also decreased the accumulation and increased the efflux of ethidium bromide from cells containing the cloned gene. Carbonyl cyanide-m-chlorophenylhydrazone (CCCP) inhibited both accumulation and efflux of ethidium bromide from these cells. The ATPase mRNA was overexpressed in the fluoroquinolone-resistant strain when exposed to ciprofloxacin. This is the first report of an ABC transporter in C. hathewayi.

Involvement of a chromomycin ABC transporter system in secretion of a deacetylated precursor during chromomycin biosynthesis

Chromomycin A(3) is an antitumour antibiotic that acts by inhibiting transcription and replication of DNA. The producer micro-organism Streptomyces griseus subsp. griseus is highly resistant to chromomycin A(3) and to the structurally related compound mithramycin upon induction with chromomycin A(3). The biosynthetic gene cluster of chromomycin contains three genes involved in self-resistance to chromomycin in S. griseus: cmrA and cmrB encode a type I ATP-binding cassette (ABC) transporter, and cmrX encodes a UvrA-like protein of ABC excision nuclease systems. These genes are linked in the chromosome, together with a gene encoding a transcriptional repressor (cmmRII). Involvement of these genes in chromomycin resistance was determined through gene inactivation, and heterologous expression in Streptomyces albus. Inactivation of cmrX produced a chromomycin-sensitive low-producer strain, while inactivation of cmmRII generated a high-chromomycin-producer strain, which was resistant to chromomycin, and also to mithramycin. Expression of either cmrA and cmrB, or cmrX, in S. albus generated strains with low chromomycin resistance; it was therefore necessary to co-express the three genes to achieve high levels of resistance. However, the CmrAB ABC transporter conferred a high level of resistance to the biosynthesis intermediate 4A,4E-O-dideacetyl-chromomycin A(3). A model is proposed for the biosynthesis of, and self-resistance to, chromomycin A(3) in S. griseus subsp. griseus.

Molecular analysis of the benastatin biosynthetic pathway and genetic engineering of altered fatty acid-polyketide hybrids

The entire gene locus encoding the biosynthesis of the potent glutathione-S-transferase inhibitors and apoptosis inducers benastatin A and B has been cloned and sequenced. The cluster identity was unequivocally proven by deletion of flanking regions and heterologous expression in S. albus and S. lividans. Inactivation and complementation experiments revealed that a KSIII component (BenQ) similar to FabH is crucial for providing and selecting the rare hexanoate PKS starter unit. In the absence of BenQ, several novel penta- and hexacyclic benastatin derivatives with antiproliferative activities are formed. In total, five new compounds were isolated and fully characterized, and the chemical analysis was confirmed by derivatization. The most intriguing observation is that the ben PKS can utilize typical straight and branched fatty acid synthase primers. If shorter straight-chain starters are utilized, the length of the polyketide backbone is increased, resulting in the formation of an extended, hexacyclic ring system reminiscent of proposed intermediates in the griseorhodin and fredericamycin pathways. Analysis and manipulation of the hybrid fatty acid polyketide pathway provides strong support for the hypothesis that the number of chain elongations is dependent on the total size of the polyketide chain that is accommodated in the PKS enzyme cavity. Our results also further substantiate the potential of metabolic engineering toward polyphenols with altered substituents and ring systems.

Identification and characterization of a putative ABC transporter PltHIJKN required for pyoluteorin production in Pseudomonas sp. M18

A putative ABC (ATP-binding cassette) transport gene cluster pltHIJKN was identified and characterized within a 7.5-kb genome region downstream of the antibiotic pyoluteorin (Plt) biosynthetic gene cluster in Pseudomonas sp. M18, a rhizosphere bacterium which is of ecological importance for controlling plant diseases caused by soil-borne fungal pathogens. The sequence similarity, conserved domains and hydrophobicity profiles strongly suggest that the pltHIJKN gene products are integrated into a typical three-component ABC export system, which consists of the inner membrane ABC transporter PltIJK, the membrane fusion protein PltH and the outer membrane efflux protein PltN. Mutant strains of M18 defective in pltH or pltI did not produce detectable levels of Plt. Overexpression of the entire pltHIJKN gene cluster resulted in a significant increase of Plt production. Heterogenous expression of the pltHIJKN gene cluster gave rise to a significant enhancement of resistance of E. coli DH5alpha to exogenous Plt. These results indicate that PltHIJKN is required for Plt biosynthesis and resistance, which is likely to be mediated by Plt export using the PltHIJKN transport system. Exogenous Plt induced the expression of both the Plt biosynthetic gene cluster and the ABC transport gene cluster pltHIJKN at the transcriptional level, suggesting that Plt biosynthesis and expression of pltHIJKN are coordinately and similarly regulated in Pseudomonas sp. M18.

