Recombinant Aliivibrio salmonicida Probable ubiquinone biosynthesis protein UbiB (ubiB)

What is the function of UbiB in Aliivibrio salmonicida?

UbiB in Aliivibrio salmonicida is a probable ubiquinone biosynthesis protein that plays a critical role in the initial steps of coenzyme Q (ubiquinone) biosynthesis. As a member of the UbiB protein kinase-like family, it is specifically required for the first monooxygenase step in the CoQ biosynthetic pathway . UbiB possesses ATPase activity and is integral to the electron transport chain and oxidative phosphorylation processes. In A. salmonicida, this protein is encoded by gene VSAL_I2998 and contributes to the bacterium's energy metabolism and potentially its virulence as a fish pathogen.

How is UbiB protein structurally characterized?

The UbiB protein from A. salmonicida contains a protein kinase-like (PKL) domain characteristic of the UbiB family . The full-length protein consists of 543 amino acids . Structurally, UbiB proteins contain conserved cysteine residues that are essential for activity, particularly in coordinating iron-sulfur clusters in related proteins such as UbiU and UbiV . The protein's structure is predicted to include membrane-spanning regions, consistent with its role in mitochondrial membrane homeostasis observed in UbiB family members .

What is the genomic context of ubiB in Aliivibrio salmonicida?

In the A. salmonicida strain LFI1238 genome, the ubiB gene is designated as VSAL_I2998 . Unlike in some other organisms where ubiB is part of an operon (such as in E. coli where it is in an operon with ubiE and yigP ), the genomic context of ubiB in A. salmonicida suggests it may be independently regulated. The genome of A. salmonicida has undergone substantial gene decay through insertion sequences, which has affected numerous metabolic pathways , though the ubiB gene appears to be intact and functional.

How does UbiB contribute to oxygen-independent ubiquinone biosynthesis?

Recent research has elucidated two pathways for ubiquinone biosynthesis: O₂-dependent and O₂-independent pathways. While UbiB is involved in the traditional O₂-dependent pathway, its family members UbiU and UbiV function in the novel O₂-independent pathway .

The O₂-independent pathway allows bacteria to synthesize ubiquinone across varying oxygen levels, which is particularly relevant for organisms like A. salmonicida that may encounter fluctuating oxygen conditions in marine environments. Studies indicate that these pathways rely on different mechanisms:

This metabolic flexibility may contribute to A. salmonicida's ability to cause infection under the low-oxygen conditions often encountered in fish tissues during cold-water vibriosis .

What is the relationship between UbiB function and iron metabolism in A. salmonicida?

A. salmonicida's response to iron limitation involves complex regulatory networks. UbiB's role in ubiquinone biosynthesis intersects with iron metabolism as several ubiquinone biosynthetic enzymes require iron-containing cofactors. Studies on A. salmonicida have shown that under iron-limited conditions, there are significant transcriptional changes affecting energy metabolism .

The bacterium produces siderophores (including the unique dihydroxamate siderophore bisucaberin) in a temperature-regulated manner, with production occurring primarily at temperatures below 10°C . This correlates with the observation that cold-water vibriosis outbreaks typically occur at temperatures below 10°C. The connection between iron acquisition systems, energy metabolism (involving ubiquinone), and virulence suggests that UbiB may indirectly influence pathogenicity through its role in maintaining cellular bioenergetics under the iron-limited conditions encountered during infection.

How might UbiB function differ under varying oxygen conditions within host environments?

A. salmonicida encounters varying oxygen conditions during infection of fish hosts. Experimental infection studies have shown that A. salmonicida rapidly establishes bacteremia (as early as 2 hours post-infection) and can persist in the blood and intestines of infected fish . This suggests adaptation to both oxygen-rich environments (blood) and relatively oxygen-poor environments (intestines).

UbiB's involvement in the O₂-dependent ubiquinone biosynthetic pathway suggests that its function might be complemented by the O₂-independent pathway (involving UbiU-UbiV) when oxygen becomes limited . This metabolic flexibility likely contributes to A. salmonicida's ability to colonize different host tissues. The bacterium shows temperature-dependent virulence (optimal at 10-12°C) , which may relate to differential expression or activity of metabolic enzymes including UbiB under these conditions.

What are the optimal expression systems for producing recombinant A. salmonicida UbiB protein?

Production of recombinant A. salmonicida UbiB requires careful consideration of expression systems due to its membrane-associated nature and potential toxicity when overexpressed. Based on studies with similar proteins, the following approach is recommended:

• Expression vector selection: pET-based vectors with tightly regulated promoters (T7 lac) allow controlled expression

• Host strain optimization: E. coli BL21(DE3) derivatives like C41(DE3) or C43(DE3) that are designed for membrane protein expression

• Culture conditions: Growth at lower temperatures (16-20°C) after induction with reduced IPTG concentration (0.1-0.5 mM)

• Solubilization strategy: Addition of mild detergents (DDM, LDAO) for membrane protein extraction

• Purification approach: IMAC (immobilized metal affinity chromatography) with histidine tags followed by size exclusion chromatography

For functional studies, it is critical to verify that the recombinant protein retains its ATPase activity, which can be assessed using established assays measuring ATP hydrolysis rates .

