Recombinant Azotobacter vinelandii Probable ubiquinone biosynthesis protein UbiB (ubiB)

What is the role of UbiB in ubiquinone biosynthesis in Azotobacter vinelandii?

UbiB functions as an ATPase in the ubiquinone (UQ) biosynthetic pathway of A. vinelandii. As demonstrated in related Proteobacteria, UbiB is one of several essential enzymes that modify the aromatic ring of the precursor 4-hydroxybenzoic acid (4-HB). The protein contributes to the series of reactions involving prenylation, decarboxylation, hydroxylation, and methylation that are required for UQ production. UbiB specifically participates in the O₂-dependent pathway of ubiquinone biosynthesis, which is critical for aerobic respiration in A. vinelandii .

What are the optimal conditions for expressing recombinant A. vinelandii UbiB in heterologous systems?

Methodological Approach:

To express recombinant A. vinelandii UbiB in heterologous systems like E. coli:

• Vector Selection: Use pET-based expression vectors with T7 promoter for high-level expression.

• Host Strain: BL21(DE3) or Rosetta(DE3) strains are recommended, particularly when rare codons are present in the A. vinelandii sequence.

• Culture Conditions:

  • Growth temperature: 30°C pre-induction, 18-20°C post-induction

  • Media: LB supplemented with 1% glucose pre-induction

  • Induction: 0.1-0.5 mM IPTG when OD₆₀₀ reaches 0.6-0.8

  • Post-induction growth: 16-18 hours

• Buffer Composition for Extraction:

  • 100 mM Tris-HCl (pH 8.0)

  • 150 mM NaCl

  • 5% glycerol

  • 1 mM DTT

  • Protease inhibitor cocktail

This methodology is adapted from approaches used for similar proteins in the ubiquinone biosynthetic pathway .

What techniques are most effective for purifying recombinant UbiB while maintaining its enzymatic activity?

Purification of recombinant UbiB requires careful handling to preserve its ATPase activity. The following step-by-step protocol is recommended:

• Initial Extraction: Lyse cells in buffer containing 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, and 1 mM DTT using either sonication or French press.

• Affinity Chromatography: Apply the cleared lysate to a Ni-NTA column if using His-tagged UbiB. Wash with 20 mM imidazole and elute with a gradient of 100-250 mM imidazole.

• Size Exclusion Chromatography: Further purify using a Superdex 200 column equilibrated with 25 mM Tris-HCl pH 7.5, 150 mM NaCl, 5% glycerol, and 0.5 mM DTT.

• Activity Preservation:

  • Maintain samples at 4°C throughout purification

  • Add 1 mM ATP to all buffers to stabilize the protein

  • Store final purified protein in small aliquots at -80°C with 10% glycerol

• Purity Assessment: SDS-PAGE analysis with Coomassie staining should show >95% purity, with validation by Western blotting using anti-UbiB antibodies.

This methodology has been adapted from successful purification protocols for other ATPases involved in biosynthetic pathways .

How can I design primers for PCR amplification and cloning of the ubiB gene from A. vinelandii?

Methodological Guide for Primer Design:

• Sequence Identification:

  • Obtain the complete sequence of A. vinelandii ubiB gene from genomic databases

  • Verify gene boundaries by comparing with annotated genomes

• Primer Design Parameters:

  • Forward primer: Include restriction site (NdeI or NcoI) at 5' end, followed by 18-25 nucleotides complementary to the start of the gene

  • Reverse primer: Include restriction site (XhoI or HindIII) at 5' end, followed by 18-25 nucleotides complementary to the end of the gene

  • For His-tag fusion, modify the reverse primer to remove stop codon if C-terminal tag is desired

• Primer Specifications:

  • Length: 25-35 nucleotides total

  • GC content: 40-60%

  • Tm: 55-65°C with <5°C difference between primers

  • Add 3-4 extra bases at the 5' end of restriction sites for efficient enzyme cutting

• Example Primer Set:

  • Forward: 5'-AAACATATGACCGCTGACCTGATCGA-3' (with NdeI site)

  • Reverse: 5'-AAACTCGAGTCACAGGTTCAGGCGCT-3' (with XhoI site)

• PCR Conditions for Amplification from A. vinelandii Genomic DNA:

  • Initial denaturation: 98°C for 2 min

  • 30 cycles of: 98°C for 10 sec, 58°C for 30 sec, 72°C for 1 min/kb

  • Final extension: 72°C for 10 min

Following amplification, verify the PCR product by agarose gel electrophoresis, purify, digest with appropriate restriction enzymes, and ligate into the prepared expression vector .

