Recombinant Burkholderia phytofirmans ATP synthase subunit b (atpF)

What is Burkholderia phytofirmans PsJN and why is its ATP synthase subunit of interest to researchers?

Burkholderia phytofirmans PsJN (recently reclassified as Paraburkholderia phytofirmans PsJN) is a well-characterized plant growth-promoting rhizobacterium (PGPR) originally isolated from onion roots . This endophytic bacterium establishes rhizospheric and endophytic colonization in various plants, stimulating growth and enhancing stress tolerance . The ATP synthase subunit b (atpF, locus Bphyt_3898) is of particular interest because:

• It is part of the F-type ATP synthase complex essential for bacterial energy metabolism

• ATP synthesis-related genes, including atpF, show significant upregulation during plant-microbe interactions, particularly under stress conditions

• Oxidative phosphorylation (which involves ATP synthase) was found to be one of the most highly activated processes in B. phytofirmans PsJN during plant drought stress

What are the optimal methods for expression and purification of recombinant B. phytofirmans atpF protein?

The optimal methodology for expression and purification of recombinant B. phytofirmans atpF involves:

Expression system selection:

• E. coli BL21(DE3) is typically used for membrane protein expression

• Codon optimization is advisable due to differences between Burkholderia and E. coli codon usage

Vector design considerations:

• Inclusion of an N-terminal or C-terminal affinity tag (His6, GST, or MBP) to facilitate purification

• Use of a vector with tightly regulated promoter (T7 or tac) to control expression

Expression conditions:

• Growth at lower temperatures (16-20°C) after induction to improve proper folding

• Extended expression time (overnight) at reduced inducer concentrations

• Addition of glycerol (5-10%) to culture media to enhance membrane protein stability

Purification protocol:

• Cell lysis preferably by French press or sonication in buffer containing 50 mM Tris-HCl pH 8.0, 150 mM NaCl

• Membrane fraction isolation by ultracentrifugation (100,000 × g, 1 hour)

• Solubilization using mild detergents like DDM (n-dodecyl-β-D-maltoside) or LDAO

• Affinity chromatography using the engineered tag

• Size exclusion chromatography for final purification

• Storage in buffer containing 50% glycerol at -20°C or -80°C

How can researchers effectively analyze atpF gene expression patterns in B. phytofirmans during plant colonization?

Effective analysis of atpF expression during plant colonization requires:

Sample preparation:

• Inoculate plants with B. phytofirmans PsJN (10⁶-10⁸ CFU/mL)

• Harvest plant tissues at different time points post-inoculation (6h, 24h, 48h, 7 days)

• Surface-sterilize tissues to eliminate epiphytic bacteria

• Homogenize tissues in RNA preservation solution

RNA extraction and enrichment:

• Extract total RNA from plant-bacteria samples

• Deplete plant rRNA using a Ribo-Zero rRNA removal kit as described in Sheibani-Tezerji et al. (2015)

• Enrich bacterial RNA using specialized kits for low-abundance microbial RNA

Expression analysis methods:

• RT-qPCR approach: Design primers specific to B. phytofirmans atpF gene (the Bphyt_3898 locus), with reference genes such as 16S rRNA or glutamine synthetase (Bphyt_2615)

• RNA-Seq approach: Perform transcriptome analysis using next-generation sequencing, followed by mapping reads to the B. phytofirmans genome

• In situ visualization: Use fluorescence in situ hybridization with atpF-specific probes to localize expression within plant tissues

Data normalization and comparative analysis:

• Compare expression levels between different plant tissues

• Analyze temporal expression patterns during colonization stages

• Compare expression under normal versus stress conditions

Research by Sheibani-Tezerji et al. demonstrated that genes associated with ATP synthase showed differential expression in PsJN colonizing drought-stressed potato plants .

What role does atpF play in B. phytofirmans' ability to promote plant growth under stress conditions?

The atpF gene, encoding ATP synthase subunit b, contributes significantly to B. phytofirmans' plant growth-promoting capabilities under stress conditions through several mechanisms:

Energy metabolism regulation:

• ATP synthase is critical for bacterial energy production via oxidative phosphorylation

• Under plant stress conditions, ATP synthase genes (including atpF) show significant upregulation

• This increased energy generation supports bacterial metabolic activities that benefit the host plant

Stress response involvement:

• Transcriptome studies revealed that oxidative phosphorylation was among the most enriched functions in B. phytofirmans under plant drought stress

• ATP synthase components participate in maintaining bacterial homeostasis during oxidative stress

• ATP generation supports the production of compatible solutes and stress protectants

Relationship to plant colonization:

• Effective energy metabolism supports bacterial motility, essential for rhizospheric and endophytic colonization

• Colonization efficiency directly correlates with growth promotion effects

Quantitative evidence:
Studies with potato and Arabidopsis plants show that ATP synthase-related genes in B. phytofirmans were among those most significantly upregulated (2-4 fold) during drought stress, with expression peaking at 6 hours post-stress induction .

