Recombinant Salmonella paratyphi A Fumarate reductase subunit D (frdD)

What is Fumarate reductase subunit D (frdD) in Salmonella paratyphi A?

Fumarate reductase subunit D (frdD) in Salmonella paratyphi A is a 13 kDa hydrophobic protein that serves as a critical component of the fumarate reductase enzyme complex. The protein is encoded by the frdD gene (locus name SSPA3861) and consists of 119 amino acids with a sequence beginning with "MINPNPKRSDE..." . This protein is anchored in the bacterial membrane and functions as part of the electron transport chain during anaerobic respiration. The fumarate reductase complex catalyzes the conversion of fumarate to succinate, which is essential for energy production under oxygen-limited conditions that Salmonella may encounter within host tissues during infection.

In contrast to the more extensively studied DmsABC enzyme complex (which uses alternative electron acceptors like methionine sulfoxide during infection), the frdD subunit specifically helps anchor the catalytic portions of the fumarate reductase complex to the membrane, enabling efficient electron transfer processes that support bacterial survival in the anaerobic environment of infected tissues .

How does recombinant frdD differ from native frdD in Salmonella paratyphi A?

Recombinant Salmonella paratyphi A Fumarate reductase subunit D differs from its native form primarily in expression system, purification process, and potential modifications:

• Expression system: Recombinant frdD is typically produced in laboratory expression systems (commonly E. coli) rather than being isolated from Salmonella paratyphi A. This allows for controlled production and higher yields.

• Tag modifications: Recombinant frdD often contains affinity tags to facilitate purification. As noted in the product information, "The tag type will be determined during production process" .

• Buffer composition: The recombinant protein is stored in specialized buffers (Tris-based with 50% glycerol) optimized for stability and function, which differs from its native membrane environment .

• Purity level: Recombinant preparations typically achieve higher purity (often >95%) compared to native protein isolations.

• Function preservation: While the amino acid sequence (residues 1-119) matches the native protein, the recombinant version may show subtle differences in folding or activity depending on the expression and purification conditions employed.

What is the role of fumarate reductase in Salmonella pathogenesis?

Fumarate reductase plays a crucial role in Salmonella pathogenesis through several mechanisms:

• Anaerobic energy production: During infection, Salmonella encounters oxygen-limited environments within host tissues. The fumarate reductase complex (including the frdD subunit) enables energy generation through anaerobic respiration, using fumarate as a terminal electron acceptor.

• Metabolic adaptation: This enzyme allows Salmonella to adapt to changing metabolic conditions within the host, contributing to bacterial persistence during infection.

• Potential contribution to virulence: While not directly associated with DNA damage like the CdtB subunit of typhoid toxin , the ability to sustain metabolism under anaerobic conditions supports bacterial replication and virulence factor production.

• Systemic infection support: Similar to the DmsABC enzyme complex that "is important for the systemic phase of the Salmonella infection" , fumarate reductase likely contributes to Salmonella's ability to disseminate and persist in systemic infections.

The importance of anaerobic respiration enzymes is highlighted by research showing that mutants lacking certain respiratory enzymes show attenuated virulence, particularly during the systemic phase of infection. This suggests that targeting these respiratory pathways could be a potential therapeutic approach.

How does frdD contribute to the electron transport chain in relation to other respiratory enzymes in Salmonella paratyphi A?

Fumarate reductase subunit D (frdD) serves as a membrane anchor in the fumarate reductase complex, which plays a specialized role in the Salmonella electron transport chain that differs from but complements other respiratory enzymes:

• Membrane localization: frdD contains hydrophobic regions that anchor the catalytic components of the fumarate reductase complex to the cytoplasmic membrane, positioning the complex optimally for electron transfer.

• Electron flow pathway: In the anaerobic electron transport chain, electrons from reduced quinones are transferred to the iron-sulfur clusters in the fumarate reductase complex, eventually reducing fumarate to succinate.

• Coordination with other respiratory enzymes:

  Respiratory Enzyme	Terminal Electron Acceptor	Oxygen Requirement	Regulatory Control
  Fumarate Reductase	Fumarate	Anaerobic	FNR, Fur
  DmsABC	Methionine sulfoxide, DMSO	Anaerobic	FNR, Fur, H₂O₂ responsive
  Nitrate Reductase	Nitrate	Anaerobic	FNR
  Cytochrome oxidases	Oxygen	Aerobic	ArcAB

• Regulatory integration: Like the DmsABC system that is controlled by FNR and Fur regulatory proteins , the fumarate reductase complex is likely regulated by similar anaerobic sensing mechanisms to ensure appropriate expression when oxygen is limited.