How do the O₂-dependent and O₂-independent ubiquinone biosynthesis pathways interact in A. vinelandii, and what role does UbiB play in this interaction?

A. vinelandii, like many Proteobacteria, possesses both O₂-dependent and O₂-independent pathways for ubiquinone biosynthesis, allowing adaptation to environments with varying oxygen concentrations. UbiB functions specifically in the O₂-dependent pathway as an ATPase, while the O₂-independent pathway involves UbiT, UbiU, and UbiV proteins.

The interaction between these pathways appears to be regulated primarily by oxygen availability:

• Pathway Switching: Under aerobic conditions, the O₂-dependent pathway predominates with active UbiB participation. As oxygen becomes limited, expression shifts toward UbiT, UbiU, and UbiV to maintain ubiquinone production via the O₂-independent pathway.

• UbiB's Regulatory Role: Beyond its enzymatic function, UbiB likely participates in sensing oxygen availability or energy status, potentially serving as a regulatory node between the two pathways.

• Pathway Complementation: Experimental evidence from related bacteria suggests that inactivation of the UbiB-dependent pathway can be partially compensated by upregulation of the O₂-independent pathway, though with reduced efficiency.

• Energetic Considerations: The UbiB-containing O₂-dependent pathway may be more energetically favorable when oxygen is plentiful, while the O₂-independent pathway represents an essential adaptation for A. vinelandii's survival in microaerobic environments, particularly important for its nitrogen-fixing capabilities.

This pathway flexibility contributes to A. vinelandii's metabolic versatility and ability to thrive across diverse environmental conditions .

What is the relationship between UbiB function and nitrogen fixation in A. vinelandii?

The relationship between UbiB function and nitrogen fixation in A. vinelandii represents a sophisticated metabolic interconnection:

• Bioenergetic Support: UbiB, through its role in ubiquinone biosynthesis, contributes to the electron transport chain efficiency. This is critical for generating sufficient ATP required for the highly energy-demanding nitrogen fixation process, which consumes approximately 16 ATP molecules per N₂ fixed.

• Oxygen Protection Mechanism: While ubiquinone primarily functions in aerobic respiration, proper regulation of electron flow through the respiratory chain helps A. vinelandii manage oxygen levels in the vicinity of nitrogenase—an enzyme highly sensitive to oxygen inactivation. This respiratory protection is vital for nitrogen fixation.

• Metabolic Coordination: Under nitrogen-fixing conditions, A. vinelandii must balance its energy usage between nitrogen fixation and other cellular processes. UbiB's contribution to efficient energy generation helps maintain this balance.

• Adaptation to Microaerobic Conditions: During nitrogen fixation, A. vinelandii often operates in microaerobic environments. The presence of both O₂-dependent (involving UbiB) and O₂-independent ubiquinone biosynthesis pathways allows the bacterium to maintain respiratory capacity across varying oxygen tensions.

• Molybdenum Utilization Connection: Research on A. vinelandii's molybdenum storage protein (MoSto) has shown that proper metal cofactor utilization is critical for nitrogenase function. Similarly, efficient ubiquinone biosynthesis through UbiB ensures proper electron flow for these metalloenzymes .

This intricate relationship between energy metabolism and nitrogen fixation highlights the importance of UbiB in A. vinelandii's ecological success as a free-living diazotroph.

How can CRISPR-Cas9 be optimized for creating precise mutations in the ubiB gene of A. vinelandii?

