Recombinant Azorhizobium caulinodans ATP synthase subunit a (atpB)

What is the genomic organization of the atpB gene in Azorhizobium caulinodans?

The atpB gene in A. caulinodans is part of the ATP synthase operon, which encodes components of the F₁F₀-ATP synthase complex. While specific information about atpB organization is limited in current literature, we can infer from the genome structure of A. caulinodans ORS571 that ATP synthase genes are likely clustered similarly to other alphaproteobacteria. The genome of A. caulinodans shows organized gene clusters for functional pathways, as seen with the chemotaxis gene cluster (che) that contains cheA, cheW, cheY, cheB, and cheR genes in a co-oriented arrangement . ATP synthase genes typically follow similar organizational patterns with conserved operon structures across bacterial species.

How does the atpB protein contribute to energy metabolism in A. caulinodans during symbiotic nitrogen fixation?

The atpB protein forms a critical component of the F₁F₀-ATP synthase complex, which is essential for energy production during nitrogen fixation. In A. caulinodans, which can fix nitrogen both in symbiotic and free-living states under microaerobic conditions , the ATP synthase complex is particularly important. During symbiotic nitrogen fixation, A. caulinodans requires significant energy to power nitrogenase activity. The atpB subunit, as part of the membrane-embedded F₀ portion of ATP synthase, facilitates proton translocation across the membrane, which drives ATP synthesis. This process is critical for supporting the energy-intensive process of converting atmospheric nitrogen to ammonia within root and stem nodules of S. rostrata. Mutational studies of energy metabolism genes in rhizobia generally show reduced symbiotic effectiveness and nitrogen fixation rates.

What primary approaches are used to express recombinant A. caulinodans atpB protein for functional studies?

For functional studies of recombinant A. caulinodans atpB protein, researchers typically employ the following methodological approaches:

• Cloning and expression system selection: The atpB gene is amplified from A. caulinodans ORS571 genomic DNA using specifically designed primers. Common expression systems include E. coli BL21(DE3) strains with pET-series vectors that provide high-level expression under IPTG induction.

• Optimization of expression conditions: Expression is typically optimized by testing various temperatures (18-37°C), IPTG concentrations (0.1-1.0 mM), and induction durations (4-24 hours). Lower temperatures often enhance proper folding of membrane proteins like atpB.

• Membrane protein purification protocol:

  • Cell lysis using French press or sonication in buffer containing glycerol and protease inhibitors

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

  • Solubilization using appropriate detergents (commonly DDM, LDAO, or Triton X-100)

  • Purification via His-tag affinity chromatography followed by size exclusion chromatography

This approach is analogous to methods used for isolating other membrane proteins in A. caulinodans, such as the transmembrane chemoreceptor TlpA1, which contains two N-terminal transmembrane regions similar to membrane-spanning segments found in atpB .

How can site-directed mutagenesis of conserved residues in A. caulinodans atpB provide insights into proton translocation mechanisms?

Site-directed mutagenesis of conserved residues in A. caulinodans atpB can reveal critical functional elements of proton translocation. A methodological approach would include:

• Identification of target residues: Using sequence alignment with well-characterized ATP synthase subunit a proteins from model organisms like E. coli to identify conserved amino acids, particularly those in transmembrane domains involved in the proton channel.

• Mutagenesis protocol:

  • Design primers containing desired mutations

  • Perform PCR-based site-directed mutagenesis using a system like Q5 or QuikChange

  • Confirm mutations by sequencing

  • Transform into expression strains

• Functional analysis of mutants:

  • Reconstitute purified mutant atpB with other ATP synthase subunits in liposomes

  • Measure proton translocation using pH-sensitive fluorescent dyes

  • Assess ATP synthesis rates using luciferase-based ATP detection assays

  • Compare results with wild-type protein to determine impact of mutations

• In vivo complementation studies:

  • Generate A. caulinodans atpB deletion mutants

  • Complement with plasmids expressing mutant atpB variants

  • Assess growth rates and ATP production in various conditions

  • Evaluate symbiotic effectiveness with S. rostrata (nodulation efficiency, nitrogen fixation rates)

This approach parallels methods used to study the function of chemotaxis proteins in A. caulinodans, where complementation of deletion mutants with wild-type genes has been used to confirm protein function .