Putative ABC transporter responsible for acetic acid resistance in Acetobacter aceti

Two-dimensional gel electrophoretic analysis of the membrane fraction of Acetobacter aceti revealed the presence of several proteins that were produced in response to acetic acid. A 60-kDa protein, named AatA, which was mostly induced by acetic acid, was prepared; aatA was cloned on the basis of its NH2-terminal amino acid sequence. AatA, consisting of 591 amino acids and containing ATP-binding cassette (ABC) sequences and ABC signature sequences, belonged to the ABC transporter superfamily. The aatA mutation with an insertion of the neomycin resistance gene within the aatA coding region showed reduced resistance to acetic acid, formic acid, propionic acid, and lactic acid, whereas the aatA mutation exerted no effects on resistance to various drugs, growth at low pH (adjusted with HCl), assimilation of acetic acid, or resistance to citric acid. Introduction of plasmid pABC101 containing aatA under the control of the Escherichia coli lac promoter into the aatA mutant restored the defect in acetic acid resistance. In addition, pABC101 conferred acetic acid resistance on E. coli. These findings showed that AatA was a putative ABC transporter conferring acetic acid resistance on the host cell. Southern blot analysis and subsequent nucleotide sequencing predicted the presence of aatA orthologues in a variety of acetic acid bacteria belonging to the genera Acetobacter and Gluconacetobacter. The fermentation with A. aceti containing aatA on a multicopy plasmid resulted in an increase in the final yield of acetic acid.

Nystatin biosynthesis and transport: nysH and nysG genes encoding a putative ABC transporter system in Streptomyces noursei ATCC 11455 are required for efficient conversion of 10-deoxynystatin to nystatin

The genes nysH and nysG, encoding putative ABC-type transporter proteins, are located at the flank of the nystatin biosynthetic gene cluster in Streptomyces noursei ATCC 11455. To assess the possible roles of these genes in nystatin biosynthesis, they were inactivated by gene replacements leading to in-frame deletions. Metabolite profile analysis of the nysH and nysG deletion mutants revealed that both of them synthesized nystatin at a reduced level and produced considerable amounts of a putative nystatin analogue. Liquid chromatography-mass spectrometry and nuclear magnetic resonance structural analyses of the latter metabolite confirmed its identity as 10-deoxynystatin, a nystatin precursor lacking a hydroxyl group at C-10. Washing experiments demonstrated that both nystatin and 10-deoxynystatin are transported out of cells, suggesting the existence of an alternative efflux system(s) for the transport of nystatin-related metabolites. This notion was further corroborated in experiments with the ATPase inhibitor sodium o-vanadate, which affected the production of nystatin and 10-deoxynystatin in the wild-type strain and transporter mutants in a different manner. The data obtained in this study suggest that the efflux of nystatin-related polyene macrolides occurs through several transporters and that the NysH-NysG efflux system provides conditions favorable for C-10 hydroxylation.

Transcriptional termination control of a novel ABC transporter gene involved in antibiotic resistance in Bacillus subtilis

In members of one of the subfamilies of the bacterial ATP binding cassette (ABC) transporters, the two nucleotide binding domains are fused as a single peptide and the proteins have no membrane-spanning domain partners. Most of the ABC efflux transporters of this subfamily have been characterized in actinomycetes, producing macrolide, lincosamide, and streptogramin antibiotics. Among 40 ABC efflux transporters of Bacillus subtilis, five proteins belong to this subfamily. None of these proteins has been functionally characterized. We examined macrolide, lincosamide, and streptogramin antibiotic resistance in insertional disruptants of the genes that encode these proteins. It was found that only a disruptant of vmlR (formerly named expZ) showed hypersensitivity to virginiamycin M and lincomycin. Expression of the vmlR gene was induced by the addition of these antibiotics in growth medium. Primer extension analysis revealed that transcription of the vmlR gene initiates at an adenosine residue located 225 bp upstream of the initiation codon. From the analysis of the vmlR and lacZ fusion genes, a 52-bp deletion from +159 to +211 resulted in constitutive expression of the vmlR gene. In this region, a typical rho-independent transcriptional terminator was found. It was suggested that the majority of transcription ends at this termination signal in the absence of antibiotics, whereas under induced conditions, RNA polymerase reads through the terminator, and transcription continues to the downstream vmlR coding region, resulting in an increase in vmlR expression. No stabilization of vmlR mRNA occurred under the induced conditions.

Secretion systems for secondary metabolites: how producer cells send out messages of intercellular communication

Many secondary metabolites (e.g. antibiotics and mycotoxins) are toxic to the microorganisms that produce them. The clusters of genes that are responsible for the biosynthesis of secondary metabolites frequently contain genes for resistance to these toxic metabolites, such as different types of multiple drug resistance systems, to avoid suicide of the producer strains. Recently there has been research into the efflux systems of secondary metabolites in bacteria and in filamentous fungi, such as the large number of ATP-binding cassette transporters found in antibiotic-producing Streptomyces species and that are involved in penicillin secretion in Penicillium chrysogenum. A different group of efflux systems, the major facilitator superfamily exporters, occur very frequently in a variety of bacteria that produce pigments or antibiotics (e.g. the cephamycin and thienamycin producers) and in filamentous fungi that produce mycotoxins. Such efflux systems include the CefT exporters that mediate cephalosporin secretion in Acremonium chrysogenum. The evolutionary origin of these efflux systems and their relationship with current resistance determinants in pathogenic bacteria has been analyzed. Genetic improvement of the secretion systems of secondary metabolites in the producer strain has important industrial applications.

