Recombinant Clavibacter michiganensis subsp. michiganensis ATP synthase subunit b (atpF)

What is Clavibacter michiganensis and why is it significant in plant pathology?

Clavibacter michiganensis is an aerobic non-sporulating Gram-positive plant pathogenic actinomycete that currently constitutes the only species within the genus Clavibacter. It is the causative agent of bacterial wilt and canker of tomato (Lycopersicon esculentum) and has spread throughout the world since its first report in the USA in 1910. This pathogen causes serious economic losses to both greenhouse and field tomato (Solanum lycopersicum) crops by either killing young plants or reducing marketable yields .

C. michiganensis has nine subspecies, with C. michiganensis subsp. michiganensis and C. michiganensis subsp. sepedonicus being the primary pathogens causing substantial economic damage to tomatoes and potatoes worldwide . The seed-transmitted nature of this pathogen contributes significantly to its global dispersion and establishment in new environments .

What is the function of ATP synthase in bacterial systems?

ATP synthase is a multi-subunit enzyme complex responsible for ATP synthesis coupled to the proton gradient across the bacterial membrane. The complex consists of two main sectors: the membrane-embedded F₀ sector and the cytoplasmic F₁ sector. The F₁ sector contains the catalytic sites for ATP synthesis, while the F₀ sector forms a proton channel. Subunit b (atpF) is part of the F₀ sector and serves as a critical stator component, connecting the F₁ and F₀ sectors and helping maintain the structural integrity of the complex during rotation of other components.

In C. michiganensis, ATP synthase (EC 3.6.3.14) plays an essential role in energy metabolism, particularly under the nutrient-limited conditions encountered during plant infection . The ATP synthase complex is composed of multiple subunits, including alpha (atpA), which has been characterized and is available as a recombinant protein .

How do ATP synthase genes contribute to phylogenetic studies of C. michiganensis?

ATP synthase genes, including those encoding various subunits such as atpD (delta subunit), have been used successfully in multi-locus sequence typing (MLST) frameworks for phylogenetic analysis of C. michiganensis. Research has shown that atpD, along with other housekeeping genes like dnaK, gyrB, ppK, recA, and rpoB, provides a robust framework for distinguishing C. michiganensis subspecies and strains .

Studies have demonstrated that C. michiganensis subsp. michiganensis is monophyletic and distinct from its closest taxonomic neighbors based on these gene sequences. Interestingly, nonpathogenic Clavibacter-like strains, while belonging to the C. michiganensis clade, are phylogenetically distinct from pathogenic strains .

What is known about recombination events in ATP synthase genes of C. michiganensis?

Analysis of ATP synthase genes provides insights into the evolutionary dynamics of C. michiganensis. For instance, the atpD gene has shown evidence of allele sharing between saprophytic strains and C. michiganensis subsp. michiganensis strains. Split graph analysis of various gene fragments, including atpD, has revealed reticulations suggesting potential recombination events .

The estimated ratio of recombination to mutation (r/m) is approximately 0.027:1 on concatenated gene fragments including atpD, indicating that point mutations rather than recombination events are the primary drivers of sequence diversity in these genes .

What expression systems are recommended for producing recombinant C. michiganensis atpF?

For the expression of recombinant C. michiganensis proteins, including ATP synthase subunits, Escherichia coli expression systems are commonly employed. Specifically, E. coli BL21(DE3) cells have proven effective for the expression of recombinant proteins from various bacterial sources . This strain is deficient in lon and ompT proteases, which helps reduce proteolytic degradation of heterologous proteins.

When expressing membrane proteins like atpF, it may be necessary to optimize culture conditions to prevent formation of inclusion bodies. This might include:

• Expression at lower temperatures (16-25°C)

• Using weaker promoters or lower inducer concentrations

• Co-expression with chaperone proteins

• Expression as fusion proteins with solubility-enhancing tags

Mammalian cell expression systems can also be considered for certain applications, especially when proper folding and post-translational modifications are crucial .

What is the recommended protocol for purifying recombinant C. michiganensis atpF?

Purification of recombinant ATP synthase subunits typically employs immobilized metal affinity chromatography (IMAC) using Ni-nitrilotriacetic acid (Ni-NTA) columns when the protein is expressed with a 6×His tag . The general purification workflow includes:

• Cell lysis: Using sonication, French press, or detergent-based methods

• Clarification: Centrifugation to remove cell debris

• IMAC purification: Loading the clarified lysate onto a Ni-NTA column

• Washing: Removing non-specifically bound proteins

• Elution: Using imidazole gradient or step elution

• Buffer exchange: Removing imidazole and adjusting to storage buffer

For membrane proteins like atpF, the addition of appropriate detergents during purification is crucial to maintain protein solubility and structure. Common detergents include n-dodecyl-β-D-maltoside (DDM), n-octyl-β-D-glucopyranoside (OG), or digitonin.

Purity should be assessed using SDS-PAGE, with target purity typically >85% for research applications .

How can CRISPR/Cas9 technology be applied to study atpF function in C. michiganensis?

CRISPR/Cas9-mediated gene editing has been successfully implemented in C. michiganensis, offering a powerful tool for studying gene function, including ATP synthase components. The system couples the expression of Cas9 and single-guide RNA with homology-directed repair templates, allowing for precise genetic modifications .

