Recombinant Salmonella typhimurium Fumarate reductase subunit D (frdD)

What role does frdD play in bacterial metabolism?

frdD functions as an essential membrane anchor component of the fumarate reductase complex, which catalyzes the reduction of fumarate to succinate during anaerobic respiration. This reaction is particularly critical when Salmonella transitions from aerobic to anaerobic metabolism during host infection .

The complex consists of four subunits (FrdA, FrdB, FrdC, and FrdD), with FrdD serving as one of the hydrophobic membrane anchor proteins that facilitates electron transfer within the complex. The complete complex enables Salmonella to utilize fumarate as a terminal electron acceptor under anaerobic conditions, allowing energy generation when oxygen is limited .

What are the optimal expression systems for recombinant frdD production?

The most common expression system for recombinant frdD is Escherichia coli, which allows for high-yield protein production. Based on established protocols, recombinant frdD is typically expressed with an N-terminal affinity tag, commonly a His-tag (either 6× or 10×) . The expression parameters include:

The lower expression temperature helps maintain proper folding of this membrane protein component, enhancing solubility and functional integrity .

What are the recommended storage conditions for purified recombinant frdD?

For optimal stability of purified recombinant frdD, researchers should follow these evidence-based storage protocols :

• Store as lyophilized powder at -20°C/-80°C for long-term storage

• Reconstitute in Tris-based buffer systems at pH 8.0

• Add cryoprotectants:

  • 50% glycerol for standard storage

  • Alternatively, 6% trehalose has shown excellent stability results

• Aliquot upon reconstitution to avoid repeated freeze-thaw cycles

• Working aliquots should be stored at 4°C and used within one week

• For reconstitution, use a concentration of 0.1-1.0 mg/mL

Multiple studies confirm that repeated freeze-thaw cycles significantly reduce protein stability and activity .

How can researchers generate and validate frdD knockout mutants in Salmonella?

Generation of frdD knockout mutants can be accomplished using the lambda red recombinase procedure, which has been successfully employed for creating various Salmonella gene deletions . The methodological approach includes:

• Primer design: Design primers containing 40-45 bp homology to the regions flanking the frdD gene and 20 bp homology to a selectable antibiotic resistance cassette (typically kanamycin or chloramphenicol).

• PCR amplification: Amplify the antibiotic resistance cassette using these primers.

• Transformation: Transform the PCR product into Salmonella expressing lambda red recombinase proteins (typically from the pKD46 plasmid).

• Selection: Select transformants on appropriate antibiotic-containing media.

• Verification: Confirm the mutation using PCR with locus-specific primers and sequencing.

• P22 phage transduction: To ensure the phenotype is linked to the mutation, transfer the mutation back into a wild-type background using P22 phage transduction .

For phenotypic validation, researchers should:

• Test growth under anaerobic conditions with different electron acceptors

• Compare growth rates in minimal media with fumarate as the sole electron acceptor

• Evaluate competitive fitness against wild-type strains in relevant in vivo models

What considerations are important when designing single-gene and multi-gene deletion collections involving frdD?

When creating systematically targeted single-gene deletion (SGD) and multi-gene deletion (MGD) mutant collections that include frdD, researchers should consider the following evidence-based design principles :

• Antibiotic marker selection: Use different antibiotic resistance markers (e.g., kanamycin versus chloramphenicol) to enable construction of all viable combinations of mutants in the same background.

• Marker orientation: Consider the orientation of the resistance cassette (sense versus antisense) relative to the deleted gene to minimize polar effects on adjacent genes.

• Deletion boundaries: Precisely define gene boundaries to avoid disrupting adjacent gene function or regulatory elements.

• Verification strategy: Implement a hierarchical screening approach, examining MGDs first followed by SGDs within target regions.

• Conservation considerations: Account for the potential essentiality of frdD under specific growth conditions, as demonstrated by the importance of the fumarate reductase complex during anaerobic growth .

Research has demonstrated that the fumarate reductase complex is part of a core set of genes required for infection across multiple host species, making it a valuable target for deletion studies aimed at understanding virulence mechanisms .

How does frdD contribute to Salmonella's metabolic adaptation during host infection?

The frdD subunit, as part of the fumarate reductase complex, plays a crucial role in Salmonella's metabolic adaptation during host infection through several mechanisms:

• Anaerobic respiration: During oxygen limitation in the intestinal lumen, frdD enables Salmonella to utilize fumarate as a terminal electron acceptor, supporting energy generation under anaerobic conditions .

