Recombinant Salmonella schwarzengrund Fumarate reductase subunit D (frdD)

What is Salmonella schwarzengrund Fumarate reductase subunit D (frdD)?

Salmonella schwarzengrund Fumarate reductase subunit D (frdD) is a 13 kDa hydrophobic protein that functions as part of the fumarate reductase enzyme complex in Salmonella schwarzengrund bacteria. The protein is encoded by the frdD gene, identified by the ordered locus name SeSA_A4610 in the Salmonella schwarzengrund (strain CVM19633) genome . The frdD subunit is one of multiple components that make up the complete fumarate reductase complex, which plays an essential role in anaerobic respiration by catalyzing the reduction of fumarate to succinate. This process is crucial for bacterial energy metabolism under oxygen-limited conditions.

What is the protein structure and sequence of S. schwarzengrund frdD?

The S. schwarzengrund frdD protein consists of 119 amino acids in its full-length form. Its primary sequence is: MINPNPKRSDEPVFWGLFGAGGMWGAIIAPVIVLLVGIMLPLGLFPGDALSFER VLTFAQSFIGRVFLFLMIVLPLWCGLHRMHHAMHDLKIHVPAGKWVFYGLAAILTVVTA IGVITL . This hydrophobic protein has been characterized with the UniProt accession number B4TSD5. The protein structure features transmembrane domains, which anchor the fumarate reductase complex to the bacterial membrane. The membrane-spanning regions are rich in hydrophobic amino acids, which facilitate its integration into the lipid bilayer.

How does frdD differ from other fumarate reductase subunits in S. schwarzengrund?

The fumarate reductase complex in S. schwarzengrund consists of multiple subunits, including frdC, which is another membrane-anchoring protein. While frdD is a 13 kDa hydrophobic protein encoded by the SeSA_A4610 locus, frdC is a 15 kDa hydrophobic protein encoded by the adjacent SeSA_A4611 locus . The amino acid sequence of frdC (MTTKRKPYVRPMTSTWWKKLPFYRFYMLREGTAVPAVWFSIELIFGLFALKH GAESWMGFVGFLQNPVVVILNLITLAAALLHTKTWFELAPKAANIIVKDEKMGPEPIIK GLWVVTAVVTVVILYVALFW) differs from that of frdD . These proteins work together as part of the membrane anchor of the fumarate reductase complex, but they have distinct structural and potentially functional roles within the complex.

What are the optimal storage and handling conditions for recombinant S. schwarzengrund frdD?

For optimal stability, recombinant S. schwarzengrund frdD should be stored at -20°C, with extended storage at -20°C or -80°C. The protein is typically supplied in a Tris-based buffer with 50% glycerol, specifically optimized for this protein's stability . It is recommended to avoid repeated freezing and thawing of the protein, as this can lead to degradation and loss of activity. For short-term work, store working aliquots at 4°C for up to one week. When handling the protein, it is advisable to maintain cold chain conditions and use sterile techniques to prevent contamination.

What techniques are used to express and purify recombinant S. schwarzengrund frdD?

Recombinant S. schwarzengrund frdD is typically expressed in bacterial expression systems using vectors that contain the frdD gene sequence (expression region 1-119) . The expression is often conducted in E. coli strains optimized for protein production. The purification process generally involves:

• Cell lysis using either mechanical disruption or chemical methods

• Initial purification using affinity chromatography (based on the tag used during expression)

• Further purification using ion exchange chromatography or size exclusion chromatography

• Buffer exchange to a storage-compatible Tris-based buffer with 50% glycerol

The tag type for the recombinant protein is determined during the production process and may include common tags such as His-tag, GST, or MBP to facilitate purification . The expression and purification protocols may need optimization depending on the specific experimental requirements.

How can researchers assess the functional activity of recombinant frdD?

The functional activity of recombinant frdD can be assessed through several experimental approaches:

• Enzymatic assays: Measuring fumarate reduction activity when combined with other subunits of the complex.

• Membrane integration studies: Analyzing the protein's ability to incorporate into artificial membrane systems.

• Protein-protein interaction assays: Investigating interactions with other fumarate reductase subunits using techniques like co-immunoprecipitation or pull-down assays.