What are the challenges and solutions for structural characterization of recombinant A. caulinodans atpB?

Structural characterization of membrane proteins like atpB presents significant challenges. The following methodological approach addresses these challenges:

Challenges:

• Low expression yields of functional protein

• Protein instability outside the membrane environment

• Difficulties in growing high-quality crystals for X-ray crystallography

• Conformational heterogeneity

Solutions and Methods:

• Expression optimization:

  • Use specialized strains like C41(DE3) or C43(DE3) designed for membrane protein expression

  • Employ fusion tags that enhance solubility (MBP, SUMO)

  • Screen multiple detergents for optimal extraction and stability

• Structural analysis techniques:

  • Cryo-EM: Particularly useful for membrane proteins where crystallization is challenging

  • Sample preparation protocol:
    a. Purify protein in amphipathic detergents or reconstitute into nanodiscs
    b. Apply to glow-discharged grids
    c. Vitrify by plunging into liquid ethane
    d. Collect images using direct electron detectors
    e. Process data using software packages like RELION or cryoSPARC

• Complementary approaches:

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to probe dynamics

  • Crosslinking coupled with mass spectrometry to identify interacting regions

  • EPR spectroscopy with site-directed spin labeling to measure distances between residues

• Comparative modeling:

  • Use structures of homologous proteins as templates

  • Validate models through mutagenesis of predicted functional residues

The structural information obtained can provide insights into how the protein's architecture supports its function in the unique symbiotic nitrogen-fixing context of A. caulinodans.

How do modifications to the recombinant atpB affect ATP synthase assembly and function in A. caulinodans under various environmental conditions?

To assess how modifications to recombinant atpB affect ATP synthase assembly and function under varying conditions, researchers can implement the following experimental framework:

Experimental design:

• Generate modified atpB variants:

  • C-terminal or N-terminal tags (His, FLAG, GFP)

  • Domain swaps with homologous proteins from related species

  • Point mutations at conserved residues

• Expression systems:

  • Homologous expression in A. caulinodans (preferred for physiological relevance)

  • Heterologous expression in E. coli for biochemical studies

• Functional assessment under varying conditions:

  Environmental Condition	Parameters to Measure	Methodology
  Oxygen levels (aerobic, microaerobic, anaerobic)	ATP synthesis rate, Proton gradient formation, Assembly efficiency	Membrane vesicle preparations, ATP luciferase assays, Fluorescent probes (ACMA)
  pH range (5.5-8.0)	Enzyme activity, Proton translocation efficiency	pH-jump experiments, ATP synthesis assays
  Temperature variations (25-40°C)	Stability, Activity	Thermal shift assays, Activity assays at different temperatures
  Presence of symbiotic signals from S. rostrata	Expression levels, Complex assembly	qRT-PCR, Blue native PAGE

• In vivo assessment:

  • Growth rates in free-living vs. symbiotic states

  • Nitrogen fixation activity (acetylene reduction assay) similar to methods used in A. caulinodans nodule studies

  • Nodulation efficiency and competitiveness

• Protein-protein interaction studies:

  • Co-immunoprecipitation to identify interaction partners

  • Blue native PAGE to assess complex assembly

  • FRET analysis with fluorescently labeled subunits

This methodological approach would provide comprehensive data on how modifications to atpB affect ATP synthase function across environmental conditions relevant to A. caulinodans' life cycle.

What purification strategies optimize yield and stability of recombinant A. caulinodans atpB protein?

Optimizing purification of recombinant A. caulinodans atpB requires specific strategies for membrane proteins. The following methodological protocol addresses yield and stability concerns:

• Cell lysis optimization:

  • Gentle lysis methods (osmotic shock, enzymatic treatments) preserve protein structure

  • Buffer composition: 50 mM Tris-HCl pH 7.5, 150-300 mM NaCl, 10% glycerol, 1 mM DTT, protease inhibitor cocktail

• Membrane isolation and solubilization:

  • Differential centrifugation to isolate membrane fractions

  • Detergent screening table for optimal solubilization:

  Detergent	Concentration	Advantages	Limitations
  DDM	1-2%	Good for initial extraction	May destabilize some complexes
  LMNG	0.5-1%	Enhanced stability	Higher cost
  Digitonin	0.5-1%	Preserves native interactions	Lower solubilization efficiency
  SMA copolymer	2.5%	Extracts protein with native lipids	pH limitations

