Recombinant Bacillus subtilis Multidrug resistance protein EbrB (ebrB)
Description
Introduction to EbrB and the EbrAB System
The EbrB protein is encoded by the ebrB gene, which was originally cloned from chromosomal DNA of Bacillus subtilis ATCC 9372. The ebrB gene works in tandem with ebrA, with both genes arranged consecutively in the bacterial genome. When expressed together, these genes produce elevated resistance against specific antimicrobial compounds, including ethidium bromide, acriflavine, pyronine Y, and safranin O . This resistance mechanism has been demonstrated not only in the native B. subtilis but also when the genes are transferred to Escherichia coli, confirming their functional role in drug resistance .
The EbrAB system represents a departure from the conventional understanding of SMR family transporters. While most characterized SMR transporters function as single polypeptides or homodimers, the EbrAB system requires both EbrA and EbrB components to confer resistance. Expression studies have clearly demonstrated that neither ebrA nor ebrB alone is sufficient for drug resistance, but when both are present, either on the same plasmid or on separate plasmids, resistance is established . This requirement for two components makes the EbrAB system a novel and intriguing model for studying multidrug resistance mechanisms.
Sequence and Physical Properties
The EbrB protein from Bacillus subtilis strain 168 consists of 117 amino acids with a molecular mass of approximately 12.3 kDa . Its amino acid sequence (MRGLLYLALAIVSEVFGSTMLKLSEGFTQAWPIAGVIVGFLSAFTFLSFSLKTIDLSSAYATWSGVGTALTAIVGFLLFGETISLKGVFGLTLVIAGVVVLNQSKAHAEDKKQTACE) reveals a highly hydrophobic protein consistent with its role as a membrane-embedded transporter . Hydropathy analysis indicates that EbrB contains four hydrophobic regions that likely form transmembrane segments, a characteristic feature of the SMR family of transporters .
Protein Family and Classification
EbrB belongs to the drug/metabolite transporter (DMT) superfamily, specifically within the Small Multidrug Resistance (SMR) family (TC 2.A.7.1) . Within this classification, it falls into the EbrA/EbrB subfamily, reflecting its unique characteristics as part of a heterodimeric transport system. The protein shows significant sequence similarity with other members of the SMR family, including EmrE from Escherichia coli, which is considered the prototypical SMR transporter . The SMR family is characterized by small proteins with typically 100-120 amino acids that confer resistance to various antimicrobial compounds through active efflux mechanisms.
Heterodimer Formation
Research indicates that EbrA and EbrB directly interact to form a functional transporter complex. Evidence suggests they likely form a heterodimer analogous to the EmrE homodimer, the well-characterized SMR transporter from E. coli . This heterodimeric structure is essential for transport function, as neither protein alone can confer drug resistance. Drug resistance profiling and binding experiments have conclusively demonstrated that both proteins must be present for proper transport activity .
The interaction between EbrA and EbrB represents an interesting example of functional complementation, where two similar but distinct proteins combine to create a functional unit with properties different from either component alone. This arrangement may provide advantages in terms of substrate specificity or regulatory control compared to homodimeric transporters.
Transport Mechanism and Substrate Specificity
The EbrAB system functions as an energy-dependent drug efflux pump that removes toxic compounds from the bacterial cell. The efflux activity is likely coupled to an influx of protons, utilizing the proton motive force as an energy source . This mechanism is consistent with other characterized SMR transporters, which typically function as proton/drug antiporters.
Table 1: Substrates of the EbrAB Multidrug Efflux Pump
| Compound | Compound Type | Resistance Observed in E. coli | Resistance Observed in B. subtilis |
|---|---|---|---|
| Ethidium bromide | Cationic dye | Yes | Yes |
| Acriflavine | Cationic dye | Yes | Yes |
| Pyronine Y | Cationic dye | Yes | Yes |
| Safranin O | Cationic dye | Yes | Yes |
| TPP Cl* | Quaternary ammonium compound | Yes | Not reported |
*TPP Cl: Tetraphenylphosphonium chloride
As shown in Table 1, the EbrAB system confers resistance to several cationic lipophilic dyes. Experimental evidence demonstrates that cells expressing both ebrA and ebrB show elevated MICs (Minimum Inhibitory Concentrations) for these compounds compared to control cells . Moreover, direct measurement of ethidium efflux in cells expressing the EbrAB system shows glucose-induced (energy-dependent) transport of this substrate, confirming the active efflux mechanism .
