Recombinant Salmonella dublin 4-hydroxybenzoate octaprenyltransferase (ubiA)

What is the functional role of ubiA in Salmonella dublin's metabolism?

The ubiA gene encodes 4-hydroxybenzoate octaprenyltransferase, which plays a crucial role in the ubiquinone (coenzyme Q) biosynthesis pathway. This enzyme catalyzes the conversion of 4-hydroxybenzoate into 3-octaprenyl-4-hydroxybenzoate by adding an octaprenyl group . Ubiquinone serves as the primary mobile electron carrier in the aerobic respiratory chain of Salmonella, making ubiA essential for energy production through oxidative phosphorylation. When ubiA is deleted or dysfunctional, Salmonella shifts to using alternative electron carriers like demethylmenaquinone and menaquinone, particularly under anaerobic conditions .

Research has demonstrated that strains with deletions in the ubiA gene exhibit compromised motility and growth. This growth impairment occurs because the inability to synthesize ubiquinone reduces the bacteria's capacity to generate proton-motive force, which is essential for flagellar biogenesis and rotation .

What expression systems are most effective for producing recombinant S. dublin ubiA protein?

For successful expression of recombinant S. dublin ubiA protein, several host systems have proven effective, including:

• E. coli expression systems: These represent the most common approach, particularly using BL21(DE3) or its derivatives with pET vectors under IPTG induction .

• Yeast expression systems: These are useful when post-translational modifications may be important.

• Baculovirus expression systems: Preferred when working with potentially toxic membrane proteins like ubiA.

• Mammalian cell expression systems: These can be used when proper folding is critical .

For optimal expression, the protocol should include:

• Codon optimization for the chosen expression host

• Addition of a suitable affinity tag (His6, GST, etc.) for purification

• Expression at lower temperatures (16-25°C) to enhance proper folding of membrane proteins

• Addition of detergents during purification to maintain protein solubility

The full-length S. dublin ubiA protein (290 amino acids) has a predicted molecular weight of approximately 32-33 kDa .

How is ubiA function related to Salmonella dublin virulence and survival?

While ubiA itself is not a classical virulence factor, its role in energy metabolism indirectly affects virulence. The relationship between ubiA function and S. Dublin virulence can be understood through several mechanisms:

• Energy production for invasion: Disruption of the ubiquinone pathway via ubiA deletion significantly reduces motility and flagellar function, which are important for host cell invasion .

• Adaptation to host environments: The ability to switch between ubiquinone and alternative electron carriers (demethylmenaquinone and menaquinone) when ubiA is dysfunctional represents a metabolic flexibility that may contribute to S. Dublin's adaptation to different host environments .

• Interaction with host immune response: The electron transport chain components have been implicated in bacterial resistance to oxidative stress encountered during host immune responses.

• Persistence: S. Dublin strains can persist within the same cattle herd for more than 20 years, suggesting that metabolic adaptability, potentially involving electron transport chain adjustments, plays a role in long-term survival .

Notably, research has shown that when ubiA is deleted, suppressor mutations frequently arise in genes encoding components of NADH:quinone oxidoreductase-1 (particularly nuoG, nuoM, and nuoN), which partially rescue growth and motility . This adaptive response highlights the critical nature of maintaining electron transport efficiency for bacterial survival.

What methodological approaches are most effective for studying the impact of ubiA mutations on electron transport chain function?

To comprehensively assess the impact of ubiA mutations on electron transport chain function, researchers should employ a multi-faceted methodological approach:

• Electron transfer activity measurements:

  • Measurement of NADH-oxidase activities in membrane fractions using spectrophotometric assays

  • Monitoring NADH depletion at 340 nm to quantify electron transfer rates

  • Comparing wild-type and mutant strains under varying substrate conditions

• Quinone pool analysis:

  • Reversed-phase HPLC characterization of membrane quinones

  • Quantification of ubiquinone, demethylmenaquinone, and menaquinone levels

  • Mass spectrometry confirmation of quinone species

• Membrane potential assessment:

  • Fluorescent dye-based assays (DiSC3(5), JC-1) to measure membrane potential

  • Comparison between wild-type, ubiA mutants, and suppressor strains

• Respiratory chain component quantification:

  • Immunoblotting to determine levels of respiratory chain complexes

  • Blue native PAGE to assess complex assembly

  • Proteomic analysis of membrane fractions

Table: Comparative analysis of electron transport chain parameters in S. Dublin strains

Research has shown that suppressor mutations in NADH:quinone oxidoreductase-1 subunits (nuoG, nuoM, nuoN) can partially compensate for the loss of ubiquinone by improving electron flow to alternative quinones . These methodological approaches allow for detailed characterization of the metabolic adaptations that occur in response to ubiA dysfunction.

How can whole-genome sequencing be optimized to investigate S. Dublin strains with varying ubiA functionality?

Optimizing whole-genome sequencing (WGS) for investigating S. Dublin strains with varying ubiA functionality requires a systematic approach:

• Sequencing strategy optimization:

  • Use high-coverage (50-100x) short-read sequencing (Illumina) for accurate SNP detection

  • Complement with long-read sequencing (PacBio, Nanopore) to resolve complex genomic regions and plasmids

  • Target deeper coverage (>100x) around the ubiA gene region and related electron transport chain genes

• Bioinformatic pipeline development:

  • Implement core-genome SNP (cgSNP) analysis for high-resolution phylogenetic studies

  • Apply hierarchical clustering at multiple thresholds (e.g., 100, 15, and 1 cgSNPs) to identify related strains

  • Use specialized tools for detecting structural variations that may affect ubiA expression

• Comparative genomic analysis:

  • Analyze ubiA gene variability across S. Dublin isolates

  • Map suppressor mutations in NADH:quinone oxidoreductase genes (nuoG, nuoM, nuoN)

  • Identify additional genomic adaptations that may compensate for ubiA dysfunction

• Integrating functional data:

  • Correlate genomic findings with phenotypic data on growth, motility, and electron transport

  • Validate the impact of identified mutations through gene editing experiments

  • Develop predictive models for strain behavior based on genomic signatures

Recent studies have demonstrated the power of WGS in epidemiological investigations of S. Dublin, identifying distinct geographical clades and persistent strains within cattle herds . For example, phylogenetic analysis of 197 Danish cattle isolates from 1996 to 2016 identified three major clades corresponding to specific geographical regions, with evidence of persistent circulation within the same herds for over 20 years .

What are the most effective experimental designs for investigating the relationship between ubiA function and antimicrobial resistance in S. Dublin?

To investigate the relationship between ubiA function and antimicrobial resistance (AMR) in S. Dublin, researchers should implement the following experimental designs:

• Generation of isogenic strain sets:

  • Create wild-type, ΔubiA, and complemented strains

  • Develop strains with point mutations in ubiA affecting function to various degrees

  • Include suppressors with mutations in NADH:quinone oxidoreductase genes

• Comprehensive AMR profiling:

  • Determine MIC values using broth microdilution for multiple antibiotic classes

  • Perform time-kill assays under aerobic and anaerobic conditions

  • Assess biofilm formation capacity and its relationship to AMR

• Genomic and plasmid analysis:

  • Characterize plasmid content using WGS and plasmid isolation techniques

  • Identify plasmid replicon types (e.g., IncFII/IncFIB, IncN) that may carry resistance genes

  • Analyze the co-occurrence of resistance genes with specific plasmid types

• Physiological studies:

  • Examine cell membrane properties in different strains

  • Measure proton motive force under different growth conditions

  • Evaluate efflux pump activity with and without functional ubiA

Research has shown that some S. Dublin isolates harbor plasmids of IncFII/IncFIB and IncN types, with the latter carrying resistance genes such as blaTEM-1, tetA, strA, and strB . The relationship between electron transport chain function and AMR is complex, as changes in membrane potential can affect drug uptake and efflux.

Table: Resistance gene distribution in S. Dublin isolates with varying ubiA functionality

How do mutations in ubiA affect the metabolic adaptation of S. Dublin to different host environments?