Molecular analysis of resistance to streptogramin A compounds conferred by the Vga proteins of staphylococci

The Vga and Msr resistance determinants, encoded by mobile genetic elements in various staphylococcal strains, belong to a family of ATP-binding cassette (ABC) proteins whose functions and structures are ill defined. Their amino acid sequences are similar to those of proteins involved in the immunity of streptomycetes to the macrolide-lincosamide-streptogramin antibiotics that they produce. Sequence analysis of the genomes of the gram-positive bacteria with low G+C contents revealed that Lmo0919 from Listeria monocytogenes is more closely related to Vga variants than to Msr variants. In the present study we compared the antibiotic resistance profiles conferred by the Vga-like proteins in two staphylococcal hosts. It was shown that Vga(A), the Vga(A) variant [Vga(A)v], and Lmo0919 can confer resistance to lincosamides and streptogramin A compounds, while only Vga(B) is able to increase the level of resistance to pristinamycin, a mixture of streptogramin A and streptogramin B compounds. By using polyclonal antibodies, we found that the Vga(A) protein colocalized with the beta subunit of the F(1)-F(0) ATPase in the membrane fractions of staphylococcal cells. In order to identify functional units in these atypical ABC proteins, such as regions that might be involved in substrate specificity and/or membrane targeting, we analyzed the resistance phenotypes conferred by various plasmids carrying parts or modified versions of the vga(A) gene and we determined the subcellular localization of the gene products. Only polypeptides composed of two ABC domains were detected in the cell membranes. No region of drug specificity was identified. Resistance properties were dependent on the integrities of both Walker B motifs.

An Enterococcus faecalis ABC homologue (Lsa) is required for the resistance of this species to clindamycin and quinupristin-dalfopristin

Enterococcus faecalis isolates are resistant to clindamycin (CLI) and quinupristin-dalfopristin (Q-D), and this is thought to be a species characteristic. Disruption of a gene (abc-23, now designated lsa, for "lincosamide and streptogramin A resistance") of E. faecalis was associated with a > or =40-fold decrease in MICs of Q-D (to 0.75 microg/ml), CLI (to 0.12 to 0.5 microg/ml), and dalfopristin (DAL) (to 4 to 8 microg/ml) for the wild-type E. faecalis parental strain (Q-D MIC, 32 microg/ml; CLI MIC, 32 to 48 microg/ml; DAL MIC, 512 microg/ml). Complementation of the disruption mutant with lsa on a shuttle plasmid resulted in restoration of the MICs of CLI, Q-D, and DAL to wild-type levels. Under high-stringency conditions, lsa was found in 180 of 180 isolates of E. faecalis but in none of 189 other enterococci. Among 19 erm(B)-lacking Enterococcus faecium strains, 9 (47%) were highly susceptible to CLI (MIC, 0.06 to 0.25 microg/ml) and had DAL MICs of 4 to 16 microg/ml; for the remaining erm(B)-lacking E. faecium strains, the CLI and DAL MICs were 4 to > 256 and 2 to > 128 microg/ml, respectively. In contrast, none of 32 erm(B)-lacking E. faecalis strains were susceptible (CLI MIC range, 16 to 32 microg/ml; DAL MIC range, > or =32 microg/ml). When lsa was introduced into an E. faecium strain initially susceptible to CLI, the MICs of CLI and DAL increased > or =60-fold and that of Q-D increased 6-fold (to 3 to 6 microg/ml). Introduction of lsa into two DAL-resistant (MICs, > 128 microg/ml), Q-D-susceptible (MICs, 0.5 and 1.5 microg/ml) E. faecium strains (CLI MICs, 12 and >256 microg/ml) resulted in an increase in the Q-D MICs from 3- to 10-fold (to 8 and >32 microg/ml), respectively. Although efflux was not studied, the similarity (41 to 64%) of the predicted Lsa protein to ABC proteins such as Vga(A), Vga(B), and Msr(A) of Staphylococcus aureus and YjcA of Lactococcus lactis and the presence of Walker A and B ATP-binding motifs suggest that this resistance may be related to efflux of these antibiotics. In conclusion, lsa appears to be an intrinsic gene of E. faecalis that explains the characteristic resistance of this species to CLI and Q-D.

