Recombinant Yersinia pestis bv. Antiqua Fumarate reductase subunit C (frdC)

What is the genomic context of frdC in Yersinia pestis bv. Antiqua?

Yersinia pestis bv. Antiqua represents one of the classical biovars of this plague-causing organism. The Antiqua strain possesses a genome of approximately 4.7 Mb encoding 4,138 open reading frames, while the related Nepal516 strain (also classified as Antiqua biovar) has a 4.5 Mb genome with 3,956 open reading frames . The fumarate reductase operon, including the frdC gene, is part of the core genome inherited from Y. pseudotuberculosis, from which Y. pestis recently diverged as a clonal lineage . Genomic analyses indicate that the frdC gene in Y. pestis functions analogously to its well-characterized counterpart in Escherichia coli, encoding a hydrophobic membrane anchor subunit of the fumarate reductase complex that is crucial for anaerobic respiration .

How does fumarate reductase function in Y. pestis compared to other bacterial species?

While specific characterization of Y. pestis fumarate reductase is limited in the provided search results, functional extrapolation can be made from the well-studied E. coli enzyme. In E. coli, fumarate reductase (FRD) is a four-subunit enzyme that catalyzes the terminal step in anaerobic respiration, with fumarate as the terminal electron acceptor . The FrdC and FrdD subunits anchor the catalytic FrdA and FrdB subunits to the inner surface of the cytoplasmic membrane and are required for interaction with quinones .

Y. pestis, existing in diverse environmental niches including the anaerobic conditions of necrotic tissues during infection, likely employs fumarate reductase in a similar manner for anaerobic energy metabolism. The conservation of this enzyme across bacterial species suggests its fundamental importance in microbial bioenergetics, particularly under oxygen-limited conditions that Y. pestis may encounter during its infectious cycle.

What are the optimal expression systems for producing recombinant Y. pestis frdC protein?

For expressing membrane proteins like FrdC, several expression systems warrant consideration:

When designing expression constructs, fusion tags (His6, MBP, SUMO) should be carefully positioned to avoid disrupting membrane insertion. Codon optimization for the expression host and inclusion of appropriate signal sequences may significantly improve expression levels. The methodology successfully employed for recombinant F1 antigen production from Y. pestis could provide a starting template, although modifications would be necessary for membrane protein expression .

What purification strategies are most effective for isolating functional recombinant FrdC protein?

Purification of membrane proteins like FrdC requires specialized approaches:

• Membrane fraction isolation: Following cell lysis, differential centrifugation separates membrane fractions (typically 100,000×g ultracentrifugation).

• Detergent solubilization: Screening multiple detergents (DDM, LMNG, CHAPS) at various concentrations is critical for maintaining protein stability and function.

• Affinity chromatography: If using His-tagged constructs, immobilized metal affinity chromatography (IMAC) with Ni-NTA resin provides initial purification, with carefully optimized imidazole gradients.

• Size exclusion chromatography: Critical for removing aggregates and ensuring homogeneity.

Similar approaches to those used for E. coli FrdC characterization should be applicable, recognizing that Y. pestis FrdC may exhibit unique biochemical properties requiring empirical optimization .

What amino acid residues are critical for quinone binding in Y. pestis FrdC?

Based on structural and functional studies of E. coli fumarate reductase, several key amino acid residues are likely critical for quinone binding in Y. pestis FrdC as well. In E. coli, replacement of FrdCE29 with Asp, Leu, Lys, or Phe significantly impaired both quinol oxidase and quinone reductase activities . Similarly, substitution of FrdCH82 with Arg, Leu, Tyr, or Glu decreased menaquinol oxidase activity but had variable effects on ubiquinone reduction .

Additional critical residues identified in E. coli FrdC include Ala-32, Phe-38, Trp-86, and Phe-87 . These residues likely participate in a QB-type binding site, similar to photosynthetic reaction centers. A thorough sequence alignment between E. coli and Y. pestis FrdC would be necessary to identify the corresponding residues in Y. pestis, followed by site-directed mutagenesis to confirm their functional relevance.

How can the membrane topology of Y. pestis FrdC be experimentally determined?