• Oxidative TCA cycle transition: Contrary to conventional understanding, recent research demonstrates that Salmonella can switch from a branched TCA cycle to a complete oxidative TCA cycle during inflammation. This switch is facilitated by inflammation-derived electron acceptors (nitrate, tetrathionate, oxygen) and provides Salmonella with a competitive advantage over the resident microbiota .

• Succinate utilization: A complete TCA cycle enables Salmonella to efficiently utilize microbiota-derived succinate as a carbon source. This ability contributes significantly to colonization in conventionally raised mice but provides no advantage in germ-free mice, indicating the importance of microbiota-derived metabolites in Salmonella's metabolic strategy .

• Host-specific adaptation: Evidence from transposon-directed insertion-site sequencing (TraDIS) indicates that fumarate reductase subunits are part of a core set of genes required for infection across multiple host species (chickens, pigs, and cattle) .

What is the relationship between frdD functionality and virulence in different host models?

Research indicates a strong correlation between fumarate reductase functionality and Salmonella virulence across different host models:

• Chicken models: Multiple independent mutations in frdA (nine separate mutants) were all significantly attenuated in chickens, suggesting that the fumarate reductase complex is essential for full virulence in avian hosts .

• Mouse models: A Salmonella enterica serovar Typhimurium succinate dehydrogenase/fumarate reductase double mutant exhibited avirulence in BALB/c mice while retaining immunogenicity, suggesting potential vaccine applications .

• Experimental evolution studies: During adaptation to mice through serial passage, S. typhimurium lineages showed increased fitness as measured by faster growth rates in mice (selection coefficients 0.11–0.58). This adaptation likely involves optimized central metabolism, including the TCA cycle pathways in which frdD participates .

• Inflammation-dependent advantages: The functional importance of the TCA cycle, including fumarate reductase activity, increases during inflammation. Wild-type Salmonella outcompetes sdh mutants in standard mouse models, but this advantage is abrogated in NOX2 (Cybb)-deficient animals and significantly reduced in iNOS (Nos2)-deficient mice, indicating that inflammation-derived electron acceptors drive the metabolic advantage .

What techniques can be used to study frdD promoter activity and gene expression regulation?

Several advanced techniques have been successfully employed to study promoter activity and gene expression regulation in Salmonella, which can be applied specifically to frdD research:

• ChIP-exo: This high-resolution technique has been used to accurately determine binding sites of RNA polymerase subunits and sigma factors in Salmonella. For frdD studies, this method can identify precise binding locations of transcription factors that regulate frdD expression .

• RNA-Seq: Transcriptome analysis via RNA-Seq can reveal expression patterns of frdD under various environmental conditions. Integration with ChIP-exo data allows for comprehensive understanding of both transcription factor binding and resulting gene expression changes .

• Dual-fluorescent reporter systems: These systems have been used to assess the proportion of Salmonella populations expressing specific genes. A similar approach with frdD promoter-driven fluorescent protein expression can visualize frdD activation patterns at the single-cell level .

• IVET (In Vivo Expression Technology): This approach has been used to identify Salmonella genes expressed in specific tissues during infection. By creating frdD-purA-lacZY fusion constructs, researchers can assess frdD promoter activity in various host environments .

• RT-qPCR: For quantitative assessment of frdD expression under different conditions, RT-qPCR provides a reliable approach. This has been effectively used to track expression of TCA cycle genes like sucA and sdhA during infection and can be applied to frdD .

What in vivo models are most appropriate for studying frdD function during infection?

Based on published research, the following in vivo models have proven valuable for studying fumarate reductase function during infection and can be specifically applied to frdD research:

• Streptomycin-treated mouse model: This model of Salmonella-induced colitis is particularly useful for studying metabolic adaptations during intestinal inflammation. Both C57BL/6 and CBA mouse strains have been successfully employed .

• Competitive infection assays: Co-infection with wild-type and frdD mutant strains allows for direct comparison of fitness under identical host conditions. This approach has revealed the importance of complete TCA cycle functionality during inflammation .

• Gnotobiotic mouse models: These models enable researchers to dissect the role of specific microbiota members in Salmonella metabolism. Mono-association with Bacteroides (a major succinate producer) can restore succinate utilization advantages in Salmonella, which involves TCA cycle components including the fumarate reductase complex .

• Immunodeficient mouse models: NOX2-deficient (Cybb-/-) and iNOS-deficient (Nos2-/-) mice provide valuable insights into how host-derived electron acceptors influence Salmonella metabolism. These models have demonstrated that both tetrathionate and nitrate likely contribute to changes in central metabolism during infection .

• Multi-host species approach: TraDIS studies across chickens, pigs, and cattle have identified core genes required across host species, with fumarate reductase genes consistently important for virulence .