How does the structure and function of B. phytofirmans atpF compare with homologous proteins in other plant-associated bacteria?

Comparative analysis of B. phytofirmans atpF reveals important structural and functional relationships with homologous proteins in other bacteria:

Sequence homology comparisons:

Functional conservation assessment:

• The core ATP synthase function is highly conserved across bacterial species

• Plant-associated beneficial bacteria show greater conservation in regulatory elements

• Stress-responsive expression patterns appear to be a shared feature among plant growth-promoting bacteria

Structural insights:

• The membrane-spanning region shows highest conservation

• The C-terminal domain that interacts with other ATP synthase subunits has species-specific variations

• These variations may reflect adaptations to different plant hosts or environmental niches

Genome analyses indicate that B. phytofirmans PsJN is closely related to other beneficial Burkholderia species like B. phymatum STM815, with high synteny (67-77%) in regions containing energy metabolism genes .

What are the most effective methods for creating and validating atpF knockout mutants in B. phytofirmans?

Creating and validating atpF knockout mutants in B. phytofirmans requires careful experimental design due to the essential nature of ATP synthase. The most effective approaches include:

Gene knockout strategies:

• Homologous recombination:

  • Design primers to amplify ~1kb regions flanking the atpF gene

  • Clone fragments into a suicide vector (pK18mobsacB)

  • Introduce vector into B. phytofirmans via triparental mating

  • Select for double-crossover events using counterselection markers

• CRISPR-Cas9 approach:

  • Design sgRNAs targeting unique regions of atpF

  • Clone into a CRISPR-compatible vector for Burkholderia

  • Introduce by electroporation or conjugation

  • Screen for mutations by PCR and sequencing

Conditional knockout strategies (recommended for essential genes):

• Create an inducible expression system with a complementary copy

• Use temperature-sensitive promoters or riboswitch-based regulation

• Generate the knockout only when the complementary copy is expressed

Validation approaches:

• Molecular verification:

  • PCR screening using primers flanking the expected deletion

  • Whole-genome sequencing to confirm genetic modifications

  • RT-qPCR to verify absence of atpF transcripts

• Functional validation:

  • Measure ATP production using luciferase-based ATP assays

  • Assess oxidative phosphorylation using oxygen consumption rates

  • Evaluate membrane potential using fluorescent probes

• Phenotypic characterization:

  • Growth curves under different carbon sources

  • Stress tolerance assays (oxidative, osmotic, pH)

  • Plant colonization efficiency quantification using methods similar to those described by Poupin et al.

Successful atpF knockouts would likely show impaired energy metabolism and reduced fitness, particularly under stress conditions. If atpF proves essential, conditional approaches become necessary.

How can researchers distinguish between the direct effects of atpF activity and its indirect effects on plant growth promotion?

Distinguishing direct from indirect effects of atpF on plant growth promotion requires multifaceted experimental approaches:

Complementation assays:

• Create an atpF knockout mutant (if viable) or a conditional knockdown

• Develop complementation strains with:

  • Native atpF gene

  • atpF variants with specific mutations

  • Heterologous atpF genes from other bacteria

• Compare plant growth parameters across treatments

Metabolomic analysis:

• Compare metabolite profiles of wild-type vs. atpF-modified strains

• Focus on energy-related metabolites (ATP/ADP ratios, NAD+/NADH)

• Analyze exudates to identify compounds potentially affecting plant physiology

Co-culture experiments:

• Design split-plate assays separating bacteria from plants

• Use semipermeable membranes allowing chemical but not physical contact

• Compare with direct inoculation to distinguish contact-dependent effects

Transcriptomic approach:

• Perform dual RNA-seq of both plant and bacteria during interaction

• Compare gene expression networks between wild-type and atpF-modified strains

• Identify plant pathways responding specifically to atpF-dependent factors

Mathematical modeling:

• Develop systems biology models integrating bacterial metabolism and plant responses

• Use these models to predict direct vs. indirect effects

• Validate predictions with targeted experiments

Studies with B. phytofirmans have shown that plant growth promotion involves multiple mechanisms, including modulation of plant hormone pathways and metabolite exchange . By systematically manipulating atpF while monitoring these pathways, researchers can determine which effects are directly linked to ATP synthase function versus those resulting from downstream metabolic changes.