Several complementary approaches can be employed to determine membrane topology:

• Cysteine scanning mutagenesis: Systematically replacing residues with cysteine and assessing their accessibility to membrane-impermeable thiol-reactive reagents.

• Fusion reporter assays: Creating fusions with reporters like GFP, PhoA, or LacZ at various positions and assessing activity based on predicted localization.

• Protease protection assays: Limited proteolysis of membrane vesicles with proteases like trypsin, followed by mass spectrometry to identify protected fragments.

• Cryo-electron microscopy: For high-resolution structural determination, particularly effective when coupled with lipid nanodisc reconstitution.

These approaches should be employed in parallel, as each has inherent limitations and biases that may affect interpretation.

What directed evolution approaches can enhance functional properties of Y. pestis FrdC?

Directed evolution represents a powerful approach for enhancing protein functionality. Based on successful applications with RecA protein , the following strategies could be applied to Y. pestis FrdC:

• Error-prone PCR: Introducing random mutations throughout the frdC gene using error-prone polymerase conditions.

• DNA shuffling: Fragmenting multiple frdC gene variants and reassembling them through PCR-based recombination.

• Site-saturation mutagenesis: Targeting specific residues identified from homology models or previous studies for comprehensive amino acid substitution.

• Selection system design: Developing a growth-based selection system where E. coli cells lacking endogenous frdC are complemented with Y. pestis frdC variants under anaerobic conditions with fumarate as the sole electron acceptor.

The approach used for RecA variants, which achieved enhanced conjugational recombination through multiple rounds of selection, provides a methodological framework that could be adapted for FrdC functional enhancement .

How can genomic analysis tools be applied to understand frdC evolution across Yersinia species?

The evolution of frdC across Yersinia species can be investigated using several approaches:

• Comparative genomics: Utilizing databases like Yersiniomics, which contains 200 genomic, 317 transcriptomic, and 62 proteomic datasets for Yersinia species .

• Phylogenetic analysis: Constructing phylogenetic trees based on frdC sequences from different Yersinia species and strains to trace evolutionary relationships.

• Different Region (DFR) analysis: Applying DFR typing methods, which have been successfully used for Y. pestis genotyping, to investigate genomic regions containing the frd operon .

• Selection pressure analysis: Calculating dN/dS ratios to assess selective pressure on the frdC gene throughout Yersinia evolution.

The table below shows available reference data for various Yersinia strains that could be utilized for such analyses:

How does FrdC functionality contribute to Y. pestis virulence and host adaptation?

The contribution of fumarate reductase to Y. pestis virulence likely involves several mechanisms:

• Metabolic flexibility: FrdC, as part of the fumarate reductase complex, enables Y. pestis to perform anaerobic respiration, which may be critical during infection of anaerobic or microaerobic host tissues.

• Biovar-specific adaptations: The antiqua biovar possesses distinct genomic features compared to other biovars, with potential implications for metabolic capabilities. The complete genome sequence of Antiqua strain reveals 4,138 open reading frames that may interact with or regulate fumarate reductase activity .

• Host niche adaptation: Different expression patterns of metabolic genes, including frdC, may contribute to adaptation to various ecological niches within hosts.

Experimental approaches to investigate these contributions would include:

• Creating precise frdC deletion mutants in Y. pestis bv. Antiqua

• Conducting comparative virulence assays in animal models

• Performing transcriptomic analysis under various oxygen tensions mimicking host conditions

• Metabolomic profiling to assess changes in central carbon metabolism

How can FrdC be targeted for vaccine or antimicrobial development against Y. pestis?

FrdC presents several properties that could be exploited for therapeutic development:

• Vaccine development: While FrdC as a membrane protein presents challenges for recombinant expression, lessons from the F1 antigen work demonstrate that structural conformation is critical for protective immunity. Multimeric forms of recombinant proteins provide superior protection compared to monomeric forms in mouse challenge models . FrdC-derived peptides or detergent-solubilized preparations could be evaluated using similar immunization protocols.

• Drug targeting approaches: The quinone binding sites in FrdC represent potential targets for specific inhibitors. The critical residues identified in E. coli FrdC (Glu-29, Ala-32, His-82, Trp-86) may have counterparts in Y. pestis that could be targeted .

• Combination approaches: Targeting fumarate reductase in combination with other virulence factors might provide synergistic protection or treatment efficacy.