How can protein-protein interaction studies be designed to investigate frdD associations within the fumarate reductase complex?

To investigate protein-protein interactions (PPIs) involving frdD, researchers can employ a modified method specifically validated for Salmonella that includes the following steps :

• HBH-tagging: Add a histidine-biotin-histidine tag to frdD via recombinant DNA techniques. This approach has been successfully used with other Salmonella proteins including HimD, PduB, and PhoP.

• In vivo cross-linking: Stabilize protein interactions through formaldehyde cross-linking. Optimal formaldehyde concentrations range from 0.5-3%, with specific optimization needed for frdD interactions.

• Tandem affinity purification: Perform purification under fully denaturing conditions using sequential Ni-NTA agarose and streptavidin purification steps. This two-step purification greatly reduces nonspecific binding.

• Mass spectrometry identification: Identify cross-linked proteins using liquid-chromatography in conjunction with tandem mass-spectrometry (LC-MS/MS).

• Appropriate controls: Include at least two different negative controls to eliminate background and nonspecific proteins as false interactions.

This methodology has been shown to successfully identify both stable and transient protein interactions in Salmonella, making it ideal for characterizing frdD's interactions with other fumarate reductase subunits and potential regulatory proteins .

How does oxidative stress impact frdD function, and what methodologies can assess this relationship?

The relationship between oxidative stress and fumarate reductase function can be investigated using the following methodologies:

• Methionine sulfoxide reductase (Msr) mutant models: Studies with pan Msr gene deletion (Δ5 msr) strains have demonstrated increased sensitivity to oxidative stress agents. Similar approaches can assess how oxidative damage affects frdD function .

• Oxidative stress assays: Challenge Salmonella wild-type and frdD mutant strains with hypochlorous acid (HOCl), chloramine T (ChT), and superoxide-generating oxidant paraquat. Comparative survival can indicate frdD's role in oxidative stress resistance .

• Quantification of oxidative damage markers: Measure malondialdehyde (MDA), protein carbonyls, and protein aggregation levels in wild-type versus frdD mutant strains under oxidative conditions .

• Neutrophil survival assays: The susceptibility of frdD mutants to neutrophils can indicate the importance of this protein during host-generated oxidative bursts .

• In vivo fitness assessment: Compare colonization ability and persistence of wild-type and frdD mutant strains in spleen and liver of mice, where bacteria encounter significant oxidative stress .

Research on other Salmonella membrane proteins suggests that methionine residues in membrane-anchoring subunits like frdD may be particularly susceptible to oxidation, potentially affecting protein function and bacterial survival under stress conditions .

What approaches can be used to investigate the role of frdD in bacterial adaptation to changing environmental conditions?

To study frdD's role in bacterial adaptation to changing environments, researchers can employ these evidence-based methodologies:

• Experimental evolution: Serial passage of Salmonella in specific environments followed by whole-genome sequencing can identify adaptive mutations in frdD or related genes. This approach has been successful in studying Salmonella adaptation to mice with observed selection coefficients of 0.11-0.58 after <200 generations .

• TraDIS (Transposon-Directed Insertion-Site Sequencing): This high-throughput approach enables simultaneous assessment of thousands of mutants across different environments. TraDIS has successfully identified roles for 2,715 different Salmonella genes across multiple host species, creating phenotype-genotype maps of unprecedented resolution .

• Dehydration tolerance assessments: This methodology, applied to various Salmonella knockout mutants, can assess how frdD contributes to environmental stress responses. The approach involves comparing survival rates of wild-type and mutant strains following dehydration and rehydration .

• Global transcriptional analysis: Microarray or RNA-seq analysis of dehydrated Salmonella has revealed involvement of genes like kdpFABC (potassium transport) in adaptation. Similar approaches can elucidate frdD regulation under various stresses .

• Adaptive mutation rate calculation: Using mathematical modeling, researchers have calculated adaptive mutation rates for Salmonella as >10^-6/cell/generation. Similar approaches can assess how frequently frdD mutations arise during adaptation to specific environments .

How can researchers integrate multiomics data to better understand frdD function in the context of the complete Salmonella metabolic network?

Integrating multiomics data for comprehensive understanding of frdD function requires:

• Combined ChIP-exo and RNA-Seq analysis: Integration of these techniques provides high-resolution information on both transcription factor binding locations and gene expression profiles. This approach has successfully characterized Salmonella sigma factor networks and can be applied to understanding frdD regulation .