Role of Conserved Glutamates
Genomic Context
The ebrA and ebrB genes are arranged in tandem in the Bacillus subtilis genome, suggesting coordinated expression and function. Sequence analysis has revealed some differences in the ebrAB region between different strains of B. subtilis. For example, the ebrAB region from B. subtilis ATCC 9372 shows only 91% identity with that from strain 168 . Despite these differences, the functional properties of the EbrAB system appear to be conserved across strains.
The ebrAB genes are preceded by a promoter-like sequence and followed by a terminator-like sequence, indicating that they form a distinct transcriptional unit . Interestingly, the native B. subtilis promoter for these genes is functional in E. coli, allowing for expression of the transport system in heterologous hosts .
Expression in Heterologous Systems
The ebrAB genes have been successfully expressed in E. coli, where they confer resistance to various antimicrobial compounds. When the genes are introduced into E. coli KAM3, a strain that lacks the major multidrug efflux pump AcrAB, cells display elevated resistance to ethidium bromide, acriflavine, pyronine Y, and safranin O . This resistance is accompanied by increased energy-dependent efflux of ethidium, confirming the functional expression of the transport system.
When reintroduced into B. subtilis, the ebrAB genes also confer elevated resistance to the same compounds, although the increase in MICs is somewhat smaller compared to that observed in E. coli . This difference may be due to the presence of intrinsic drug efflux pumps in B. subtilis, such as Bmr and the native EbrAB system, which may partially mask the effect of the introduced genes.
Relation to Other SMR Family Transporters
The B. subtilis genome encodes seven homologues of the SMR family, six of which are paired in three distinct operons . This arrangement suggests that heterodimeric SMR transporters may be more common than previously thought. Besides EbrAB, another well-characterized two-component SMR transporter in B. subtilis is the YkkCD system, which also requires both components for drug resistance .
The EbrAB system shows significant sequence similarity with other members of the SMR family, including EmrE from E. coli and Smr from Staphylococcus aureus . It also shares similarities with the SugE proteins found in various bacterial species, including E. coli, Proteus vulgaris, and Citrobacter freundii . These relationships provide insights into the evolutionary history of these transporters and suggest functional similarities.
Unique Features of EbrAB
What distinguishes the EbrAB system from many other SMR transporters is its requirement for two components to form a functional unit. Most characterized SMR transporters, such as EmrE, function as homodimers, where two identical subunits combine to form the active transporter. In contrast, EbrAB represents a heterodimeric system where two different but related proteins must interact to create a functional transporter .
This heterodimeric organization may provide advantages in terms of substrate specificity or regulatory control. The functional differentiation of the two components, as evidenced by the distinct roles of their conserved glutamates, suggests a sophisticated transport mechanism with specialized functions for each subunit.
Bacillus subtilis as a Recombinant Protein Production Host
Bacillus subtilis has emerged as a promising microbial host system for efficient recombinant protein production. As a Gram-positive bacterium, it does not produce endotoxins, making it suitable for various biomedical applications . This has led to its designation as "generally regarded as safe" (GRAS) by the US Food and Drug Administration and "qualified presumption of safety" (QPS) by the European Food Safety Authority .
Potential Applications in Drug Development
The study of multidrug resistance proteins like EbrB provides valuable insights into mechanisms of antibiotic resistance, a growing concern in clinical settings. Knowledge of the structure and function of such proteins can inform the development of inhibitors that might be used in combination with antibiotics to overcome resistance.
The unique heterodimeric nature of the EbrAB system presents an interesting target for drug development. Disrupting the interaction between EbrA and EbrB could potentially inhibit the transport function, sensitizing bacteria to antimicrobial compounds. Understanding the detailed mechanism of this system could therefore contribute to strategies for combating antibiotic resistance.
Product Specs
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Note: All our proteins are shipped with standard blue ice packs. If you require dry ice shipping, please inform us in advance as additional fees will apply.
Generally, liquid form has a shelf life of 6 months at -20°C/-80°C. Lyophilized form has a shelf life of 12 months at -20°C/-80°C.
Target Background
KEGG: bsu:BSU17290
STRING: 224308.Bsubs1_010100009516
Q&A
What is the EbrAB efflux pump in Bacillus subtilis and how does it function?
The EbrAB system in Bacillus subtilis is a two-component multidrug efflux pump that belongs to the small multidrug resistance (SMR) family of transporters. Unlike other known SMR family members that function with a single protein component, EbrAB requires both EbrA and EbrB proteins working together to confer drug resistance . The system functions by actively extruding toxic compounds from bacterial cells using the electrochemical potential of H+ as the driving force. This was confirmed through experiments where the addition of CCCP (a proton conductor) greatly reduced the efflux activity, while the addition of Na+ or Li+ had no significant effect .