What experimental approaches can determine UbiB's role in ubiquinone biosynthesis in A. salmonicida?

To experimentally determine UbiB's role in ubiquinone biosynthesis in A. salmonicida, a multi-faceted approach combining genetics, biochemistry, and analytical techniques is recommended:

• Gene knockout studies: Creating a ΔubiB mutant using homologous recombination or CRISPR-Cas9 systems

• Complementation analysis: Reintroducing wild-type or mutated ubiB genes to verify phenotype restoration

• Metabolite profiling: HPLC-MS analysis to quantify ubiquinone and biosynthetic intermediates, particularly octaprenylphenol which accumulates in ubiB mutants

• Isotope labeling: Using 13C-labeled precursors to track metabolic flux through the ubiquinone pathway

• Enzymatic assays: In vitro reconstitution of UbiB activity using purified recombinant protein and appropriate substrates

• Protein-protein interaction studies: Co-immunoprecipitation or bacterial two-hybrid assays to identify interaction partners within the ubiquinone biosynthetic complex

This combined approach would provide comprehensive insights into the precise biochemical function of UbiB in A. salmonicida and potentially reveal species-specific adaptations .

How can advanced bioinformatics approaches enhance our understanding of UbiB function?

Advanced bioinformatics approaches can significantly deepen our understanding of A. salmonicida UbiB function through:

• Comparative genomics: Analyzing UbiB sequences across Vibrionaceae to identify conserved domains and species-specific variations

• Structural prediction: Using AlphaFold2 or RoseTTAFold to generate high-confidence structural models highlighting functional domains

• Molecular dynamics simulations: Investigating protein dynamics and potential substrate binding sites

• Metabolic network analysis: Integrating UbiB into genome-scale metabolic models of A. salmonicida to predict effects of ubiB mutation on cellular metabolism

• Transcriptomic data mining: Analyzing existing microarray or RNA-seq datasets to identify conditions affecting ubiB expression

• Phylogenetic analysis: Tracing the evolutionary history of UbiB across bacterial species to identify functional conservation and divergence

These computational approaches can generate testable hypotheses about UbiB function and guide experimental design, particularly for understanding how UbiB contributes to A. salmonicida's adaptation to its ecological niche as a fish pathogen .

How does UbiB contribute to A. salmonicida virulence and host adaptation?

UbiB's contribution to A. salmonicida virulence likely stems from its role in maintaining cellular bioenergetics during infection. Studies of A. salmonicida pathogenicity reveal several key insights:

• Energy requirements during infection: A. salmonicida rapidly establishes bacteremia within 2 hours post-infection, suggesting high energy demands met partly through ubiquinone-dependent respiration

• Temperature-dependent virulence: Cold-water vibriosis occurs primarily at temperatures below 10°C, coinciding with conditions where specific metabolic adaptations (including those involving ubiquinone) may be critical

• Survival in iron-limited environments: Host iron restriction is a key defense mechanism, requiring metabolic adaptations where ubiquinone-dependent respiration may provide energy advantages

• Oxygen adaptation: A. salmonicida colonizes various host tissues with different oxygen tensions, suggesting reliance on flexible respiratory systems

The bacterium shows a characteristic infection pattern: initial bacteremia, followed by intestinal colonization, where it can persist as a reservoir. This suggests that UbiB-dependent energy metabolism may support both acute and chronic phases of infection .

What is the relationship between UbiB function and environmental adaptation in A. salmonicida?

UbiB's role in ubiquinone biosynthesis likely contributes significantly to A. salmonicida's environmental adaptation:

• Temperature adaptation: A. salmonicida is a psychrophilic organism, with optimal growth at temperatures below 15°C. UbiB may have evolved specific properties to function efficiently at these lower temperatures

• Salinity response: Studies show that key regulatory proteins in A. salmonicida (such as LitR) are salinity-sensitive, suggesting that metabolic processes including energy production may be modulated by environmental salinity

• Biofilm formation: A. salmonicida can form biofilms under certain conditions, which may require metabolic adaptations including altered respiratory pathways

• Oxygen fluctuations: Marine environments experience oxygen gradients that A. salmonicida navigates using flexible respiratory systems

The bacterium's genomic analysis reveals numerous pseudogenes created by insertion sequences, suggesting genome reduction during adaptation to its specific niche. UbiB has apparently been maintained as functional, highlighting its importance for survival .

How does A. salmonicida UbiB compare to homologs in other bacterial species?