How might atpF expression patterns inform our understanding of B. phytofirmans' adaptation to different plant hosts?

Analysis of atpF expression across different plant hosts provides valuable insights into B. phytofirmans' host adaptation mechanisms:

Host-specific expression patterns:
Transcriptomic studies reveal that ATP synthase genes, including atpF, show differential expression patterns depending on the plant host:

Bioenergetic adaptation signatures:

• Expression timing correlates with colonization stages (attachment, penetration, establishment)

• Expression levels reflect metabolic demands of different plant microenvironments

• Regulatory elements in the atpF operon may contain host-specific response elements

Functional implications:

• Variable expression suggests B. phytofirmans optimizes energy production based on host metabolism

• Co-expression with other genes indicates integration with broader adaptive responses

• The ability to modulate ATP synthesis efficiency may be central to versatile host range

This research direction could lead to the development of host-optimized B. phytofirmans strains with enhanced plant growth-promoting capabilities for specific crops or conditions.

What is the relationship between atpF activity and iron homeostasis in B. phytofirmans during plant colonization?

Recent research has revealed a fascinating and complex relationship between ATP synthase activity and iron homeostasis in B. phytofirmans during plant colonization:

Evidence for interconnection:

• Transcriptome analyses show co-regulation between ATP synthase components and iron acquisition systems

• PsJN-inoculated plants demonstrate enhanced iron uptake and accumulation

• ECF sigma factors involved in iron transport regulation (particularly ECF_164, orthologous to EcfI) are expressed in PsJN during plant colonization

Molecular mechanisms:

• Energy-dependent iron transport:

  • ATP synthase provides energy for active transport of iron

  • Siderophore biosynthesis and transport systems are ATP-dependent

• Regulatory crosstalk:

  • Iron status affects ATP synthase gene expression

  • Fur-like transcription regulators coordinate iron homeostasis with energy metabolism

  • Under oxidative stress, iron regulation is adjusted to prevent damage via Fenton chemistry

• Functional integration in plant colonization:

  • Iron acquisition is critical for bacterial competitiveness in planta

  • ATP generation supports siderophore production

  • Enhanced iron uptake by bacteria may also benefit host plants

Quantitative relationships:
Studies show that PsJN-inoculated Arabidopsis plants accumulated significantly more iron (40-60% increase) compared to non-inoculated controls, and this coincided with expression of bacterial ferritin and siderophore biosynthesis genes .

This relationship explains how B. phytofirmans simultaneously manages its energy metabolism and micronutrient acquisition during plant colonization, contributing to both bacterial fitness and plant growth promotion through improved metal nutrient status.

What are the most common challenges in purifying and stabilizing recombinant B. phytofirmans atpF, and how can they be addressed?

Researchers working with recombinant B. phytofirmans atpF encounter several technical challenges:

Challenge 1: Protein aggregation and insolubility

• Cause: atpF is naturally membrane-associated with hydrophobic regions

• Solutions:

  • Use fusion partners that enhance solubility (MBP, SUMO, or Trx tags)

  • Express at lower temperatures (16°C) with reduced inducer concentrations

  • Add solubilizing agents like sarkosyl (0.5-1%) during lysis, followed by dilution

  • Include 5-10% glycerol in all buffers to stabilize protein structure

Challenge 2: Low expression yields

• Cause: Codon bias, toxicity to host, or protein instability

• Solutions:

  • Optimize codons for E. coli expression

  • Use specialized strains like C41(DE3) designed for membrane proteins

  • Test different promoter systems (T7, tac, or arabinose-inducible)

  • Co-express with chaperones (GroEL/ES, DnaK/J)

Challenge 3: Protein degradation during purification

• Cause: Exposure to proteases or inherent instability

• Solutions:

  • Include protease inhibitor cocktail in all buffers

  • Maintain samples at 4°C throughout purification

  • Add stabilizing agents like glycerol, sucrose, or specific metal ions

  • Reduce the number of purification steps to minimize handling time

Challenge 4: Maintaining native conformation

• Cause: Loss of lipid environment or interacting partners

• Solutions:

  • Purify in the presence of appropriate detergents (DDM, LDAO)

  • Consider nanodiscs or liposome reconstitution for functional studies

  • Co-purify with known interacting partners from the ATP synthase complex

Storage recommendations:
Store in Tris-based buffer with 50% glycerol at -20°C for extended storage, with working aliquots at 4°C for up to one week. Repeated freeze-thaw cycles should be avoided .

How can researchers address conflicting data on atpF function between in vitro studies and in planta observations?