• Metabolic flexibility: The presence of multiple terminal electron acceptor systems, including fumarate reductase, provides Salmonella with metabolic flexibility to adapt to diverse host environments, contributing to its pathogenic success.

This integrated respiratory network allows Salmonella to optimize energy production under varying oxygen conditions encountered during infection, with frdD contributing specifically to fumarate-based respiration.

What are the structural features of frdD that contribute to its function in the fumarate reductase complex?

The structural features of Salmonella paratyphi A frdD that enable its function in the fumarate reductase complex include:

The amino acid composition is notably hydrophobic (note the prevalence of glycine, alanine, valine, leucine, isoleucine, and phenylalanine in the sequence), which is consistent with its membrane-embedded nature . This hydrophobicity is essential for maintaining the proper positioning of the complex within the membrane to facilitate electron transfer from quinones to the catalytic sites.

How does the expression of frdD and other respiratory enzymes vary during different stages of Salmonella paratyphi A infection?

The expression of frdD and other respiratory enzymes shows dynamic regulation throughout Salmonella paratyphi A infection stages:

• Initial colonization (intestinal lumen):

  • Moderate expression of aerobic respiratory enzymes

  • Beginning upregulation of anaerobic respiratory genes

  • Environmental sensing systems activate appropriate respiratory pathways

• Epithelial invasion (gallbladder):

  • Transition to microaerobic conditions triggers increased expression of fumarate reductase

  • Research using gallbladder organoids shows Salmonella can "invade epithelial cells" creating localized anaerobic microenvironments

  • Bacterial adaptation to decreased oxygen availability

• Intracellular survival (macrophages):

  • Strong induction of anaerobic respiratory enzymes including fumarate reductase

  • Similar to DmsABC which shows "100-fold higher transcription levels" under anaerobic conditions

  • Oxidative stress from host defense mechanisms further modulates expression

• Systemic dissemination:

  • Maximum expression of anaerobic respiratory enzymes

  • Evidence from DmsABC studies indicates anaerobic enzymes are "important for the systemic phase of the Salmonella infection"

  • Adaptation to diverse tissue environments with varying oxygen tensions

This dynamic regulation is orchestrated by multiple transcription factors, including FNR (fumarate and nitrate reduction) and Fur (ferric uptake regulator), which respond to oxygen availability and iron status respectively . The ability to modulate respiratory enzyme expression, including frdD, enables Salmonella to adapt its metabolism to changing host environments, contributing to its pathogenic success across diverse host tissues.

What are the optimal conditions for expression and purification of recombinant Salmonella paratyphi A frdD?

Optimal expression and purification of recombinant Salmonella paratyphi A frdD requires careful consideration of several parameters:

• Expression system selection:

  • E. coli BL21(DE3) or C43(DE3) strains are recommended for membrane protein expression

  • Codon-optimized gene sequence improves expression efficiency

  • pET or pBAD vector systems with tunable induction provide controlled expression

• Culture conditions:

  • Lower temperature (16-25°C) during induction reduces inclusion body formation

  • Rich media (such as Terrific Broth) supports higher biomass

  • Induction at mid-log phase (OD600 0.6-0.8) optimizes protein yield

• Membrane extraction:

  • Gentle cell lysis using enzymatic methods or French press

  • Membrane fraction isolation through differential ultracentrifugation

  • Detergent screening to identify optimal solubilization conditions

• Purification strategy:

  • Affinity chromatography using appropriate tags (His, GST, or MBP)

  • Size exclusion chromatography for final polishing

  • Buffer optimization containing stabilizing agents (glycerol at 50% as used in commercial preparations)

• Quality control assessments:

  • SDS-PAGE and Western blotting for purity and identity verification

  • Mass spectrometry for sequence confirmation

  • Circular dichroism to assess secondary structure integrity

Storage recommendations include maintaining the purified protein at -20°C in Tris-based buffer with 50% glycerol, avoiding repeated freeze-thaw cycles, and keeping working aliquots at 4°C for up to one week as indicated in the product specifications .

What experimental approaches can be used to study the role of frdD in Salmonella paratyphi A pathogenesis?