• Time-course studies: Examining growth phase-specific changes in frdD expression and protein interactions can reveal dynamic roles during infection progression. This approach has identified distinctive patterns of sigma factor networks under various conditions .

• Metabolomic integration: Correlating changes in metabolite concentrations (particularly TCA cycle intermediates like succinate and fumarate) with transcriptomic data can provide insights into the functional consequences of frdD activity .

• Cross-species comparative analysis: Comparing binding sites and regulons between different bacterial species (e.g., Salmonella vs. E. coli) can reveal evolutionarily conserved aspects of frdD regulation .

• Network analysis: Constructing regulatory networks that place frdD in the context of global metabolism can identify non-obvious connections to other pathways and cellular processes .

The integration of these approaches has proven valuable in understanding how bacterial metabolism adapts during infection, revealing, for example, that an oxidative central metabolism enables Salmonella to utilize microbiota-derived succinate during intestinal inflammation .

What statistical approaches are most appropriate for analyzing frdD expression data from different experimental conditions?

For robust statistical analysis of frdD expression data, researchers should consider:

• Differential expression analysis: For RNA-Seq data, employ statistical methods like DESeq2 or edgeR that account for the negative binomial distribution of count data. When comparing multiple conditions, use false discovery rate (FDR) correction for multiple testing .

• Significance thresholds: Apply a P-value cutoff of <0.05 with FDR correction, and a fold-change threshold of ≥1.8 for defining differential expression, as successfully employed in Salmonella transcriptomic studies .

• Quantitative RT-PCR validation: Validate RNA-Seq findings using qRT-PCR with appropriate reference genes. For Salmonella, the rsuA gene (coding for 16S rRNA pseudouridylate synthase A) has proven suitable as it shows stable expression across various conditions .

• Time-series analysis: For temporal expression data, apply specialized statistical methods like EDGE or maSigPro that account for time-dependent patterns rather than simple pairwise comparisons .

• Normalization methods: For ChIP-exo data, use signal-to-noise ratio calculations to determine binding intensity, and employ appropriate peak-calling algorithms to identify significant binding events .

These approaches have successfully identified condition-specific gene expression patterns in Salmonella, revealing, for example, how TCA cycle gene expression changes in response to different electron acceptors during infection .

What are the most promising avenues for developing antimicrobial strategies targeting frdD or the fumarate reductase complex?

Based on the critical role of fumarate reductase in Salmonella pathogenesis, several promising antimicrobial strategies emerge:

• Inhibitor development: Design of small molecules that specifically target the membrane anchor subunits (frdC and frdD) could disrupt electron transfer within the complex. This approach is supported by structural data showing these subunits are critical for complex integrity .

• Attenuated vaccine development: Research demonstrates that Salmonella strains with mutations in both succinate dehydrogenase and fumarate reductase are avirulent while remaining immunogenic in BALB/c mice. This suggests potential for developing live attenuated vaccines based on TCA cycle mutations .

• Anti-virulence approaches: Rather than killing bacteria directly, targeting metabolic adaptation pathways like the fumarate reductase complex could reduce virulence without imposing strong selective pressure for resistance development .

• Combination therapies: Pairing fumarate reductase inhibitors with compounds that target inflammation-derived electron acceptor utilization could synergistically reduce Salmonella's competitive advantage during infection .

• Host-directed therapies: Strategies that modify the inflammatory environment to reduce the availability of alternative electron acceptors could indirectly target pathways dependent on fumarate reductase activity .

The validity of these approaches is supported by research demonstrating that disruption of central metabolism significantly impairs Salmonella virulence across multiple host species .

How might advances in structural biology techniques further our understanding of frdD function and interaction dynamics?

Emerging structural biology techniques offer significant opportunities for advancing frdD research:

• Cryo-electron microscopy: This technique can provide high-resolution structures of the entire fumarate reductase complex in different functional states, revealing how frdD contributes to complex assembly and function. The current AlphaFold structure prediction (pLDDT 98.17) provides a starting point for more detailed structural characterization .

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): This approach can identify regions of frdD that undergo conformational changes during substrate binding or protein-protein interactions, providing dynamic structural information.

• Integrative structural biology: Combining complementary techniques (X-ray crystallography, NMR, SAXS, crosslinking-MS) can overcome limitations of individual methods, especially for membrane proteins like frdD.

• Molecular dynamics simulations: Using the AlphaFold-predicted structure as a starting point, simulations can predict how frdD behaves in a membrane environment and how mutations might affect function .

• In situ structural studies: Techniques like cellular cryo-electron tomography could eventually visualize the fumarate reductase complex within intact Salmonella cells, revealing native arrangements and interactions with other cellular components.