Oviedomycin, an unusual angucyclinone encoded by genes of the oleandomycin-producer Streptomyces antibioticus ATCC11891

Our investigations on the discovery of novel natural metabolites using type II polyketide synthase gene probes (actI/III) yielded an unusual angucyclinone, oviedomycin (2), when applied to the oleandomycin (1) producer Streptomyces antibioticus ATCC11891. The novel natural product was produced using S. albus R(-)M(-) as a host strain, into which a cosmid containing the oviedomycin gene cluster was transformed. Its structure was elucidated by NMR spectroscopy and mass spectrometry.

Global analysis of growth phase responsive gene expression and regulation of antibiotic biosynthetic pathways in Streptomyces coelicolor using DNA microarrays

The eubacterial species Streptomyces coelicolor proceeds through a complex growth cycle in which morphological differentiation/development is associated with a transition from primary to secondary metabolism and the production of antibiotics. We used DNA microarrays and mutational analysis to investigate the expression of individual genes and multigene antibiotic biosynthetic pathways during these events. We identified expression patterns in biosynthetic, regulatory, and ribosomal protein genes that were associated highly specifically with particular stages of development. A knowledge-based algorithm that correlates temporal changes in expression with chromosomal position identified groups of contiguous genes expressed at discrete stages of morphological development, inferred the boundaries of known antibiotic synthesis gene loci, and revealed novel physical clusters of coordinately regulated genes. Microarray analysis of RNA from cells mutated in genes regulating synthesis of the antibiotics actinorhodin (Act) and undecylprodigiosin (Red) identified proximate and distant sites that contain putative ABC transporter and two-component system genes expressed coordinately with genes of specific biosynthetic pathways and indicated the existence of two functionally and physically discrete regulons in the Red pathway.

Functional analysis of OleY L-oleandrosyl 3-O-methyltransferase of the oleandomycin biosynthetic pathway in Streptomyces antibioticus

Oleandomycin, a macrolide antibiotic produced by Streptomyces antibioticus, contains two sugars attached to the aglycon: L-oleandrose and D-desosamine. oleY codes for a methyltransferase involved in the biosynthesis of L-oleandrose. This gene was overexpressed in Escherichia coli to form inclusion bodies and in Streptomyces lividans, producing a soluble protein. S. lividans overexpressing oleY was used as a biotransformation host, and it converted the precursor L-olivosyl-erythronolide B into its 3-O-methylated derivative, L-oleandrosyl-erythronolide B. Two other monoglycosylated derivatives were also substrates for the OleY methyltransferase: L-rhamnosyl- and L-mycarosyl-erythronolide B. OleY methyltransferase was purified yielding a 43-kDa single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native enzyme showed a molecular mass of 87 kDa by gel filtration chromatography, indicating that the enzyme acts as a dimer. It showed a narrow pH range for optimal activity, and its activity was clearly stimulated by the presence of several divalent cations, being maximal with Co(2+). The S. antibioticus OleG2 glycosyltransferase is proposed to transfer L-olivose to the oleandolide aglycon, which is then converted into L-oleandrose by the OleY methyltransferase. This represents an alternative route for L-oleandrose biosynthesis from that in the avermectin producer Streptomyces avermitilis, in which L-oleandrose is transferred to the aglycon by a glycosyltransferase.

The role of ABC transporters in antibiotic-producing organisms: drug secretion and resistance mechanisms

Knowledge about biosynthetic gene clusters from antibiotic-producing actinomycetes is continuously increasing and the presence of an ABC transporter system is a fairly general phenomenon in most of these clusters. These transporters are involved in the secretion of the antibiotic through the cell membrane and also contribute to self resistance to the produced antibiotic.

Molecular properties of bacterial multidrug transporters

One of the mechanisms that bacteria utilize to evade the toxic effects of antibiotics is the active extrusion of structurally unrelated drugs from the cell. Both intrinsic and acquired multidrug transporters play an important role in antibiotic resistance of several pathogens, including Neisseria gonorrhoeae, Mycobacterium tuberculosis, Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Vibrio cholerae. Detailed knowledge of the molecular basis of drug recognition and transport by multidrug transport systems is required for the development of new antibiotics that are not extruded or of inhibitors which block the multidrug transporter and allow traditional antibiotics to be effective. This review gives an extensive overview of the currently known multidrug transporters in bacteria. Based on energetics and structural characteristics, the bacterial multidrug transporters can be classified into five distinct families. Functional reconstitution in liposomes of purified multidrug transport proteins from four families revealed that these proteins are capable of mediating the export of structurally unrelated drugs independent of accessory proteins or cytoplasmic components. On the basis of (i) mutations that affect the activity or the substrate specificity of multidrug transporters and (ii) the three-dimensional structure of the drug-binding domain of the regulatory protein BmrR, the substrate-binding site for cationic drugs is predicted to consist of a hydrophobic pocket with a buried negatively charged residue that interacts electrostatically with the positively charged substrate. The aromatic and hydrophobic amino acid residues which form the drug-binding pocket impose restrictions on the shape and size of the substrates. Kinetic analysis of drug transport by multidrug transporters provided evidence that these proteins may contain multiple substrate-binding sites.