• Chromatography strategy:

  • IMAC (Immobilized Metal Affinity Chromatography) using His-tagged atpB

  • Ion exchange chromatography as intermediate purification step

  • Size exclusion chromatography for final polishing and buffer exchange

• Stability enhancement:

  • Addition of lipids (POPC, POPE) at 0.1-0.2 mg/ml during purification

  • Use of amphipols (A8-35) or nanodiscs for detergent-free environments

  • Glycerol (10-20%) and specific ions (Mg²⁺, 5 mM) to stabilize structure

• Quality assessment:

  • Thermal shift assays to optimize buffer conditions

  • SEC-MALS for homogeneity analysis

  • Activity assays to confirm functional integrity

This approach draws from membrane protein purification strategies that have proven effective for transmembrane proteins with similar architecture to atpB, such as the transmembrane chemoreceptors in A. caulinodans .

How can isotopic labeling of recombinant atpB facilitate structural and functional studies?

Isotopic labeling of recombinant A. caulinodans atpB enables advanced structural and functional analyses using various spectroscopic techniques. A comprehensive methodological approach includes:

• Expression systems for isotopic labeling:

  • Minimal media supplemented with ¹⁵N-ammonium chloride, ¹³C-glucose, and/or ²H₂O

  • Optimization of growth conditions in isotope-enriched media:

    • Lower temperature (25-30°C)

    • Extended induction times (16-24 hours)

    • Higher aeration rates

• Selective labeling strategies:

  • SAIL (Stereo-Array Isotope Labeling) for specific amino acids

  • Segmental labeling for specific domains

  • Methyl-group labeling of Ile, Leu, Val residues in deuterated background

• Applications and analytical methods:

  Technique	Isotope Labeling	Information Obtained	Experimental Setup
  NMR Spectroscopy	¹⁵N, ¹³C, ²H	Residue-specific structural dynamics, Binding interfaces	TROSY experiments for large membrane proteins, Selective pulse schemes
  Mass Spectrometry	¹⁵N, ¹³C	Conformational changes, PTMs, H/D exchange patterns	LC-MS/MS analysis of peptide fragments
  Neutron Scattering	²H (deuteration)	Membrane protein-lipid interactions	Small-angle neutron scattering with contrast matching

• Functional probing with isotope labels:

  • Measure proton translocation using ¹⁸O-water exchange

  • Assess energy coupling with ³²P-labeled ATP

  • Site-specific fluorine-19 (¹⁹F) labeling for conformational changes

• Data analysis approaches:

  • Integration of chemical shift data with molecular modeling

  • Distance restraints from paramagnetic relaxation enhancement

  • Correlation of dynamics data with functional states

This methodological approach provides a comprehensive framework for using isotopic labeling to interrogate the structure-function relationships of recombinant A. caulinodans atpB.

What are the kinetic parameters of recombinant A. caulinodans atpB and how do they compare with ATP synthase complexes from other nitrogen-fixing bacteria?

Determining kinetic parameters of recombinant A. caulinodans atpB involves comparative analysis with other nitrogen-fixing bacteria. A thorough methodological approach includes:

• Preparation of proteoliposomes:

  • Reconstitution of purified atpB with other ATP synthase subunits

  • Lipid composition optimization (E. coli polar lipids with POPC/POPE)

  • Creation of proton gradient by acid-base transition or valinomycin/K⁺

• Enzymatic activity measurements:

  • ATP synthesis rates using luciferase-based assays

  • ATP hydrolysis monitoring phosphate release with malachite green

  • Proton pumping measured with pH-sensitive fluorescent dyes (ACMA)

• Kinetic parameter determination:

  Parameter	Experimental Approach	Typical Values in Nitrogen-Fixers
  K<sub>m</sub> for ADP	Varying ADP concentrations at fixed P<sub>i</sub>	0.1-0.5 mM
  K<sub>m</sub> for P<sub>i</sub>	Varying P<sub>i</sub> at fixed ADP	1-5 mM
  V<sub>max</sub>	Substrate saturation curves	10-50 μmol·min⁻¹·mg⁻¹
  H⁺/ATP ratio	Comparison of ATP formed and protons translocated	3-4 H⁺/ATP
  pH-dependency	Activity measurements across pH range 5.5-8.0	Optimum typically pH 7.0-7.5