What structural characteristics define the EbrB protein?
EbrB, like other members of the SMR family, contains four hydrophobic transmembrane regions. Hydropathy plots calculated using the Eisenberg method show that EbrB possesses these transmembrane segments as expected for SMR family proteins, with a distinctive hydrophilic region at the carboxyl terminus . The protein shows significant sequence similarity to other members of the SMR family of drug transporters, such as EmrE of Escherichia coli and Smr of Staphylococcus aureus, as well as SugEs of E. coli, Proteus vulgaris, and Citrobacter freundii .
What antimicrobial compounds does the EbrAB system confer resistance against?
The EbrAB system provides resistance against multiple structurally distinct compounds. Experimental data shows that B. subtilis cells expressing ebrAB genes exhibit elevated resistance to:
| Antimicrobial Compound | Fold Increase in MIC in E. coli | Fold Increase in MIC in B. subtilis |
|---|---|---|
| Ethidium bromide | 8× | Several-fold |
| Acriflavine | 4× | Several-fold |
| Pyronine Y | 4× | Several-fold |
| Safranin O | 4× | Several-fold |
| Tetraphenylphosphonium chloride | 4× | Not reported |
These compounds are typically cationic, lipophilic molecules that intercalate with DNA or disrupt membrane functions .
What are effective methods for cloning and expressing recombinant ebrB in B. subtilis?
Successful cloning and expression of ebrB in B. subtilis involves several critical steps:
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Gene isolation: The ebrB gene can be isolated from chromosomal DNA of B. subtilis strains using PCR amplification with primers designed based on the known sequence. For initial cloning, researchers have successfully used strains such as B. subtilis ATCC 9372 .
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Vector selection: For expression in B. subtilis, shuttle vectors like pHY300PLK that can replicate in both E. coli and B. subtilis are recommended. This allows initial cloning work in E. coli followed by transfer to B. subtilis .
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Promoter selection: For high-level expression, the B. subtilis rrnO promoter has proven effective. This promoter, combined with appropriate ribosome binding sites (RBS), enables strong expression in vegetative cells .
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Chromosomal integration: For stable expression, integration into the B. subtilis chromosome can be achieved through homologous recombination. The amyE gene locus is commonly used for this purpose, with selection provided by antibiotic resistance markers such as chloramphenicol resistance (cat) .
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Expression verification: Expression should be verified through functional assays (drug resistance testing) and protein detection methods such as Western blotting.
When designing constructs, attention must be paid to the requirement that both ebrA and ebrB are necessary for function, as neither gene alone is sufficient for conferring drug resistance .
How can researchers quantitatively measure efflux activity of recombinant EbrB proteins?
Quantitative measurement of EbrB-mediated efflux activity can be accomplished through several established methodologies:
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Fluorescent substrate efflux assay: This is the gold standard method for characterizing efflux pump activity. Energy-starved cells are loaded with a fluorescent substrate (typically ethidium bromide). After adding an energy source (glucose), the efflux rate is monitored by measuring the decrease in fluorescence over time using a spectrofluorometer .
Protocol outline:
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Grow cells to mid-log phase
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Harvest and wash cells in buffer to remove growth medium
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Incubate cells in the presence of CCCP to deplete cellular energy
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Load cells with ethidium bromide (typical concentration: 10 μM)
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Wash cells to remove extracellular ethidium
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Add glucose (20 mM) to energize cells
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Monitor fluorescence decrease (excitation 500 nm, emission 580 nm)
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Include control experiments with CCCP to confirm energy-dependence
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Minimum Inhibitory Concentration (MIC) determinations: While less direct than efflux assays, MIC testing provides valuable functional data. Compare MICs of various substrates between strains expressing EbrB versus control strains. Typical concentrations for testing range from 0.5-32 μg/ml for ethidium bromide, with similar ranges for other substrates .
What approaches are recommended for studying protein-protein interactions between EbrA and EbrB?
Given that both EbrA and EbrB are required for function, protein-protein interaction studies are crucial. Recommended approaches include:
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Co-expression experiments: As demonstrated in the research, expressing ebrA and ebrB on separate compatible plasmids can restore drug resistance, confirming their functional interaction . This approach involves:
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Creating plasmids with one gene per plasmid (e.g., pTS93 carrying ebrA and pBET51 carrying ebrB)
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Transforming both plasmids into the same host
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Testing for restoration of drug resistance
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Bacterial two-hybrid systems: These can be employed to detect direct protein interactions.
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Crosslinking studies: Chemical crosslinking followed by mass spectrometry can identify interaction sites.