Identification and expression of genes involved in biosynthesis of L-oleandrose and its intermediate L-olivose in the oleandomycin producer Streptomyces antibioticus

A 9.8-kb DNA region from the oleandomycin gene cluster in Streptomyces antibioticus was cloned. Sequence analysis revealed the presence of 8 open reading frames encoding different enzyme activities involved in the biosynthesis of one of the two 2, 6-deoxysugars attached to the oleandomycin aglycone: L-oleandrose (the oleW, oleV, oleL, and oleU genes) and D-desosamine (the oleNI and oleT genes), or of both (the oleS and oleE genes). A Streptomyces albus strain harboring the oleG2 glycosyltransferase gene integrated into the chromosome was constructed. This strain was transformed with two different plasmid constructs (pOLV and pOLE) containing a set of genes proposed to be required for the biosynthesis of dTDP-L-olivose and dTDP-L-oleandrose, respectively. Incubation of these recombinant strains with the erythromycin aglycon (erythronolide B) gave rise to two new glycosylated compounds, identified as L-3-O-olivosyl- and L-3-O-oleandrosyl-erythronolide B, indicating that pOLV and pOLE encode all enzyme activities required for the biosynthesis of these two 2,6-dideoxysugars. A pathway is proposed for the biosynthesis of these two deoxysugars in S. antibioticus.

Identification of an AfsA homologue (BarX) from Streptomyces virginiae as a pleiotropic regulator controlling autoregulator biosynthesis, virginiamycin biosynthesis and virginiamycin M1 resistance

Virginiae butanolide (VB)-BarA of Streptomyces virginiae is one of the newly discovered pairs of a gamma-butyrolactone autoregulator and the corresponding receptor protein of the Streptomyces species, and has been shown to regulate the production of antibiotic virginiamycin (VM) in S. virginiae. A divergently transcribed barX gene is situated 259 bp upstream of the barA gene, and the BarX protein has been shown to be highly homologous (39.8% identity, 74. 6% similarity) to S. griseus AfsA. Although AfsA is thought to be a biosynthetic enzyme for A-factor, another member of the family of gamma-butyrolactone autoregulators, the in vivo function of S. virginiae BarX was investigated in this study by phenotypic and transcriptional comparison between wild-type S. virginiae and a barX deletion mutant. With the same growth rate as wild-type S. virginiae on both solid and liquid media, the barX mutant showed no apparent changes in its morphological behaviour, indicating that barX does not participate in morphological control in S. virginiae. However, the barX mutant became more sensitive to virginiamycin M1 than did the wild-type strain (minimum inhibitory concentration, 50 microgram ml-1 compared with > 200 microgram ml-1) and exhibited reduced VB and VM production. The VM production was not restored by exogenous addition of VB, suggesting that BarX per se is not a biosynthetic enzyme of VBs but a pleiotropic regulatory protein controlling VB biosynthesis. DNA sequencing of a 5.6 kbp downstream region of barX revealed the presence of five open reading frames (ORFs): barZ, encoding a BarB-like regulatory protein; orf2, encoding a Streptomyces coelicolor RedD-like pathway specific regulator; varM, encoding a homologue of ATP-dependent transporters for macrolide antibiotics; orf4, encoding a homologue of beta-ketoacyl ACP/CoA reductase; and orf5, encoding a homologue of dNDP-glucose dehydratase. Reverse transcription polymerase chain reaction (RT-PCR) analyses of the downstream five genes together with those of the three upstream genes (barA, barB, encoding a regulatory protein; and varS, encoding a virginiamycin S specific transporter) revealed that, in the barX mutant, the transcriptions of barZ, orf2, varM and orf5 were completely repressed and those of barB and varS were derepressed. Because free BarA (BarA in the absence of VB) in wild-type S. virginiae represses the transcription of bicistronic barB-varS operon through binding to a specific DNA sequence (BarA-responsive element, BARE) overlapping the barB transcriptional start site, the derepression of barB-varS transcription in the barX mutant suggested that the in vivo function of BarA was impaired by the lack of BarX protein. Gel-shift assays revealed that BarA easily lost its DNA-binding activity in the absence of BarX but that the defect was restored by the presence of recombinant BarX as a fusion with maltose-binding protein (MBP-BarX), whereas MBP-BarX itself showed no DNA-binding activity, indicating that BarX is likely to be a co-repressor of BarA, enforcing the DNA-binding activity of BarA through protein-protein interactions.