• Comparative analysis framework:

  • Direct comparison with ATP synthase from:

    • Bradyrhizobium japonicum

    • Sinorhizobium meliloti

    • Rhizobium leguminosarum

  • Correlation of kinetic parameters with nitrogen fixation efficiency

  • Analysis of adaptations to microaerobic conditions

• Analysis of regulation:

  • Effect of nucleotides (ATP/ADP ratio)

  • Influence of membrane potential

  • Response to oxygen concentration changes

This methodological approach provides a comprehensive analysis of the kinetic properties of A. caulinodans atpB in comparison with other nitrogen-fixing bacteria, highlighting adaptations specific to A. caulinodans' unique symbiotic lifestyle with S. rostrata.

How does the expression of recombinant atpB in A. caulinodans influence symbiotic efficiency with Sesbania rostrata?

The impact of recombinant atpB expression on symbiotic efficiency can be assessed through a systematic functional analysis approach:

• Construction of expression systems:

  • Native promoter-controlled expression versus constitutive/inducible systems

  • Integration of recombinant atpB into the genome versus plasmid-based expression

  • Creation of atpB variants with modified regulatory elements

• Symbiotic efficiency assessment protocol:

  • Plant inoculation methods:

    • Surface-sterilized S. rostrata seeds germinated under aseptic conditions

    • Inoculation with wild-type and recombinant atpB-expressing A. caulinodans strains

    • Growth under controlled conditions (light/dark cycles, temperature, humidity)

  • Nodulation assessment parameters:

    • Nodule number and distribution on roots and stems

    • Nodule morphology and development timeline

    • Competitive nodulation assays with mixed inoculation of wild-type and recombinant strains

• Nitrogen fixation quantification:

  • Acetylene reduction assay (ARA) to measure nitrogenase activity, similar to methods used for assessing A. caulinodans nodules in previous studies

  • ¹⁵N isotope dilution measurements for precise quantification

  • Plant growth parameters (height, dry weight, nitrogen content)

• Molecular analysis of symbiotic interaction:

  • Transcriptome analysis of bacteroids expressing recombinant atpB

  • Proteome analysis focusing on energy metabolism proteins

  • ATP/ADP ratio measurements in bacteroids

  • Membrane potential assessment using fluorescent probes

• Data interpretation framework:

  Parameter	Method	Expected Outcome with Optimized atpB
  Nodulation rate	Counting nodules at 7, 14, 21 days post-inoculation	Faster nodulation, increased nodule numbers
  Nitrogen fixation	ARA assay (μmol C₂H₄ produced h⁻¹ g⁻¹ fresh nodules)	Higher nitrogenase activity
  ATP production	Luciferase-based ATP quantification	Increased ATP levels in bacteroids
  Plant growth	Dry weight measurement, N content analysis	Enhanced growth, higher nitrogen content

What methodologies can detect conformational changes in recombinant atpB during the catalytic cycle?

Detecting conformational changes in recombinant atpB during its catalytic cycle requires sophisticated biophysical techniques. A comprehensive methodological approach includes:

• Site-directed spin labeling coupled with EPR spectroscopy:

  • Strategic introduction of cysteine residues at key positions

  • Labeling with nitroxide spin labels (MTSL)

  • Continuous wave EPR to monitor local environment changes

  • DEER (Double Electron-Electron Resonance) for measuring distance changes between labeled sites

• Fluorescence-based approaches:

  • Site-specific labeling with environment-sensitive fluorophores

  • FRET pairs at strategic locations to monitor distance changes

  • Protocol details:

    • Introduce cysteine pairs at predicted mobile interfaces

    • Label with donor/acceptor fluorophores (Alexa488/Alexa594)

    • Reconstitute labeled protein into liposomes

    • Measure FRET efficiency changes upon energization

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS):

  • Exposure of protein to D₂O at different catalytic states

  • Quenching, digestion, and LC-MS analysis

  • Identification of regions with altered solvent accessibility

• Time-resolved structural methods:

  Technique	Information Obtained	Experimental Setup
  Time-resolved cryo-EM	Catalytic state snapshots	Rapid mixing/freezing devices, Classification of particles by conformational state
  TR-SAXS	Global conformational changes	Microfluidic mixing devices, Synchrotron radiation sources
  Single-molecule FRET	Real-time conformational dynamics	Surface immobilization, TIRF microscopy, Hidden Markov modeling

• Computational approaches integrated with experimental data:

  • Molecular dynamics simulations constrained by experimental distances

  • Normal mode analysis to identify collective motions

  • Markov state modeling of conformational transitions

This methodological framework enables detailed characterization of the conformational changes that atpB undergoes during proton translocation and ATP synthesis, providing insights into the molecular mechanism of energy conservation in A. caulinodans.

How can high-throughput mutagenesis approaches identify critical residues in recombinant A. caulinodans atpB for proton translocation?

High-throughput mutagenesis can systematically identify critical residues in atpB involved in proton translocation. A comprehensive methodological approach includes:

• Library generation strategies:

  • Error-prone PCR with controlled mutation rates

  • Saturation mutagenesis of conserved regions

  • Alanine-scanning mutagenesis of transmembrane segments

  • CRISPR-Cas9 based genomic library creation

• Expression and screening system:

  • E. coli strain lacking functional ATP synthase as expression host

  • Growth complementation assays on minimal media

  • Fluorescence-based high-throughput screening for proton translocation

• Functional screening methodologies:

  • ATP synthesis activity measurement in 96-well format

  • Proton translocation efficiency using pH-sensitive reporters

  • Growth rate determination under various energy conditions

• Data analysis framework:

  Analysis Approach	Methodology	Outcome
  Sequence-function mapping	Deep sequencing of pre- and post-selection libraries	Enrichment scores for each variant
  Mutational sensitivity profiles	Calculation of fitness scores for each position	Identification of critical vs. tolerant positions
  Structural clustering	Mapping of critical residues on homology models	Visualization of functional hotspots
  Evolutionary conservation analysis	Comparison with homologs from diverse species	Correlation of conservation with functional importance

• Validation of high-throughput findings:

  • Detailed biochemical characterization of selected variants

  • Creation of point mutations in A. caulinodans genome

  • Assessment of mutant phenotypes in symbiotic context with S. rostrata

  • Molecular dynamics simulations of critical residues

• Integration with proton path models:

  • Development of testable hypotheses for proton translocation mechanism

  • Correlation with existing models from other F-type ATP synthases

  • Identification of A. caulinodans-specific adaptations

This methodological approach provides a systematic framework for identifying and characterizing critical residues in atpB involved in proton translocation, which underlies ATP synthesis and energy conservation in A. caulinodans during both free-living and symbiotic nitrogen fixation.

How does the sequence and structure of A. caulinodans atpB compare with homologs from other nitrogen-fixing and non-nitrogen-fixing bacteria?

A comprehensive comparative analysis of A. caulinodans atpB with homologs from other bacteria provides evolutionary insights. The methodological approach includes:

• Sequence analysis framework:

  • Multiple sequence alignment using MUSCLE or MAFFT algorithms

  • Phylogenetic tree construction using maximum likelihood methods

  • Conservation analysis using ConSurf or Rate4Site

• Comparative structural analysis:

  • Homology modeling based on available ATP synthase structures

  • Structural superposition and RMSD calculation

  • Identification of conserved motifs and variable regions

• Functional domain comparison:

  Domain Feature	Analysis Method	Expected Findings
  Transmembrane helices	TMHMM or Phobius prediction, hydropathy analysis	Conservation pattern of membrane-spanning regions
  Proton channel residues	Structural alignment with characterized homologs	Conservation of charged/polar residues in channel
  Subunit interface regions	Contact prediction, coevolution analysis	Co-evolving residue pairs at interface sites
  Regulatory elements	Sequence motif identification	Lineage-specific regulatory features

• Evolutionary pressure analysis:

  • dN/dS ratio calculation for selective pressure

  • Identification of positively selected sites

  • Correlation with functional domains

• Taxonomic distribution:

  • Comparison across alphaproteobacteria with diverse lifestyles

  • Special focus on rhizobial species with different host specificities

  • Correlation with symbiotic vs. free-living nitrogen fixation capabilities

This analytical framework reveals how A. caulinodans atpB has evolved in the context of its unique lifestyle as both a free-living and symbiotic nitrogen fixer that can nodulate both roots and stems of S. rostrata , possibly highlighting adaptations specific to these capabilities.