The impact of ubiA mutations on S. Dublin's metabolic adaptation to different host environments involves complex physiological responses:

• Metabolic pathway rewiring:

  • When ubiA is mutated, S. Dublin shifts from ubiquinone to alternative electron carriers (demethylmenaquinone and menaquinone)

  • This shift affects the efficiency of ATP production and potentially alters carbon source utilization

  • Experimental approach: Conduct comparative metabolomics with wild-type and ubiA mutant strains under conditions mimicking different host environments (intestinal, intracellular, bloodstream)

• Adaptation to oxygen availability:

  • ubiA mutations particularly impact aerobic respiration, while anaerobic respiration is less affected

  • S. Dublin faces varying oxygen levels in different host tissues

  • Methodology: Compare growth kinetics and gene expression in wild-type and mutant strains under aerobic, microaerobic, and anaerobic conditions

• Response to host defense mechanisms:

  • Changes in electron transport chain function may alter bacterial susceptibility to oxidative stress

  • Experimental design: Challenge wild-type and ubiA mutants with reactive oxygen species and neutrophil killing assays

• Nutritional adaptation:

  • ubiA mutants show differential ability to use certain carbon sources (e.g., L-malate)

  • Protocol: Phenotype microarray analysis comparing substrate utilization between wild-type and ubiA mutant strains

One important finding is that suppressor mutations in NADH:quinone oxidoreductase-1 genes (nuoG, nuoM, nuoN) can partially restore growth and motility in ubiA mutants . This suggests that S. Dublin can adaptively reconfigure its electron transport chain to maintain viability under metabolic stress. For example, a study found that the nuoG(Q297K) mutation improved electron flow activity of NADH:quinone oxidoreductase-1 to alternative quinones in cells with a ubiA deletion .

What analytical challenges exist in differentiating between direct and indirect effects of ubiA disruption on S. Dublin pathogenesis?

Differentiating between direct and indirect effects of ubiA disruption on S. Dublin pathogenesis presents several analytical challenges:

• Pleiotropic effects of metabolic disruption:

  • Challenge: ubiA disruption affects central energy metabolism, which has cascading effects on numerous cellular processes

  • Solution: Apply systems biology approaches combining transcriptomics, proteomics, and metabolomics to map the network of effects

  • Method: Time-course experiments capturing immediate versus delayed responses to ubiA disruption

• Separating growth defects from true virulence effects:

  • Challenge: Growth impairment alone can appear as reduced virulence

  • Solution: Design growth-normalized experiments and include appropriate controls

  • Approach: Use in vivo competition assays between wild-type and ubiA mutants, alongside complemented strains

• Host-specific effects:

  • Challenge: S. Dublin adaptations to cattle versus human hosts may differ

  • Solution: Compare infection models in both bovine and human cell lines/tissues

  • Methodology: Ex vivo organ culture systems from different hosts

• Distinguishing adaptation from compensation:

  • Challenge: Determining whether suppressor mutations (e.g., in nuoG) represent pre-existing adaptations or de novo compensatory mutations

  • Solution: Single-cell sequencing approaches and fluctuation tests

  • Protocol: Measure mutation rates and characterize the population structure before and after selective pressure

• Temporal dynamics of host-pathogen interaction:

  • Challenge: Different phases of infection may be differentially affected by ubiA dysfunction

  • Solution: Stage-specific analysis of bacterial gene expression and host response

  • Design: In vivo time-course experiments with sampling at multiple infection stages

Research has shown that S. Dublin with ubiA deletion experiences varying degrees of fitness costs depending on the environment. For example, while motility is significantly reduced in soft tryptone agar, the impact on growth in liquid media may be less pronounced, especially when suppressor mutations arise . Additionally, S. Dublin's transmission between cattle herds and persistence within individual herds for decades suggests complex adaptation mechanisms that may involve metabolic flexibility .

What are the optimal protocols for purifying active recombinant S. dublin ubiA for functional studies?