Identification of a gene cluster for antibacterial polyketide-derived antibiotic biosynthesis in the nystatin producer Streptomyces noursei ATCC 11455

Streptomyces noursei ATCC 11455 produces the antifungal polyene antibiotic nystatin containing the deoxysugar moiety mycosamine. Part of the deoxythymidyl diphosphate (TDP)-glucose dehydratase gene (gdhA) known to be involved in deoxysugar biosynthesis was amplified by PCR from genomic DNA of S. noursei ATCC 11455. A gene library for S. noursei was made and screened with the gdhA probe. Several overlapping phage clones covering about 30 kb of the S. noursei genome were physically mapped. A partial DNA sequencing analysis of this region resulted in the identification of several putative genes typical of macrolide antibiotic biosynthetic gene clusters. A gene-transfer system for 5. noursei has been established, and gene deletion or disruption experiments within the putative biosynthetic gene cluster were performed. All of the knock-out mutants retained the ability to produce nystatin, suggesting that the identified gene cluster is not involved in biosynthesis of this antibiotic. Culture extracts from the wild-type strain and three knock-out mutants were analysed by TLC followed by a bioassay against Micrococcus luteus. Two antibacterial compounds were found to be synthesized by the wild-type strain while only one was produced by the mutants. This provided evidence for the involvement of the identified gene cluster in the biosynthesis of a presumably novel antibacterial macrolide antibiotic in S. noursei.

Interspecies complementation in Saccharopolyspora erythraea : elucidation of the function of oleP1, oleG1 and oleG2 from the oleandomycin biosynthetic gene cluster of Streptomyces antibioticus and generation of new erythromycin derivatives

Two glycosyltransferase genes, oleG1 and oleG2, and a putative isomerase gene, oleP1, have previously been identified in the oleandomycin biosynthetic gene cluster of Streptomyces antibioticus. In order to identify which of these two glycosyltransferases encodes the desosaminyltransferase and which the oleandrosyltransferase, interspecies complementation has been carried out, using two mutant strains of Saccharopolyspora erythraea, one strain carrying an internal deletion in the eryCIII (desosaminyltransferase) gene and the other an internal deletion in the eryBV (mycarosyltransferase) gene. Expression of the oleG1 gene in the eryCIII deletion mutant restored the production of erythromycin A (although at a low level), demonstrating that oleG1 encodes the desosaminyltransferase required for the biosynthesis of oleandomycin and indicating that, as in erythromycin biosynthesis, the neutral sugar is transferred before the aminosugar onto the macrocyclic ring. Significantly, when an intact oleG2 gene (presumed to encode the oleandrosyltransferase) was expressed in the eryBV deletion mutant, antibiotic activity was also restored and, in addition to erythromycin A, new bioactive compounds were produced with a good yield. The neutral sugar residue present in these compounds was identified as L-rhamnose attached at position C-3 of an erythronolide B or a 6-deoxyerythronolide B lactone ring, thus indicating a relaxed specificity of the oleandrosyltransferase, OleG2, for both the activated sugar and the macrolactone substrate. The oleP1 gene located immediately upstream of oleG1 was likewise introduced into an eryCII deletion mutant of Sac. erythraea, and production of erythromycin A was again restored, demonstrating that the function of OleP1 is identical to that of EryCII in the biosynthesis of dTDP-D-desosamine, which we have previously proposed to be a dTDP-4-keto-6-deoxy-D-glucose 3, 4-isomerase.

Dispensable ribosomal resistance to spiramycin conferred by srmA in the spiramycin producer Streptomyces ambofaciens

Streptomyces ambofaciens produces the macrolide antibiotic spiramycin, an inhibitor of protein synthesis, and possesses multiple resistance mechanisms to the produced antibiotic. Several resistance determinants have been isolated from S. ambofaciens and studies with one of them, srmA, which hybridized with ermE (the erythromycin-resistance gene from Saccharopolyspora erythraea), are detailed here. The nucleotide sequence of srmA was determined and the mechanism by which its product confers resistance was characterized. The SrmA protein is a methyltransferase which introduces a single methyl group into A-2058 (Escherichia coli numbering scheme) in the large rRNA, thereby conferring an MLS (macrolide-lincosamide-streptogramin type B) type I resistance phenotype. A mutant of S. ambofaciens in which srmA was inactivated was viable and still produced spiramycin, indicating that srmA is dispensable, at least in the presence of the other resistance determinants.

A silent ABC transporter isolated from Streptomyces rochei F20 induces multidrug resistance

In the search for heterologous activators for actinorhodin production in Streptomyces lividans, 3.4 kb of DNA from Streptomyces rochei F20 (a streptothricin producer) were characterized. Subcloning experiments showed that the minimal DNA fragment required for activation was 0.4 kb in size. The activation is mediated by increasing the levels of transcription of the actII-ORF4 gene. Sequencing of the minimal activating fragment did not reveal any clues about its mechanism; nevertheless, it was shown to overlap the 3' end of two convergent genes, one of whose translated products (ORF2) strongly resembles that of other genes belonging to the ABC transporter superfamily. Computer-assisted analysis of the 3.4-kb DNA sequence showed the 3' terminus of an open reading frame (ORF), i.e., ORFA, and three complete ORFs (ORF1, ORF2, and ORFB). Searches in the databases with their respective gene products revealed similarities for ORF1 and ORF2 with ATP-binding proteins and transmembrane proteins, respectively, which are found in members of the ABC transporter superfamily. No similarities for ORFA and ORFB were found in the databases. Insertional inactivation of ORF1 and ORF2, their transcription analysis, and their cloning in heterologous hosts suggested that these genes were not expressed under our experimental conditions; however, cloning of ORF1 and ORF2 together (but not separately) under the control of an expressing promoter induced resistance to several chemically different drugs: oleandomycin, erythromycin, spiramycin, doxorubicin, and tetracycline. Thus, this genetic system, named msr, is a new bacterial multidrug ABC transporter.