Methodological Framework for CRISPR-Cas9 Editing of A. vinelandii ubiB:

• sgRNA Design Parameters:

  • Target 20-nucleotide sequences within ubiB with an adjacent NGG PAM site

  • Avoid sequences with off-target sites (>3 mismatches) in the A. vinelandii genome

  • For precise mutations, design the sgRNA to cut within 10 bp of the desired mutation site

  • Recommended tools: CHOPCHOP or E-CRISP with A. vinelandii genome as reference

• Repair Template Design:

  • For point mutations: Create 70-90 bp homology arms on each side of the cut site

  • For gene deletions: 500-1000 bp homology arms

  • Introduce silent mutations in the PAM site or sgRNA target region to prevent re-cutting

• Delivery System Optimization:

  • Construct a single plasmid containing both Cas9 and sgRNA under appropriate promoters

  • Use the broad-host-range pBBR1 backbone with spectinomycin resistance marker

  • Temperature-sensitive replication origin recommended for plasmid curing

• Transformation Protocol:

  • Prepare competent A. vinelandii cells in exponential phase (OD₆₀₀ = 0.4-0.6)

  • Optimize electroporation conditions: 2.5 kV, 200 Ω, 25 μF

  • Use CaCl₂ treatment (100 mM) for 20 minutes before electroporation

  • Recovery in SOC medium for 4-6 hours before plating

• Screening Strategy:

  • PCR amplification of the target region followed by Sanger sequencing

  • RFLP analysis if the mutation creates or removes a restriction site

  • Mismatch cleavage assay (T7E1) for initial screening of large colonies

• Efficiency Considerations:

  • Expected efficiency: 5-15% for point mutations, 1-5% for gene deletions

  • Inclusion of a counter-selection marker can increase efficiency

  • Co-transformation with recombinase genes (RecA, RecX) may enhance homologous recombination

This methodology can be adapted for creating precise mutations to study UbiB structure-function relationships or for introducing tagged versions of the protein for localization studies .

What assays can be used to measure UbiB ATPase activity in vitro?

Several robust assays can be employed to measure the ATPase activity of purified recombinant UbiB:

• Malachite Green Phosphate Assay:

  • Principle: Quantifies inorganic phosphate (Pi) released from ATP hydrolysis

  • Protocol:

    • Incubate 1-5 μg purified UbiB with 1-5 mM ATP in reaction buffer (50 mM Tris-HCl pH 7.5, 5 mM MgCl₂, 1 mM DTT)

    • Sample aliquots (50 μl) at regular intervals (0-30 min)

    • Add malachite green reagent and measure absorbance at 630 nm

  • Detection Range: 0.1-10 nmol Pi

  • Advantages: High sensitivity, adaptable to microplate format

• Coupled Enzyme Assay:

  • Principle: Couples ATP hydrolysis to NADH oxidation via pyruvate kinase and lactate dehydrogenase

  • Protocol:

    • Reaction mixture: 50 mM Tris-HCl pH 7.5, 5 mM MgCl₂, 1 mM DTT, 1 mM PEP, 0.3 mM NADH, 2-5 U/ml PK, 2-5 U/ml LDH

    • Add 1-5 μg UbiB and 1-5 mM ATP to initiate reaction

    • Monitor decrease in absorbance at 340 nm continuously

  • Advantages: Real-time measurement, high reproducibility

• Radioactive [γ-³²P]ATP Assay:

  • Principle: Measures release of ³²P from labeled ATP

  • Protocol:

    • Incubate UbiB with [γ-³²P]ATP (specific activity ~3000 Ci/mmol)

    • Stop reaction with acid at various timepoints

    • Separate unreacted ATP from Pi by thin-layer chromatography

    • Quantify radioactivity by phosphorimaging

  • Advantages: Highest sensitivity, useful for kinetic analyses

Expected Values for Active UbiB:

Control reactions should include heat-inactivated UbiB and reactions without protein to establish baseline ATP hydrolysis rates .

How can I investigate UbiB-protein interactions in the ubiquinone biosynthetic pathway of A. vinelandii?