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Site-directed mutagenesis: Creating mutations at potential interaction sites can help map the regions essential for EbrA-EbrB interaction.
How does gene amplification of ebrB affect multidrug resistance patterns in B. subtilis?
Gene amplification of multidrug transporters has been observed in bacteria selected for drug resistance, similar to the P-glycoprotein gene amplification seen in mammalian multidrug-resistant cells . For EbrB, research indicates:
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Strains selected for elevated resistance often show increased copy numbers of the resistance genes.
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This amplification correlates with increased expression levels and enhanced drug efflux activity.
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The relationship between gene copy number and resistance level is not always linear, suggesting regulatory factors may also play important roles.
Experimental approach for studying gene amplification:
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Select B. subtilis cells on increasing concentrations of EbrAB substrates (e.g., rhodamine 6G)
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Analyze gene copy number through quantitative PCR
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Measure protein expression levels using Western blot
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Correlate these findings with MIC values and efflux activity
What are effective strategies for creating knockout mutants of ebrB to study loss of function?
Creating knockout mutants is essential for understanding the physiological role of EbrB. Effective strategies include:
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Direct gene disruption: Insert antibiotic resistance cassettes within the ebrB gene through homologous recombination. This can be accomplished using plasmids that cannot replicate in B. subtilis, forcing integration.
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Marker-free deletion: Use Cre-lox or similar systems to create clean deletions without antibiotic markers.
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CRISPR-Cas9 methods: These newer approaches allow precise genome editing in B. subtilis.
When studying ebrB knockouts, it's critical to consider potential redundancy with other multidrug transporters in B. subtilis, such as Bmr, which may mask phenotypes .
How can researchers design experiments to identify novel inhibitors of the EbrAB efflux system?
A systematic approach to identify inhibitors includes:
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High-throughput screening: Design a cell-based assay where B. subtilis cells expressing EbrAB are grown in the presence of a known substrate (e.g., ethidium bromide) at a concentration below the MIC. Add candidate inhibitors and identify compounds that restore sensitivity.
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Competitive inhibition assays: Using the fluorescent substrate efflux assay, test whether candidate compounds inhibit ethidium efflux.
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Structure-based drug design: Though challenging without crystal structures, homology models of EbrB based on related transporters can guide rational inhibitor design.
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Validation strategies: Confirm that potential inhibitors specifically target EbrAB rather than affecting membrane integrity or cellular energetics through:
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Testing effects on proton gradient
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Confirming lack of general membrane disruption
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Testing specificity by examining effects on other transport systems
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How does the EbrAB system in B. subtilis compare to other bacterial multidrug resistance mechanisms?
The EbrAB system presents several distinctive features compared to other multidrug transporters:
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Two-component requirement: Unlike most SMR family members that function with a single protein, EbrAB requires both components for activity .
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Substrate profile: The EbrAB system shares substrate specificity with other SMR transporters but has some unique preferences.
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Evolutionary relationship: EbrA and EbrB show sequence similarity to other SMR family transporters such as EmrE and SugE but have evolved as a cooperative two-component system .
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Functional similarity to mammalian systems: Despite lacking sequence similarity to P-glycoprotein, the bacterial multidrug efflux system transports similar drugs and is sensitive to similar inhibitors as the mammalian multidrug transporter .
What are the implications of EbrAB research for understanding the evolution of multidrug resistance mechanisms?
The EbrAB system provides valuable insights into the evolution of multidrug resistance:
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The development of a two-component system from single-component transporters suggests evolutionary pathways for increasing functional complexity.
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The observation that functionally similar transporters can evolve from different protein families (as seen in the comparison between bacterial SMR transporters and mammalian P-glycoprotein) demonstrates convergent evolution in response to similar selective pressures .
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The presence of homologs across different bacterial species suggests horizontal gene transfer may play a role in the spread of these resistance mechanisms.
How can the B. subtilis genome be manipulated to study ebrB function in different genetic contexts?
B. subtilis offers several sophisticated tools for genome manipulation to study ebrB:
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Subgenome creation: The B. subtilis genome can be artificially dissected into a main genome and a subgenome, which might be useful for studying ebrB function in isolation or in different genetic backgrounds .
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Promoter swapping: The native promoter of ebrB can be replaced with inducible promoters to control expression levels.
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Fusion proteins: Creating fusion proteins with reporter tags (such as the approach used with CotB-GST fusions in other B. subtilis studies) can aid in localization and functional studies .
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Chromosomal integration: Various tools allow the integration of modified ebrB constructs at different loci, enabling studies of positional effects on expression and function .