Comparative analysis of A. salmonicida UbiB with homologs in other bacterial species reveals both conservation and divergence:

What insights do phylogenetic analyses of UbiB provide about A. salmonicida evolution?

Phylogenetic analysis of UbiB sequences provides several insights into A. salmonicida evolution:

• Recent speciation: A. salmonicida and A. logei show highly conserved UbiB sequences, suggesting a recent common ancestor

• Horizontal gene transfer: Unlike the bisucaberin siderophore biosynthesis genes, which were acquired by horizontal gene transfer, UbiB appears to have been vertically inherited within the Vibrionaceae family

• Selective pressure: The maintenance of functional UbiB despite extensive pseudogenization in A. salmonicida indicates strong selective pressure to maintain ubiquinone biosynthesis

• Specialization signals: Subtle amino acid changes in A. salmonicida UbiB compared to free-living Vibrio species may reflect adaptation to its more specialized lifestyle as a fish pathogen

The evolution of UbiB within A. salmonicida appears to have balanced conserving essential bioenergetic functions while potentially adapting to the specific environmental constraints of its ecological niche .

How have oxygen-dependent and oxygen-independent ubiquinone biosynthesis pathways co-evolved in proteobacteria?

The co-evolution of O₂-dependent and O₂-independent ubiquinone biosynthesis pathways in proteobacteria reflects adaptation to environments with varying oxygen availability:

• Ancient origins: Both pathways appear to have ancient origins within proteobacteria, suggesting long-term importance for survival across oxygen gradients

• Complementary distribution: Many proteobacteria, including those in the Vibrionaceae family, possess both pathways, allowing ubiquinone biosynthesis over the entire O₂ range

• Specialized adaptations: The O₂-independent pathway relies on 4Fe-4S clusters in proteins like UbiU and UbiV, reflecting adaptation to low-oxygen environments

• Regulatory integration: Evidence suggests coordinated regulation of both pathways to optimize ubiquinone production based on environmental conditions

For A. salmonicida, which encounters varying oxygen conditions during its lifecycle in marine environments and within fish hosts, maintaining both pathways likely provides a significant selective advantage, allowing energy production across diverse environmental conditions .

What are promising approaches for characterizing the UbiB interaction network in A. salmonicida?

Future characterization of the UbiB interaction network in A. salmonicida would benefit from these approaches:

• Protein complex purification: Using tagged UbiB to isolate native complexes followed by mass spectrometry

• Bacterial two-hybrid or BACTH assays: Systematic screening for protein-protein interactions

• Crosslinking mass spectrometry: Identifying transient interactions within membrane environments

• Co-evolution analysis: Computational prediction of interaction partners based on correlated evolutionary patterns

• Microscopy techniques: Fluorescence microscopy with tagged proteins to visualize localization and potential interaction sites

• Proteomic analysis: Comparing wild-type and ΔubiB mutant membrane proteomes to identify altered protein assemblies

These approaches would help determine whether A. salmonicida UbiB participates in a multi-protein complex similar to the ubiquinone biosynthetic complexes identified in other bacteria, and identify any species-specific interaction partners that may contribute to its unique metabolic adaptations .

How might emerging technologies advance our understanding of UbiB function in A. salmonicida?

Emerging technologies offer exciting opportunities to advance our understanding of UbiB function:

• Cryo-electron microscopy: Determining the structure of UbiB within native membrane environments

• Single-cell analysis: Tracking ubiquinone metabolism in individual bacteria during host infection

• CRISPR-based screens: Identifying synthetic lethal interactions with ubiB to map functional relationships

• Metabolic flux analysis: Using stable isotopes and mass spectrometry to quantify the contribution of UbiB to cellular metabolism

• In situ expression profiling: RNA-FISH or similar techniques to visualize ubiB expression during infection

• Ribosome profiling: Determining translational regulation of UbiB under various environmental conditions

These technologies could provide unprecedented insights into how UbiB functions within the context of living cells and during infection, potentially revealing new strategies for targeting A. salmonicida in aquaculture settings .

What potential applications exist for recombinant A. salmonicida UbiB in research and biotechnology?

Recombinant A. salmonicida UbiB has several potential research and biotechnological applications:

• Structural biology platform: As a representative of the poorly characterized UbiB family, structural studies could provide insights applicable across bacteria and eukaryotes

• Antimicrobial development: High-throughput screening platform for identifying inhibitors targeting fish pathogens

• Biosensor development: UbiB-based biosensors for detecting environmental conditions relevant to aquaculture

• Metabolic engineering: Components for synthetic biology approaches to optimize bacterial energy production

• Vaccine development: Knowledge of UbiB's role in virulence could inform development of attenuated vaccine strains

• Aquaculture diagnostics: Antibodies against UbiB could be used in diagnostic tests for early detection of A. salmonicida