To investigate UbiB-protein interactions within the ubiquinone biosynthetic pathway, a multi-technique approach is recommended:

• Co-Immunoprecipitation (Co-IP):

  • Method: Generate antibodies against A. vinelandii UbiB or use epitope-tagged UbiB

  • Protocol Highlights:

    • Cross-link proteins in vivo using 1% formaldehyde for 10 minutes

    • Lyse cells using buffer containing 50 mM Tris-HCl pH 7.5, 150 mM NaCl, 1% NP-40, protease inhibitors

    • Immunoprecipitate with anti-UbiB antibodies or anti-tag antibodies

    • Identify co-precipitated proteins by mass spectrometry

  • Controls: Include non-specific antibodies and lysate from ΔubiB strains

• Bacterial Two-Hybrid Assay:

  • Method: BACTH system using T25 and T18 fragments of adenylate cyclase

  • Implementation:

    • Clone ubiB and candidate interactor genes into pKT25 and pUT18C vectors

    • Co-transform into E. coli BTH101 reporter strain

    • Screen on X-gal plates and quantify interaction by β-galactosidase assay

  • Expected Results: Positive interactions yield blue colonies and high β-galactosidase activity

• Pull-Down Assays with Recombinant Proteins:

  • Method: His-tagged UbiB and GST-tagged potential interactors

  • Protocol:

    • Express and purify both proteins separately

    • Incubate together in binding buffer (20 mM HEPES pH 7.5, 150 mM NaCl, 0.1% Triton X-100)

    • Pull down complexes using Ni-NTA or glutathione resin

    • Analyze by SDS-PAGE and Western blotting

• Microscale Thermophoresis (MST):

  • Method: Label UbiB with fluorescent dye and titrate with potential interactors

  • Advantages: Requires small sample volumes, measures interactions in solution

  • Data Analysis: Calculate dissociation constants (Kd) from binding curves

• Chemical Cross-Linking Coupled with Mass Spectrometry:

  • Method: Treat A. vinelandii cells or protein complexes with cross-linkers (e.g., DSS, BS3)

  • Analysis: Digest cross-linked proteins and identify peptide pairs by LC-MS/MS

  • Outcome: Provides spatial constraints and interaction interfaces

Predicted Interaction Partners:
Based on studies in related systems, likely UbiB interactors include:

• Other ubiquinone biosynthesis enzymes (e.g., UbiA, UbiG, UbiX)

• Proteins involved in the O₂-dependent hydroxylation pathway

• Potential membrane anchoring proteins for the biosynthetic complex

This comprehensive approach will provide complementary data to construct a reliable protein-protein interaction network for UbiB .

What are the best methods for analyzing the effect of ubiB mutations on ubiquinone production and bacterial growth under varying oxygen conditions?

Comprehensive Methodology for Analyzing ubiB Mutant Phenotypes:

• Construction of Defined ubiB Mutations:

  • Site-directed mutations targeting:

    • ATPase active site residues

    • Conserved motifs across UbiB homologs

    • Potential protein-protein interaction interfaces

  • Generate complete deletion mutant (ΔubiB) as reference

  • Create complementation strains with wild-type ubiB under native and inducible promoters

• Ubiquinone Quantification:

  • Extraction Protocol:

    • Harvest cells (10-50 mg wet weight)

    • Extract with chloroform:methanol (2:1)

    • Concentrate under N₂ stream

    • Resuspend in ethanol for analysis

  • HPLC-MS Analysis:

    • Column: C18 reverse-phase (150 × 2.1 mm, 3 μm particle size)

    • Mobile phase: Methanol:isopropanol (3:1) with 10 mM ammonium acetate

    • Detection: UV absorbance (275 nm) and MS in positive ion mode

    • Quantification: External standard curve with commercial ubiquinone-10

• Growth Assays Under Defined Oxygen Conditions:

  • Aerobic Conditions:

    • Shake flasks with 1:5 culture:flask volume ratio

    • 250 rpm, 30°C

  • Microaerobic Conditions:

    • Sealed vessels with defined O₂ headspace (1-5%)

    • Slow shaking (50-100 rpm)

  • Anaerobic Conditions:

    • Sealed anaerobic chambers with O₂ scavenging system

    • Monitor growth by OD₆₀₀ measurements

• Competitive Index Assay:

  • Co-culture wild-type and mutant strains (1:1 ratio)

  • Sample at various timepoints during growth

  • Determine relative abundance using strain-specific markers

  • Calculate competitive index as described in Macho et al. (2007)

• Oxygen Consumption Rate Measurement:

  • Clark-type oxygen electrode to measure respiratory capacity

  • Standardize by cell number or protein content

  • Compare basal and maximum (uncoupled) respiration rates

• Data Analysis and Interpretation:

This methodological framework allows comprehensive phenotypic characterization of UbiB's role in A. vinelandii under varying oxygen conditions and provides insights into structure-function relationships of this important biosynthetic enzyme .