For atpF studies, the following CRISPR/Cas9 applications are feasible:

• Gene knockout: Complete deletion of atpF to study its essentiality and impact on bacterial viability

• Site-directed mutagenesis: Introduction of specific mutations to study structure-function relationships

• Unmarked modifications: Using the codA::upp cassette as a counterselectable marker to create clean genetic modifications without antibiotic resistance markers

The unmarked CRISPR/Cas9-mediated system is particularly valuable as it allows template plasmids to be reused repeatedly, facilitating the editing of multiple genes in the same strain .

What methods are used to verify the functionality of recombinant C. michiganensis atpF?

Verifying the functionality of recombinant ATP synthase subunit b requires several complementary approaches:

• ATP hydrolysis assay: Measuring the ATPase activity of reconstituted ATP synthase complexes containing the recombinant atpF

• Proton pumping assays: Using pH-sensitive fluorescent dyes to monitor proton translocation

• Membrane potential measurements: Assessing the ability of the ATP synthase complex to generate or utilize membrane potential

• Complementation studies: Testing whether the recombinant atpF can restore function in atpF-deficient bacterial strains

• Structural integrity assessment: Using circular dichroism (CD) spectroscopy to confirm proper secondary structure formation

These functional assays should be performed alongside appropriate controls, including wild-type atpF and known non-functional mutants.

What strategies can overcome poor expression yields of recombinant C. michiganensis atpF?

Poor expression yields are a common challenge when working with membrane proteins like atpF. Several strategies can improve expression:

When using mammalian expression systems, consider optimizing transfection efficiency, cell density at transfection, and harvest timing to maximize protein yields .

How can researchers address protein instability issues with purified recombinant atpF?

Stability of purified atpF can be enhanced through several approaches:

• Buffer optimization:

  • Test various pH conditions (typically pH 7.0-8.0)

  • Optimize salt concentration (150-300 mM NaCl)

  • Include stabilizing agents (5-10% glycerol, 1-5 mM DTT or TCEP)

• Detergent selection:

  • Test multiple detergent types (DDM, LMNG, GDN)

  • Optimize detergent concentration (typically 2-3× CMC)

  • Consider detergent mixtures for enhanced stability

• Storage conditions:

  • Aliquot to avoid freeze-thaw cycles

  • Store at -80°C for long-term or 4°C for short-term use

  • Consider flash-freezing in liquid nitrogen

• Additives:

  • Lipids (POPC, E. coli total lipid extract)

  • Osmolytes (trehalose, sucrose)

  • Specific ligands or substrate analogs

For recombinant ATP synthase subunits, avoiding repeated freeze-thaw cycles is particularly important, and working aliquots should be stored at 4°C for up to one week .

What are common pitfalls in experimental design when studying recombinant C. michiganensis atpF interactions?

When studying interactions involving recombinant atpF, researchers should be aware of several experimental pitfalls:

• Detergent interference: Detergents used to solubilize atpF may disrupt native protein-protein interactions. Consider detergent screening or nanodiscs/liposome reconstitution for interaction studies.

• Tag interference: His-tags or other fusion tags may affect protein interactions or function. Include controls with tag-cleaved proteins or differently tagged constructs.

• Non-native conformations: Recombinant atpF may not adopt its native conformation when expressed in isolation. Consider co-expression with interacting partners.

• Buffer incompatibilities: Interaction partners may have different buffer requirements. Establish compromise buffer conditions that maintain stability of all proteins involved.

• Aggregation artifacts: Aggregated protein can lead to false-positive interaction results. Always include size exclusion chromatography or dynamic light scattering to verify monodispersity.

• Lack of membrane environment: For membrane proteins like atpF, interactions may depend on the lipid environment. Consider reconstitution into liposomes or nanodiscs.

Careful experimental design with appropriate controls and validation using multiple complementary techniques can help avoid these pitfalls.

How can structural studies of recombinant C. michiganensis atpF contribute to antimicrobial development?

ATP synthase is an essential enzyme for bacterial survival, making it a potential target for antimicrobial development. Structural studies of C. michiganensis atpF could reveal unique features that differentiate it from host ATP synthases, enabling the design of specific inhibitors.

Approaches for structural characterization include:

• X-ray crystallography of the entire ATP synthase complex or subcomplexes containing atpF

• Cryo-electron microscopy to visualize the complex in different conformational states

• NMR spectroscopy for dynamic studies of specific domains

• Molecular dynamics simulations to understand conformational changes

Understanding the structure-function relationship of atpF could lead to the identification of:

• Unique binding pockets for small molecule inhibitors

• Critical residues for protein-protein interactions that could be targeted

• Conformational changes essential for ATP synthase function

Such structural insights could inform the development of novel antimicrobials specifically targeting plant pathogens like C. michiganensis without affecting beneficial microorganisms.

What is the potential for using atpF as a diagnostic marker for C. michiganensis detection?

ATP synthase genes have shown promise as phylogenetic markers for distinguishing C. michiganensis subspecies . The atpF gene, like other ATP synthase components, could potentially serve as a diagnostic target with several advantages:

• Evolutionary conservation: As an essential gene, atpF is less likely to be lost or undergo major sequence changes

• Subspecies specificity: Sequence variations in atpF between subspecies could enable specific detection

• Copy number: Present as a single copy, reducing variability in quantitative assays

Potential diagnostic applications include:

• PCR-based detection systems targeting atpF sequence variations

• LAMP (Loop-mediated isothermal amplification) assays for field-deployable diagnostics

• Sequence-specific antibodies for immunological detection

• CRISPR-Cas diagnostic systems targeting distinctive atpF sequences

Development of such diagnostic tools would require careful validation against known C. michiganensis strains and closely related bacteria to ensure specificity and sensitivity.