Recombinant Salmonella agona Fumarate reductase subunit C (frdC)

What is Salmonella agona Fumarate Reductase Subunit C (frdC) and what is its primary function?

Fumarate reductase subunit C (frdC) in Salmonella agona is a membrane protein that plays a critical role in anchoring the catalytic components of the fumarate reductase complex to the cytoplasmic membrane . This complex is essential for anaerobic respiration, allowing the bacterium to use fumarate as a terminal electron acceptor when oxygen is limited. The protein consists of 131 amino acids and is typically encoded by the frdC gene within the fumarate reductase operon . As part of the fumarate reductase complex, frdC contributes to the bacterium's ability to generate energy under anaerobic conditions, which may be particularly important during infection when oxygen availability can be limited.

What experimental techniques are available for studying frdC expression and activity?

Multiple experimental approaches can be employed to study frdC expression and activity:

• Spectrophotometric assays: Fumarate reductase activity can be measured using benzyl viologen as an artificial electron donor. Under anaerobic conditions, the oxidation kinetics of reduced benzyl viologen can be monitored at 585 nm following the addition of fumarate. Activity is typically expressed as nmol of benzyl viologen oxidized min⁻¹mg⁻¹ .

• Succinate dehydrogenase assays: For examining the reverse reaction, researchers can use dichlorophenolindophenol (DCPIP) as an artificial electron acceptor. The reduction of DCPIP can be monitored at 600 nm, with activity expressed as μmol DCPIP reduced min⁻¹mg⁻¹ protein .

• Genetic manipulation: Creating frdC knockout strains (e.g., frdC::cat) allows for comparative studies of growth patterns, metabolic activities, and virulence between wild-type and mutant strains .

• Recombinant protein expression: Full-length recombinant frdC can be expressed with affinity tags (such as His-tag) in expression systems like E. coli for purification and subsequent biochemical characterization .

How does the structure of frdC contribute to the function of the fumarate reductase complex?

FrdC is classified as a multi-pass membrane protein that is integrated into the cell inner membrane . Its hydrophobic nature allows it to serve as an anchor for the catalytic components of the fumarate reductase complex. The protein likely adopts a structure with multiple transmembrane helices that span the cytoplasmic membrane, providing a stable platform for the attachment of the catalytic subunits (frdA and frdB). This arrangement ensures proper positioning of the catalytic components relative to the membrane and the electron transport chain, facilitating efficient electron transfer during anaerobic respiration. The exact number and arrangement of transmembrane domains may vary between bacterial species, reflecting adaptations to different membrane compositions and metabolic requirements.

What role might frdC play in S. agona's ability to transition from acute to persistent infection?

Salmonella Agona has been identified as capable of transitioning from acute gastroenteritis to persistent infection, with significant implications for public health . The role of frdC in this transition may be multifaceted:

• Metabolic adaptation: During the transition to persistence, S. Agona must adapt to changing nutrient availability within the host. The fumarate reductase complex, with frdC as an essential component, may enable metabolic flexibility by allowing the bacterium to use alternative electron acceptors when oxygen is limited .

• Genomic variation: Studies of S. Agona persistence have revealed increased single nucleotide polymorphism (SNP) variation and genomic structure changes during early persistent infection . These genetic changes may affect the expression or function of metabolic genes including frdC, potentially contributing to the establishment of persistence.

• Biofilm contribution: S. Agona is recognized as a strong biofilm former, although interestingly, isolates from patients with convalescent and temporary carriage showed significantly reduced biofilm formation compared to isolates from acute illness . As metabolic genes often influence biofilm formation, changes in frdC expression or function may contribute to these observed differences.

The fumarate reductase complex's dual functionality (as seen in some bacteria) could provide metabolic versatility that supports S. Agona's adaptation to the changing host environment during the transition from acute to persistent infection.

How do modifications to the frdC gene affect antimicrobial resistance profiles?

Antimicrobial resistance in Salmonella Agona is a growing concern, with multidrug-resistant isolates identified carrying numerous resistance genes . While frdC itself is not typically associated with direct antimicrobial resistance mechanisms, alterations in metabolic pathways can indirectly influence susceptibility:

What molecular mechanisms enable the dual functionality of fumarate reductase as both a reductase and dehydrogenase?

In some bacteria, such as Helicobacter pylori, the fumarate reductase complex can function bidirectionally, catalyzing both the reduction of fumarate to succinate and the oxidation of succinate to fumarate . This dual functionality as fumarate reductase and succinate dehydrogenase involves several molecular mechanisms:

• Structural adaptations: The orientation and arrangement of the catalytic sites within the complex, partially determined by frdC anchoring, may allow for the binding of substrates in either reaction direction.

• Cofactor interactions: The interaction between the complex and electron carriers (such as menaquinone or ubiquinone) may be sufficiently flexible to permit electron flow in either direction, depending on the redox conditions.

• Regulatory controls: Expression of the fumarate reductase operon is typically regulated by environmental conditions, particularly oxygen availability. The dual functionality may be controlled through transcriptional regulation that responds to changing metabolic needs.

The experimental evidence for this dual functionality comes from enzymatic assays that measure both fumarate reductase activity (using reduced benzyl viologen as an electron donor) and succinate dehydrogenase activity (using DCPIP as an electron acceptor) . The activity levels in wild-type vs. mutant strains (frdA::cat and sdhA::cat) provide insights into the molecular basis of this dual functionality.

What genomic adaptations in S. agona might influence frdC expression during persistent infection?