Multiple experimental approaches can be employed to investigate frdD's role in Salmonella paratyphi A pathogenesis:

• Genetic manipulation techniques:

  • Creation of frdD knockout mutants using CRISPR-Cas9 or lambda-red recombination

  • Complementation studies with wildtype or modified frdD

  • Site-directed mutagenesis to examine specific functional domains

• In vitro infection models:

  • Gallbladder organoid systems as used for typhoid toxin studies

  • Polarized monolayers in air-liquid interphase for extended infection studies

  • Macrophage infection assays to assess intracellular survival

• Comparative virulence assessments:

  • Competition assays between wildtype and ΔfrdD mutants (similar to approaches used for DmsABC studies)

  • Single infection experiments in different mouse models

  • Assessment in immunocompromised mice to evaluate role in specific immune contexts

• Transcriptomic and proteomic analyses:

  • RNA-Seq to determine transcriptional changes under different oxygen conditions

  • Quantitative proteomics to assess frdD expression during infection

  • Chromatin immunoprecipitation (ChIP) to identify regulatory factors

• Biochemical activity assays:

  • Membrane vesicle preparations to measure fumarate reductase activity

  • Oxygen consumption measurements using different electron acceptors

  • Electron transport chain component interactions

• Host response evaluation:

  • Analysis of DNA damage in infected cells (γH2AX staining)

  • Cell cycle progression assessment following infection

  • Measurement of oxidative stress markers in host cells

These approaches can be integrated to provide a comprehensive understanding of how frdD contributes to Salmonella pathogenesis across different infection stages and host environments.

How can the interaction between frdD and other subunits of the fumarate reductase complex be studied?

Investigating the interactions between frdD and other fumarate reductase subunits requires specialized techniques for membrane protein complexes:

• Co-immunoprecipitation approaches:

  • Epitope tagging of frdD or partner subunits (FrdA, FrdB, FrdC)

  • Gentle solubilization using detergents like n-dodecyl-β-D-maltoside (DDM)

  • Antibody-based pulldown followed by Western blot or mass spectrometry analysis

• Crosslinking mass spectrometry:

  • Chemical crosslinkers (DSS, BS3) to capture transient interactions

  • Photoactivatable amino acids for site-specific crosslinking

  • LC-MS/MS analysis to identify crosslinked peptides and interaction sites

• Förster Resonance Energy Transfer (FRET) analysis:

  • Fluorescent protein fusions to frdD and partner subunits

  • Live-cell imaging to monitor protein-protein interactions

  • Acceptor photobleaching FRET to quantify interaction strength

• Bacterial two-hybrid systems:

  • Modified BACTH system optimized for membrane protein interactions

  • Split-ubiquitin assays for membrane protein interaction mapping

  • Systematic screening of interaction domains

• Structural biology approaches:

  • Cryo-electron microscopy of the purified complex

  • X-ray crystallography of the stabilized complex or subdomains

  • NMR studies of specific interaction domains

• Surface plasmon resonance:

  • Immobilization of purified frdD on sensor chips

  • Real-time binding measurements with other subunits

  • Determination of binding kinetics and affinity constants

• Computational modeling:

  • Molecular dynamics simulations of the complex in membrane environment

  • Protein-protein docking to predict interaction interfaces

  • Sequence conservation analysis to identify critical interaction residues

These approaches can be combined to build a comprehensive model of how frdD contributes to the assembly and function of the complete fumarate reductase complex in Salmonella paratyphi A.

How can understanding frdD function contribute to developing new antimicrobial strategies against Salmonella paratyphi A?

Understanding frdD function offers several promising avenues for novel antimicrobial strategies against Salmonella paratyphi A:

• Target-based drug design:

  • Identification of small molecules that interfere with frdD membrane insertion

  • Development of peptidomimetics that disrupt fumarate reductase complex assembly

  • Design of competitive inhibitors of fumarate binding sites

• Metabolic vulnerability exploitation:

  • Creating compounds that selectively inhibit anaerobic respiration

  • Developing dual-targeting approaches that simultaneously inhibit multiple respiratory pathways

  • Engineering prodrugs activated by fumarate reductase activity to deliver antimicrobials

• Vaccine development strategies:

  • Evaluating frdD as a potential vaccine antigen

  • Creating attenuated strains with modified respiratory capabilities

  • Designing subunit vaccines targeting multiple respiratory complex components

• Host-directed therapeutics:

  • Modulating host environments to create unfavorable conditions for anaerobic respiration

  • Targeting host factors that interact with bacterial respiratory machinery

  • Enhancing host antimicrobial responses that specifically impact respiratory enzymes

• Diagnostic applications:

  • Developing rapid detection methods based on fumarate reductase activity

  • Utilizing Paratype genotyping tools to identify strains with specific respiratory enzyme variants

  • Creating biomarkers for tracking treatment efficacy

These approaches are particularly valuable given that Salmonella Paratyphi A is "becoming resistant to antimicrobials and has no licensed vaccines" . The essential nature of anaerobic respiration for systemic infection makes respiratory enzymes including fumarate reductase attractive targets for developing novel intervention strategies.