Purifying active recombinant S. dublin ubiA presents challenges due to its membrane-associated nature. The following optimized protocol addresses these challenges:

• Expression system selection:

  • Use E. coli C41(DE3) or C43(DE3) strains specifically developed for membrane protein expression

  • Employ vectors with tightly regulated promoters (e.g., pET or pBAD systems)

  • Include a C-terminal His6 or His8 tag to facilitate purification while minimizing interference with function

• Optimized culture conditions:

  • Grow cells at 20-25°C after induction to slow expression and improve folding

  • Add 0.5% glucose during initial growth to suppress leaky expression

  • Induce with 0.1-0.5 mM IPTG (for T7 promoter systems) when OD600 reaches 0.6-0.8

  • Consider auto-induction media for gentler protein expression

• Membrane preparation and solubilization:

  • Harvest cells and prepare spheroplasts using lysozyme/EDTA treatment

  • Disrupt cells by sonication or French press (low pressure for membrane proteins)

  • Isolate membrane fraction by differential centrifugation (40,000 × g for 1 hour)

  • Solubilize membranes with mild detergents (critical step):

    • Primary option: 1% n-dodecyl-β-D-maltoside (DDM)

    • Alternatives: 1% digitonin or 1% Triton X-100

  • Include 20% glycerol and 1 mM DTT to stabilize the protein

• Purification strategy:

  • Metal affinity chromatography using Ni-NTA or TALON resin

  • Washing steps with 20-40 mM imidazole to reduce non-specific binding

  • Elution with 250-300 mM imidazole in buffer containing 0.05% DDM

  • Size exclusion chromatography using Superdex 200 for final purification

• Activity preservation:

  • Add 50% glycerol to the final storage buffer

  • Store aliquots at -20°C or -80°C

  • Avoid repeated freeze-thaw cycles

  • For extended experiments, maintain working aliquots at 4°C for up to one week

Table: Troubleshooting common purification issues with recombinant ubiA

The amino acid sequence of S. dublin ubiA (UniProt: B5FQQ8) indicates a highly hydrophobic protein with multiple transmembrane domains, requiring careful handling to maintain native conformation and activity .

How can researchers effectively design experiments to study the interplay between ubiA function and virulence plasmids in S. Dublin?

Designing experiments to investigate the interplay between ubiA function and virulence plasmids in S. Dublin requires sophisticated approaches:

• Strain construction strategy:

  • Generate a matrix of strains with variations in:

    • ubiA functionality (wild-type, deletion, point mutations)

    • Presence/absence of virulence plasmids

    • Combinations of both variables

  • Use precision gene editing tools (λ-Red recombination, CRISPR-Cas9) for clean genetic manipulation

  • Verify constructs by whole-genome sequencing to detect any off-target effects

• Plasmid characterization:

  • Identify and characterize virulence plasmids present in S. Dublin:

    • 83-kb virulence plasmid containing spv genes found in most isolates

    • Additional plasmids (e.g., 49-kb and 87-kb) found in some isolates

  • Perform plasmid profiling using both physical isolation methods and WGS

  • Analyze plasmid content across different geographical isolates

• Functional interaction assessment:

  • Measure plasmid maintenance stability in backgrounds with varied ubiA functionality

  • Determine if electron transport chain disruption affects virulence gene expression

  • Investigate whether metabolic stress from ubiA mutation influences plasmid transfer rates

• In vivo and in vitro virulence testing:

  • Tissue culture invasion and persistence assays

  • Macrophage survival assays

  • Bovine infection models (both cell culture and animal models where ethically approved)

  • Comparative genomics of clinical isolates with varying ubiA function

Research has shown that virulence plasmids in S. Dublin contain important pathogenicity determinants. All S. Dublin isolates typically carry an 83-kb virulence plasmid, and some strains (particularly those in Danish clade II) harbor additional 49-kb and 87-kb plasmids . The 49-kb plasmid carries resistance genes (blaTEM-1, tetA, strA, strB) and has been detected in both cattle and human isolates .

Table: Virulence plasmid distribution in S. Dublin with varying ubiA functionality

This experimental design allows researchers to determine whether the metabolic stress caused by ubiA dysfunction influences virulence plasmid maintenance, transfer, or expression, potentially revealing new interactions between core metabolism and virulence.