What experimental approaches can determine if the atpB protein in A. caulinodans has adapted specifically for microaerobic nitrogen fixation?

To investigate potential adaptations of atpB for microaerobic nitrogen fixation, researchers can employ the following experimental approaches:

• Comparative functional analysis:

  • Heterologous expression of atpB from various sources in an A. caulinodans atpB knockout strain

  • Assessment of ATP synthase activity under different oxygen concentrations

  • Measurement of nitrogen fixation efficiency with different atpB variants

• Oxygen sensitivity characterization:

  • Purified enzyme assays across oxygen gradients (0-21% O₂)

  • Oxygen consumption measurements in membrane vesicles

  • ROS production assessment under varying oxygen tensions

• Domain swap experiments:

  • Creation of chimeric atpB proteins with domains from aerobes and anaerobes

  • Functional characterization in vivo and in vitro

  • Identification of domains responsible for oxygen adaptation

• Response to oxygen experimental design:

  Parameter	Method	Expected Adaptation Signs
  Kinetic parameters at low O₂	ATP synthesis assays in controlled O₂ environment	Optimized activity under microaerobic conditions
  Structural stability	CD spectroscopy, thermal shift assays	Enhanced stability under low O₂ tension
  Redox sensitivity	Activity under different redox potentials	Reduced sensitivity to oxidative damage
  Proton gradient utilization	Measurement of PMF threshold for activity	Ability to function at lower PMF typical in microaerobic conditions

• Transcriptional and post-translational regulation:

  • qRT-PCR analysis of atpB expression under varying O₂ levels

  • Identification of oxygen-responsive regulatory elements

  • Characterization of post-translational modifications under different O₂ conditions

• In planta experiments:

  • Creation of A. caulinodans strains with atpB variants

  • Inoculation of S. rostrata under different oxygen conditions

  • Assessment of nodulation and nitrogen fixation efficiency

This comprehensive methodological approach can reveal whether the atpB protein in A. caulinodans has evolved specific adaptations for functioning optimally under the microaerobic conditions required for nitrogen fixation, both in free-living state and within root and stem nodules .

What methodologies can determine the stoichiometry and subunit interactions of recombinant atpB within the complete ATP synthase complex?

Determining the stoichiometry and interactions of recombinant atpB within the ATP synthase complex requires sophisticated structural and biophysical approaches. A comprehensive methodological framework includes:

• Isolation of intact ATP synthase complex:

  • Gentle solubilization using digitonin or styrene maleic acid lipid particles (SMALPs)

  • Affinity purification using tagged subunits (preferably not atpB)

  • Size exclusion chromatography to maintain complex integrity

• Stoichiometry determination methods:

  • Quantitative mass spectrometry with isotope-labeled standards

  • Densitometric analysis of SDS-PAGE with purified subunit standards

  • Native mass spectrometry of intact complexes

• Interaction mapping techniques:

  Technique	Application	Experimental Details
  Chemical cross-linking coupled with MS	Identification of proximity between subunits	MS-cleavable crosslinkers, LC-MS/MS analysis of crosslinked peptides
  Blue native PAGE	Assessment of complex integrity	Comparison of wild-type and mutant complexes
  Co-immunoprecipitation	Verification of specific interactions	Antibodies against atpB and partner subunits
  FRET analysis	Dynamic interactions in membrane environment	Fluorescently labeled subunits reconstituted in liposomes

• Structural characterization:

  • Cryo-EM of the intact ATP synthase complex

  • Subcomplex analysis using negative stain EM

  • Integrative structural modeling combining various data sources

• Functional validation of interactions:

  • Mutational analysis of predicted interface residues

  • Suppressor mutation screening to identify compensatory changes

  • Correlation of structural data with ATP synthesis activity

This methodological approach provides a comprehensive framework for understanding how atpB integrates into the ATP synthase complex and interacts with other subunits to enable efficient energy conservation in A. caulinodans, which is critical for supporting the energy demands of nitrogen fixation both in free-living conditions and within symbiotic nodules of S. rostrata .