Two glycosyltransferases and a glycosidase are involved in oleandomycin modification during its biosynthesis by Streptomyces antibioticus

A 5.2 kb region from the oleandomycin gene cluster in Streptomyces antibioticus located between the oleandomycin polyketide synthase gene and sugar biosynthetic genes was cloned. Sequence analysis revealed the presence of three open reading frames (designated oleI, oleN2 and oleR). The oleI gene product resembled glycosyltransferases involved in macrolide inactivation including the oleD product, a previously described glycosyltransferase from S. antibioticus. The oleN2 gene product showed similarities with different aminotransferases involved in the biosynthesis of 6-deoxyhexoses. The oleR gene product was similar to several glucosidases from different origins. The oleI, oleR and oleD genes were expressed in Streptomyces lividans. OleI and OleD intracellular proteins were partially purified by affinity chromatography in an UDP-glucuronic acid agarose column and OleR was detected as a major band from the culture supernatant. OleI and OleD showed oleandomycin glycosylating activity but they differ in the pattern of substrate specificity: OleI being much more specific for oleandomycin. OleR showed glycosidase activity converting glycosylated oleandomycin into active oleandomycin. A model is proposed integrating these and previously reported results for intracellular inactivation, secretion and extracellular reactivation of oleandomycin.

ABC transporters in antibiotic-producing actinomycetes

Many antibiotic-producing actinomycetes possess at least one ABC (ATP-binding cassette) transporter which forms part of the antibiotic biosynthetic pathway and in most cases confers resistance to the drug in an heterologous host. Three types of antibiotic ABC transporters have been so far described in producer organisms. In Type I two genes are involved, one encoding a hydrophilic ATP-binding protein with one nucleotide-binding domain and the other encoding a hydrophobic membrane protein. In Type II transporters only a gene encoding the hydrophilic ATP-binding protein with two nucleotide-binding domains is present and no gene encoding a hydrophobic membrane protein has been found. In Type III only one gene is involved which encodes both the hydrophilic and hydrophobic components. Possibly these ABC transporters are responsible for secretion of the antibiotics outside the cells. A comparative analysis of the ATP-binding components of the different antibiotic ABC transporters and analysis of the amino acid distances between the so-called Walker motifs suggests that the three types of transporters have probably evolved from a common ancestor containing a single nucleotide-binding domain.

Staphylococcal resistance to streptogramins and related antibiotics

Streptogramin and related antibiotics are mixtures of two compounds, A and B (e.g. Dalfopristin and Quinupristin), particularly against Gram-positive bacteria. Staphylococci resistant to these mixtures are always resistant to the A compounds but are not necessarily resistant to the B compounds. Resistance to A compounds and to the mixtures is conferred by acetyltransferases or ATP-binding proteins via unknown mechanisms. Several genes encoding each of the two categories of protein have been characterized and regularly detected on plasmids. Genes encoding lactonases, which inactivate B compounds, have been occasionally detected on these plasmids. Staphylococci which harbour plasmids conferring resistance to A compounds should not be treated with the mixtures even if they appear susceptible in vitro. Indeed, susceptibility to the mixtures of staphylococci carrying resistance to A compounds has often been attributed to partial loss of the plasmids conferring this resistance. When staphylococci are constitutively resistant to B compounds, the in vitro activities of the mixtures should be evaluated, because they are better correlated than MICs with their efficacy in therapy.

Characterization of a new staphylococcal gene, vgaB, encoding a putative ABC transporter conferring resistance to streptogramin A and related compounds

The Staphylococcus aureus plasmid gene, vgaB, conferring resistance to streptogramin (SgA) and related compounds (PIIA, virginiamycin M, mikamycin A, synergistin A, Dalfopristin) was cloned and sequenced. This gene potentially encodes a 552-aa protein, VgaB, of 61,327 Da, which exhibits a significant similarity with the ATP-binding domains of numerous proteins. VgaB has two ATP-binding domains containing each of the A and the B motifs described by Walker et al. [Walker, J.E., Saraste, M., Runswick, M.J., Gay, N.J., 1982. Distantly related sequences in the alpha- and beta-subunits of ATP synthase, myosin, kinases and other ATP-requiring enzymes and a common nucleotide binding fold. EMBO J., 1, 945-951], but does not include TM hydrophobic domains. The 155-amino-acid sequence between the two ATP-binding domains of VgaB is richer in Glu than the rest of the protein. The vgaB gene was found in 21 of the 52 SgA(R) and independent wt staphylococci investigated. In each of the 21 staphylococci, vgaB was carried on a plasmid of 50-90 kb also harboring the vatB gene encoding an acetyltransferase inactivating SgA. In all plasmids, vgaB and vatB have the same relative positions.