Reconciling discrepancies between in vitro and in planta observations of atpF function requires systematic investigation and methodological considerations:

Common sources of discrepancy:

Methodological approaches to resolve contradictions:

• Controlled environment gradients:

  • Create experimental systems with increasing complexity from pure culture to plant-bacteria interface

  • Use microfluidic devices to allow precise control of environmental parameters

  • Compare atpF expression and function across this gradient

• In situ analysis techniques:

  • Employ bacterial biosensors expressing fluorescent proteins under atpF promoter control

  • Use fluorescence microscopy to visualize expression directly in plant tissues

  • Apply single-cell techniques to capture heterogeneity within bacterial populations

• Multi-omics integration:

  • Combine transcriptomics, proteomics, and metabolomics data

  • Build a systems biology model accounting for contextual differences

  • Identify factors that explain divergent observations

• Synthetic biological approaches:

  • Create chimeric systems with controlled components

  • Design minimal systems that recapitulate specific aspects of plant-microbe interactions

  • Test atpF function in these defined contexts

Case study example:
In studies of B. phytofirmans, gene expression in culture versus in planta conditions showed significant differences. For example, when comparing ATP synthase complex expression in cultures to expression in potato colonization, researchers found 1.5-3 fold higher expression levels in planta, particularly under stress conditions . This demonstrates the importance of validating functional data across different experimental contexts.

How might engineering of atpF contribute to developing improved B. phytofirmans strains for agricultural applications?

Strategic engineering of atpF could lead to enhanced B. phytofirmans strains with improved agricultural benefits:

Potential engineering approaches:

• Expression level optimization:

  • Modify promoter strength to enhance ATP production

  • Create strains with stress-responsive atpF expression

  • Engineer post-transcriptional regulatory elements for tissue-specific expression

• Protein engineering strategies:

  • Enhance protein stability for improved function under field conditions

  • Modify subunit interfaces to increase ATP synthase efficiency

  • Engineer variants optimized for different pH and temperature ranges

• Metabolic integration:

  • Co-engineer atpF with iron acquisition pathways for synergistic effects

  • Coordinate expression with stress response systems

  • Balance energy production with other plant-beneficial functions

Expected agricultural benefits:

Development pathway:

• Create and screen atpF variant libraries in laboratory conditions

• Test promising candidates in controlled plant experiments

• Evaluate field performance under various environmental stresses

• Assess impacts on plant growth metrics and stress tolerance

Studies with B. phytofirmans have already demonstrated that the bacterial energy metabolism significantly impacts its ability to confer stress tolerance in various plants, including drought tolerance in Arabidopsis and salt tolerance in tomato . Engineering atpF for optimized expression could enhance these beneficial effects.

What novel analytical techniques are emerging for studying the structural dynamics of bacterial ATP synthase complexes in situ?

Cutting-edge analytical techniques are revolutionizing our ability to study ATP synthase structural dynamics in their native contexts:

Advanced structural biology approaches:

• Cryo-electron tomography (Cryo-ET):

  • Enables visualization of ATP synthase complexes within intact bacterial cells

  • Preserves native membrane environment and protein-protein interactions

  • Recent advances allow sub-nanometer resolution of large complexes in situ

  • Can be combined with focused ion beam milling to access intracellular structures

• Single-particle cryo-electron microscopy:

  • Achieves near-atomic resolution of purified ATP synthase complexes

  • Captures different conformational states during catalytic cycle

  • Recently applied to bacterial F-type ATP synthases revealing rotary mechanism details

• Integrative structural modeling:

  • Combines multiple experimental data sources (cryo-EM, XL-MS, HDX-MS)

  • Creates comprehensive structural models of entire ATP synthase complexes

  • Accounts for dynamic aspects of subunit interactions

Live-cell imaging innovations:

• FRET-based approaches:

  • Engineer fluorescent protein pairs into specific ATP synthase subunits

  • Monitor conformational changes in real-time in living cells

  • Detect responses to environmental changes or stress conditions

• Super-resolution microscopy:

  • Techniques like STORM and PALM break the diffraction limit

  • Allow visualization of individual ATP synthase complexes within bacterial membranes

  • Can track dynamic assembly/disassembly processes

• Correlative light and electron microscopy (CLEM):

  • Combines fluorescence localization with ultrastructural context

  • Enables targeting of specific ATP synthase variants in heterogeneous populations

  • Particularly valuable for studying ATP synthase in bacteria colonizing plant tissues

Emerging methodologies:

• Mass photometry:

  • Analyzes mass distribution of membrane protein complexes

  • Requires minimal sample amounts

  • Can detect different oligomeric states or assembly intermediates

• Native mass spectrometry:

  • Analyzes intact membrane protein complexes

  • Preserves non-covalent interactions

  • Determines subunit stoichiometry and lipid interactions