Experimental evaluation would require:

• Development of high-throughput screening assays for FrdC inhibitors

• Structure-based drug design utilizing homology models or experimentally determined structures

• Validation of candidates in cellular and animal models of Y. pestis infection

What techniques can resolve contradictory data regarding FrdC membrane topology?

When facing contradictory data regarding FrdC membrane topology, several advanced analytical approaches can help resolve discrepancies:

• HDX-MS (Hydrogen-Deuterium Exchange Mass Spectrometry): This technique can identify solvent-exposed regions versus membrane-protected regions with high precision.

• SMFS (Single Molecule Force Spectroscopy): Using atomic force microscopy to directly measure unfolding forces of membrane-embedded versus soluble domains.

• EPR (Electron Paramagnetic Resonance) spectroscopy with site-directed spin labeling: This approach can determine distances between specific residues and their accessibility to the aqueous environment.

• Molecular dynamics simulations: Computational modeling of FrdC in membrane environments can provide dynamic information about stable topological configurations.

• Cross-linking mass spectrometry: Using membrane-impermeable versus membrane-permeable cross-linkers to experimentally verify residue exposure.

Integration of multiple orthogonal approaches typically provides the most reliable topology model, as each method has distinct biases and limitations.

What are the optimal approaches for studying quinone-FrdC interactions in native-like environments?

Studying quinone-FrdC interactions in native-like environments requires specialized techniques:

• Nanodiscs or liposome reconstitution: Incorporating purified FrdC into lipid bilayers that mimic the bacterial membrane composition, followed by functional assays.

• MicroScale Thermophoresis (MST): For measuring binding affinities between detergent-solubilized FrdC and fluorescently labeled quinone derivatives.

• Surface Plasmon Resonance (SPR): With careful immobilization strategies to maintain FrdC in a membrane-mimetic environment.

• Isothermal Titration Calorimetry (ITC): For direct measurement of binding thermodynamics in detergent micelles.

• Solid-state NMR: For structural characterization of quinone binding sites within the membrane environment.

These approaches should be conducted with careful consideration of detergent effects, lipid composition, and protein orientation to ensure physiologically relevant results.

How can multi-omics data be integrated to understand FrdC regulation in Y. pestis bv. Antiqua?

The integration of multi-omics data for understanding FrdC regulation would involve:

• Transcriptomic analysis: Utilizing RNA-Seq data from the Yersiniomics database, which contains 151 biological conditions analyzed with microarrays and 425 RNA-Seq runs , to identify conditions that regulate frdC expression.

• Proteomic correlation: Analyzing mass spectrometry data, particularly the 62 proteomic datasets available for Yersinia species , to determine if transcriptional changes translate to protein level alterations.

• ChIP-Seq for transcription factor binding: Identifying regulatory proteins that interact with the frdC promoter region.

• Metabolomic integration: Correlating metabolite levels, particularly TCA cycle intermediates, with FrdC expression and activity.

• Network analysis: Constructing regulatory networks that position FrdC within the broader context of Y. pestis metabolism, particularly under anaerobic conditions.

Analytical platforms like Cytoscape, MetaboAnalyst, and KEGG Pathway Mapping can facilitate this integration, while machine learning approaches can identify non-obvious regulatory relationships.

What computational models best predict the impact of mutations on FrdC stability and function?

Several computational approaches can predict mutational impacts on FrdC:

• Molecular dynamics simulations: Providing atomistic insights into how mutations affect protein dynamics and stability within membrane environments.

• Machine learning algorithms: Tools like PROVEAN, PolyPhen-2, and DeepDDG that have been trained on experimental mutation data to predict functional consequences.

• Rosetta membrane protein modeling: Energy-based calculations that are specifically parameterized for membrane proteins.

• Evolutionary coupling analysis: Identifying co-evolving residues that may have functional relationships, helping predict when mutations might be compensatory versus deleterious.

• Quantum mechanics/molecular mechanics (QM/MM) methods: For detailed modeling of electron transfer reactions at quinone binding sites.

The optimal approach typically involves consensus predictions from multiple methods, validated against experimental data from model systems like the E. coli fumarate reductase, where extensive mutational analysis has been performed .