Genomic studies of S. Agona isolates from different stages of infection have revealed important adaptations that may influence frdC expression during persistence:

• Genomic rearrangements: Analysis of 207 S. Agona isolates from different infection stages revealed that while most (195) maintained a conserved genomic arrangement (GS1.0), 12 isolates showed rearranged structures. These rearranged isolates were typically associated with early convalescent carriage (3 weeks to 3 months) . Such genomic rearrangements could potentially affect the expression of metabolic genes including frdC.

• SNP variation: Increased SNP variation was observed during the early period of persistent infection, suggesting a population expansion after acute S. Agona infection . These genetic changes may represent adaptive mutations that influence metabolic pathways, potentially including alterations in frdC expression or function.

• Immune evasion mechanisms: The genomic changes observed during persistent infection may reflect immune evasion mechanisms . Changes in metabolic gene expression, including frdC, could contribute to altered antigen presentation or metabolic states that help the bacterium evade host immune responses.

These genomic adaptations highlight the dynamic nature of S. Agona populations during the establishment of persistent infection and suggest potential mechanisms by which frdC expression and function might be modulated during this process.

What assays can be used to measure fumarate reductase activity in S. agona samples?

Several robust assays can be employed to measure fumarate reductase activity in S. agona samples:

Benzyl Viologen-Based Assay:
This is the standard method for measuring fumarate reductase activity under anaerobic conditions .

Protocol Overview:

• Prepare reaction mixture containing 75 mM sodium phosphate buffer (pH 6.8), 0.2 mM benzyl viologen, and 1-5 μg of cell extract in 1-ml quartz cuvettes with stoppers.

• Maintain anaerobic conditions by flushing N₂ gas through the cuvette.

• Inject freshly prepared 20 mM sodium dithionite until the absorbance at 585 nm reaches 0.8-0.9 (representing half-reduced benzyl viologen).

• Add anaerobic solution of sodium fumarate (final concentration 5 mM).

• Record benzyl viologen oxidation kinetics at 585 nm.

• Calculate activity as nmol of benzyl viologen oxidized min⁻¹mg⁻¹ using the extinction coefficient of 8.65 cm⁻¹mM⁻¹ .

Succinate Dehydrogenase Assay (For Bidirectional Activity):
This assay measures the reverse reaction (succinate oxidation) and can be used to assess dual functionality .

Protocol Overview:

• Prepare reaction mixture containing 50 mM Tris-HCl buffer (pH 7.2), 0.25 mM DCPIP, 0.4 mM phenazine methosulfate, and cell extract.

• Make the mixture anoxic by sparging with N₂ gas.

• Start the reaction by adding 20 mM sodium succinate (pH 7.4).

• Record DCPIP-dependent reduction kinetics at 600 nm.

• Express the rate as μmol DCPIP reduced min⁻¹mg protein⁻¹ using a molar extinction coefficient for DCPIP of 2.1 × 10⁴ cm⁻¹ .

How can researchers effectively study the role of frdC in S. agona persistence?

To investigate the role of frdC in S. Agona persistence, researchers should consider a multi-faceted approach:

Genetic Manipulation Strategies:

• Gene knockout studies: Create frdC knockout strains using methods compliant with NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acids . Compare these knockout strains with wild-type in terms of:

  • Growth under aerobic vs. anaerobic conditions

  • Ability to establish persistent infection in model systems

  • Biofilm formation capacity

• Complementation studies: Reintroduce wild-type or mutated frdC genes to knockout strains to confirm phenotype restoration or identify critical residues.

• Reporter fusion constructs: Create frdC-reporter gene fusions to monitor expression levels under different environmental conditions and during different infection stages.

In vitro Persistence Models:

• Biofilm formation assays: Assess biofilm capacity using crystal violet assays following growth in rich media, comparing isolates from acute infection versus persistent carriage .

• Environmental stress resistance: Evaluate survival under conditions that mimic host environments (low pH, nutrient limitation, antimicrobial peptides).

• Viable but non-culturable (VBNC) state induction: Investigate conditions that trigger entry into the VBNC state, which has been observed in S. Agona and may contribute to persistence .

In vivo Studies:

• Animal infection models: Use appropriate animal models to compare the ability of wild-type and frdC mutant strains to establish persistent infections.

• Time-course studies: Isolate bacteria at different time points during infection to assess genomic and phenotypic changes, with particular attention to frdC expression and fumarate reductase activity.

• Tissue-specific analyses: Examine frdC expression in bacteria isolated from different host tissues to identify potential tissue-specific regulation.

What are the key considerations when designing experiments to investigate frdC interactions with antimicrobial compounds?

When investigating potential interactions between frdC and antimicrobial compounds, researchers should consider:

Experimental Design Framework:

• Expression system selection: Use recombinant systems that accurately represent the native conformation and function of frdC, such as membrane vesicles or reconstituted proteoliposomes.

• Compound screening approach: Implement high-throughput screening methods to identify compounds that specifically interact with frdC or inhibit fumarate reductase activity.

• Confirmation studies: Validate hits from primary screens using secondary assays that assess direct binding (e.g., surface plasmon resonance) and functional inhibition (e.g., enzymatic assays).

Activity Assessment:

Resistance Development Monitoring:

• Serial passage experiments: Expose S. Agona to sub-lethal concentrations of promising compounds over multiple generations to assess resistance development.

• Genomic analysis: Sequence isolates with reduced susceptibility to identify potential resistance mechanisms, particularly those involving frdC mutations.

• Cross-resistance profiling: Evaluate whether resistance to frdC-targeting compounds confers resistance to other antimicrobials, which may indicate shared resistance mechanisms.