What are the promising approaches for studying the structure-function relationship of UbiB in A. vinelandii?

Several cutting-edge approaches show promise for elucidating the structure-function relationship of A. vinelandii UbiB:

• Cryo-EM Structure Determination:

  • Advantages over X-ray crystallography for membrane-associated proteins like UbiB

  • Expected resolution: 2.5-3.5 Å for purified UbiB

  • Special consideration: Incorporation into nanodiscs to maintain native-like lipid environment

  • Target: Visualization of ATP binding pocket and potential substrate interaction sites

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS):

  • Maps protein dynamics and conformational changes

  • Application: Compare apo-UbiB versus ATP-bound states

  • Expected outcome: Identification of flexible regions that may be involved in substrate recognition

• Site-Directed Mutagenesis Coupled with Activity Assays:

  • Systematic mutation of conserved residues in:

    • Walker A and B motifs (ATP binding)

    • Putative substrate binding regions

    • Predicted protein-protein interaction interfaces

  • Correlation of structural features with enzymatic activity

• Molecular Dynamics Simulations:

  • Time scale: 500 ns to microsecond simulations

  • Systems: UbiB in membrane environment, with/without nucleotides and potential substrates

  • Analysis: Conformational changes, water/ion accessibility, binding energy calculations

  • Integration with experimental data for validation

• In vivo Crosslinking and Mass Spectrometry:

  • Photo-activatable unnatural amino acids incorporated at specific positions

  • UV-induced crosslinking to capture transient interactions

  • MS/MS analysis to identify interaction partners and interfaces

• Proposed Structural Model and Functional Implications:

These complementary approaches will provide a comprehensive understanding of how UbiB's structure enables its function in ubiquinone biosynthesis and potentially reveal novel therapeutic targets for antibacterial development .

How might findings from A. vinelandii UbiB research contribute to our understanding of ubiquinone biosynthesis in other organisms, including humans?

Research on A. vinelandii UbiB has significant translational potential across species:

• Evolutionary Conservation and Divergence:

  • UbiB belongs to a protein family conserved from bacteria to humans (human ortholog: CABC1/ADCK3/COQ8A)

  • Comparing A. vinelandii UbiB with homologs provides insights into:

    • Core conserved functions maintained throughout evolution

    • Species-specific adaptations in ubiquinone biosynthesis

    • Structural elements preserved across phylogenetic distance

• Pathways Comparison: O₂-Dependent vs. O₂-Independent:

  • A. vinelandii's dual pathway system offers a unique model for understanding:

    • Adaptation to varying oxygen environments

    • Regulatory mechanisms governing pathway switching

    • Potential alternative routes in human cells under hypoxic conditions

  • Such knowledge has implications for understanding human diseases involving mitochondrial dysfunction

• Mitochondrial Disease Relevance:

  • Human CABC1/ADCK3/COQ8A mutations cause cerebellar ataxia and CoQ10 deficiency

  • A. vinelandii UbiB research may elucidate:

    • Functional consequences of disease-associated mutations

    • Potential bypass mechanisms to restore ubiquinone production

    • Therapeutic strategies to enhance CoQ10 biosynthesis

• Metabolic Engineering Applications:

  • Understanding UbiB function enables:

    • Engineering microbial strains for enhanced CoQ10 production

    • Creating oxygen-independent biosynthetic pathways

    • Developing synthetic biology approaches for ubiquinone production

• Interspecies Functional Complementation:

  • Cross-species complementation experiments involving A. vinelandii UbiB and orthologs from:

    • Other bacteria (e.g., E. coli, P. aeruginosa)

    • Yeast (e.g., S. cerevisiae Coq8)

    • Human CABC1/ADCK3/COQ8A

  • Such experiments reveal functional conservation and species-specific requirements

• Comparative Structural Insights:

This cross-species perspective highlights how mechanistic insights from A. vinelandii UbiB research can inform our understanding of ubiquinone biosynthesis across domains of life, with potential implications for human health and disease .