Mechanisms of multidrug transporters

Drug resistance, mediated by various mechanisms, plays a crucial role in the failure of the drug-based treatment of various infectious diseases. As a result, these infectious diseases re-emerge rapidly and cause many victims every year. Another serious threat is imposed by the development of multidrug resistance (MDR) in eukaryotic (tumor) cells, where many different drugs fail to perform their therapeutic function. One of the causes of the occurrence of MDR in these cells is the action of transmembrane transport proteins that catalyze the active extrusion of a large number of structurally and functionally unrelated compounds out of the cell. The mode of action of these MDR transporters and their apparent lack of substrate specificity is poorly understood and has been subject to many speculations. In this review we will summarize our current knowledge about the occurrence, mechanism and molecular basis of (multi-)drug resistance especially as found in bacteria.

Interaction between ATP, oleandomycin and the OleB ATP-binding cassette transporter of Streptomyces antibioticus involved in oleandomycin secretion

The OleB protein of Streptomyces antibioticus, oleandomycin (OM) producer, constitutes an ATP-binding cassette transporter containing two nucleotide-binding domains and is involved in OM resistance and its secretion in this producer strain. We have characterized some properties of the first nucleotide-binding domain of OleB using an overexpressed fusion protein (MBP-OleB') between a maltose-binding protein (MBP) and the first half of OleB (OleB'). Extrinsic fluorescence of the base-modified fluorescent nucleotide analogue 1,N6-ethenoadenosine 5'-triphosphate (epsilon ATP) and 2'(3')-o-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate was determined in the presence of MBP and the fusion protein MBP-OleB', and it was found that epsilon ATP binds to MBP-OleB' with a stoichiometry of 0.9. Measurements of the intrinsic fluorescence of the MBP-OleB' fusion protein indicated that ATP induces a decrease in the accessibility of the MBP-OleB' tryptophans to acrylamide, an indication of a folding effect. This conclusion was confirmed by the fact that ATP also induces considerable stabilization against guanidine chloride denaturation of MBP-OleB'. Two effects were found to be associated with the presence of Mg2+ ions: (1) an increase in the quenching of MBP-OleB' intrinsic fluorescence by ATP; and (2) an increase in the accessibility of MBP-OleB' tryptophans to acrylamide. Significant changes in the intrinsic fluorescence of the fusion protein were also observed in the presence of OM, demonstrating the existence of interaction between the transporter and the antibiotic in the absence of any hydrophobic membrane component.

Topological studies of the membrane component of the OleC ABC transporter involved in oleandomycin resistance in Streptomyces antibioticus

The OleC ABC transporter of Streptomyces antibioticus is constituted by an ATP-binding protein (OleC) and a hydrophobic protein (OleC5). Here we present experimental evidence demonstrating that the OleC5 protein is an integral membrane protein and we propose a topological model for its integration into the membrane. This model is based on the generation of hybrid proteins between different regions of OleC5 and a Escherichia coli beta-lactamase (BlaM) and the determination of the minimal inhibitory concentrations to ampicillin in these constructions. Fusions were generated both by cloning specific fragments of oleC5 and by creating ExoIII nested deletions of the gene. In the topological model proposed there will be six alpha-helix transmembrane regions, two cytoplasmic and four periplasmic loops and a hydrophobic linker domain.

Characterization of the ATPase activity of the N-terminal nucleotide binding domain of an ABC transporter involved in oleandomycin secretion by Streptomyces antibioticus

The oleB gene of Streptomyces antibioticus, oleandomycin producer, encodes an ABC transporter containing two putative ATP-binding domains and is involved in oleandomycin resistance and secretion in this organism. We have overexpressed in Escherichia coli the N-terminal nucleotide-binding domain of OleB (OleB') as a fusion protein and purified the fusion protein by affinity chromatography. The fusion protein showed ATPase activity dependent on the presence of Mg2+ ions. ATPase activity was resistant to specific inhibitors of P-, F-, and V-type ATPase whereas sodium azide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-C1) were strong inhibitors. The change of Lys71, located within the Walker A motif of the OleB' protein, to Gln or Glu caused a loss of ATPase activity, whereas changing to Gly did not impair the activity. The results suggest that the intrinsic ATPase activity of purified fusion protein can be clearly distinguished from other ATP-hydrolysing enzymes, including ion-translocating ATPases or ABC-traffic ATPases, both on the basis of inhibition by different agents and since it hydrolyzes ATP without interacting with a hydrophobic membrane component.