What is the relationship between frdD function and the genotoxic effects observed during Salmonella paratyphi A infection?

While frdD is not directly involved in DNA damage induction, its function may indirectly contribute to the genotoxic effects observed during Salmonella paratyphi A infection through several mechanisms:

• Metabolic support for toxin production:

  • Efficient anaerobic respiration via fumarate reductase provides energy for the synthesis and secretion of the typhoid toxin

  • Research shows that the CdtB subunit of typhoid toxin "directly induces DNA breaks in host cells"

  • Energy production through anaerobic respiration may be critical for sustained toxin expression

• Bacterial persistence enhancement:

  • Fumarate reductase activity enables longer-term infection

  • Extended infections allow for "an initial arrest of the cell cycle" followed by continued proliferation "despite the DNA damage"

  • Persistent bacteria provide ongoing sources of genotoxic factors

• Indirect oxidative stress modulation:

  • Respiratory activity influences the redox state of the bacterial cell

  • Changes in bacterial metabolism may affect the host cellular environment

  • Similar to how the DmsABC complex uses "methionine sulfoxide as a substrate" , fumarate reductase activity may influence oxidative stress markers

• Potential contribution to inflammation:

  • Bacterial respiratory activity sustains infection, prolonging inflammatory responses

  • Chronic inflammation contributes to DNA damage in host cells

  • This connection supports "the epidemiological link between Salmonella infection and GBC (gallbladder cancer)"

• Spatial distribution effects:

  • Enabling bacterial growth in specific tissue microenvironments

  • Different tissue localization influences patterns of host cell DNA damage

  • Contributes to the observation that damage "extended to neighboring, non-infected cells"

This relationship highlights how bacterial respiratory metabolism, while not directly genotoxic, supports pathogenic processes that lead to host DNA damage and potential long-term consequences such as cancer risk.

How does frdD compare between different Salmonella serovars and what are the implications for host specificity?

Comparative analysis of frdD across Salmonella serovars reveals important evolutionary patterns with implications for host specificity:

• Sequence conservation patterns:

  • Core functional domains show high conservation across serovars

  • Variable regions correlate with adaptation to different host environments

  • Specific amino acid substitutions may reflect adaptation to unique host conditions

• Expression regulation differences:

  • Promoter region variations influence expression timing and magnitude

  • Regulatory network integration varies between host-adapted and broad-host serovars

  • Different responses to host-specific environmental signals

• Comparative functional characteristics:

  Serovar	frdD Sequence Identity	Oxygen Tension Adaptation	Host Range	Associated Diseases
  S. Paratyphi A	Reference (100%)	Low oxygen adaptation	Human-restricted	Paratyphoid fever
  S. Typhi	High (~95%)	Low oxygen adaptation	Human-restricted	Typhoid fever
  S. Typhimurium	Moderate (~85%)	Variable oxygen adaptation	Broad-host	Gastroenteritis, systemic infection
  S. Gallinarum	Moderate (~83%)	Avian-adapted	Avian-restricted	Fowl typhoid

• Co-evolution with virulence factors:

  • In human-adapted serovars like Paratyphi A, frdD functions alongside specialized virulence factors like the typhoid toxin

  • The absence of typhoid toxin in S. Typhimurium means it "lacks the typhoid toxin produced by the human serovars Typhi and Paratyphi A"

  • This co-evolution influences tissue tropism and disease manifestation

• Genotyping implications:

  • Paratype genotyping tools can segregate "Salmonella Paratyphi A population into three primary and nine secondary clades, and 18 genotypes"

  • Variations in respiratory genes may contribute to these phylogenetic patterns

  • Such tools facilitate "surveillance studies tracking Salmonella Paratyphi A across the globe"

These comparative differences in frdD and associated respiratory systems contribute to the distinct host ranges and disease presentations of various Salmonella serovars, offering insights into evolutionary adaptation strategies and potential targets for serovar-specific interventions.