What are the most significant recent advances in UbiB research and remaining knowledge gaps?

Recent advances in UbiB research have significantly expanded our understanding of this protein's role in ubiquinone biosynthesis, while also highlighting several critical knowledge gaps that warrant further investigation.

Key Advances:

• Pathway Elucidation: The discovery of parallel O₂-dependent and O₂-independent pathways for ubiquinone biosynthesis has revolutionized our understanding of bacterial bioenergetics adaptation. The role of UbiB in the O₂-dependent pathway has been firmly established .

• Functional Characterization: Confirmation of UbiB's ATPase activity and its essential role in ubiquinone biosynthesis under aerobic conditions has provided mechanistic insights into its function.

• Evolutionary Conservation: Comparative genomics has revealed UbiB homologs across diverse bacterial species and eukaryotes, indicating fundamental importance in cellular bioenergetics.

• Regulatory Mechanisms: Emerging evidence suggests sophisticated regulation of ubiquinone biosynthesis pathways in response to environmental oxygen availability, with UbiB being a key component of this regulatory network.

Remaining Knowledge Gaps:

• Structural Information: Despite functional characterization, high-resolution structural data for A. vinelandii UbiB remains unavailable, limiting our understanding of its precise molecular mechanism.

• Substrate Specificity: The exact substrate(s) of UbiB and the reaction(s) it catalyzes or facilitates beyond ATP hydrolysis remain incompletely characterized.

• Protein-Protein Interactions: The composition and dynamics of the ubiquinone biosynthetic complex in A. vinelandii, including UbiB's interaction partners, requires further investigation.

• Regulatory Control: Mechanisms governing UbiB expression and activity in response to varying oxygen levels and other environmental factors need clarification.

• Species-Specific Adaptations: How UbiB function in A. vinelandii differs from homologs in other organisms, particularly in context of the bacterium's unique diazotrophic lifestyle, remains unexplored.

Addressing these knowledge gaps will require interdisciplinary approaches combining structural biology, biochemistry, genetics, and systems biology to fully elucidate UbiB's role in ubiquinone biosynthesis and cellular adaptation to environmental changes .

What methodological improvements would advance research on recombinant A. vinelandii UbiB?

Advancing research on recombinant A. vinelandii UbiB requires methodological improvements across several technical domains:

• Protein Expression and Purification:

  • Development of specialized expression systems for membrane-associated proteins

  • Optimization of detergent-free purification using styrene-maleic acid lipid particles (SMALPs)

  • Establishment of high-yield fermentation protocols specific for A. vinelandii proteins

  • Creation of fusion constructs that maintain native protein conformation while enhancing stability

• Functional Assays:

  • Development of high-throughput assays for UbiB ATPase activity

  • Creation of fluorescent or bioluminescent reporters for real-time monitoring of UbiB function

  • Establishment of in vitro reconstitution systems for complete ubiquinone biosynthetic pathway

  • Implementation of isotope labeling approaches to track metabolic flux through the pathway

• Genetic Manipulation Tools:

  • Refinement of CRISPR-Cas9 protocols specifically optimized for A. vinelandii

  • Development of inducible promoter systems with fine-tuned expression control

  • Creation of landing pad integration sites for consistent heterologous gene expression

  • Establishment of high-efficiency transformation protocols for larger constructs

• Imaging and Localization:

  • Adaptation of super-resolution microscopy techniques for A. vinelandii cells

  • Development of specific fluorescent probes for ubiquinone and pathway intermediates

  • Implementation of correlative light and electron microscopy approaches

  • Creation of non-disruptive protein tagging strategies compatible with A. vinelandii

• Computational Resources:

  • Development of A. vinelandii-specific codon optimization algorithms

  • Creation of machine learning tools to predict protein-protein interactions in the ubiquinone pathway

  • Refinement of homology modeling approaches for A. vinelandii proteins

  • Implementation of molecular dynamics force fields optimized for membrane-associated proteins