• Spectroscopic methods: Using circular dichroism or fluorescence spectroscopy to assess proper protein folding.

These methods provide complementary information about different aspects of frdD functionality, from its structural integrity to its role in the assembled enzyme complex.

What role does frdD play in S. schwarzengrund virulence and pathogenesis?

The role of frdD in S. schwarzengrund virulence is complex and not fully elucidated. As part of the fumarate reductase complex, frdD contributes to bacterial energy metabolism under anaerobic conditions, which may be relevant during infection of host tissues where oxygen is limited. While the search results don't directly address frdD's role in virulence, they do highlight that S. schwarzengrund isolates possess various virulence factors and antimicrobial resistance genes that contribute to their pathogenicity .

Studies on S. schwarzengrund have revealed that isolates from both food and clinical sources have similar virulome profiles and are capable of invading human Caco-2 cells . The presence of virulence genes like the aerobactin operon (iucABCD and iutA) has been identified in some S. schwarzengrund isolates, particularly those carrying IncFIB-IncFIC(FII) fusion plasmids . Although these plasmids did not significantly enhance invasion or persistence in human Caco-2 cells, they may confer advantages in other infection contexts or host environments.

How is S. schwarzengrund frdD related to antimicrobial resistance?

While frdD itself is not directly associated with antimicrobial resistance, S. schwarzengrund isolates often carry multiple antimicrobial resistance genes. A comprehensive study analyzing 2,058 S. schwarzengrund isolates found that 61.7% carried at least one antimicrobial resistance gene . The most common resistance genes included:

These resistance genes are often carried on plasmids, with approximately 51.5% of isolates carrying multiple transfer genes associated with IncFIB-FIC plasmids . Additionally, other plasmid types like IncI1 (4.9%), IncHI2 (3.0%), and IncHI1 (1.2%) were detected in at least 1% of the strains .

Although fumarate reductase subunits themselves are not typically associated with antimicrobial resistance mechanisms, the metabolic adaptability conferred by the fumarate reductase complex may indirectly contribute to bacterial survival under stress conditions, including exposure to certain antimicrobials.

What experimental models are suitable for studying S. schwarzengrund frdD function in vivo?

Several experimental models can be employed to study S. schwarzengrund frdD function in vivo:

• Cell culture models: Human intestinal epithelial cell lines like Caco-2 cells have been used to study S. schwarzengrund invasion and persistence . These models can be adapted to investigate the role of frdD through comparison of wild-type and frdD-mutant strains.

• Animal models: Poultry models are particularly relevant since S. schwarzengrund has been frequently isolated from chicken sources . Mice models may also be used as a more accessible laboratory system.

• Ex vivo tissue models: Intestinal tissue explants can provide a more complex environment that better mimics in vivo conditions while allowing controlled experimental manipulation.

• Genetic models: Construction of isogenic mutants lacking frdD or expressing modified versions can help elucidate the protein's specific functions through comparative studies.

Each model system has advantages and limitations, and the choice depends on the specific research questions being addressed. For example, studies focusing on food safety aspects might prioritize poultry models, while those investigating human infection might favor human cell lines or humanized mouse models.

How can comparative genomics inform our understanding of frdD conservation and evolution?

Comparative genomic approaches can provide valuable insights into the evolution and conservation of frdD across different Salmonella serovars and related bacterial species. Analysis methods include:

• Sequence alignment and phylogenetic analysis: Comparing frdD sequences across different bacterial species to understand evolutionary relationships and selective pressures.

• Synteny analysis: Examining the genomic context of the frdD gene to identify conserved gene clusters or operons.

• SNP-based phylogenetic analysis: Studies have used single nucleotide polymorphism (SNP) analysis to investigate the relationships between S. schwarzengrund isolates, revealing that those carrying certain plasmids (like the IncFIB-IncFIC(FII) fusion plasmid) form distinct subclades .

Comparative genomic analyses could potentially reveal how fumarate reductase genes have been maintained or modified across different ecological niches and host adaptations. For instance, the core gene phylogeny of IncFIB-IncFIC(FII) fusion plasmids suggests that those found in S. schwarzengrund might be descended from plasmids of avian pathogenic isolates, indicating potential adaptation to avian hosts .

What are the current limitations in studying S. schwarzengrund frdD?

Several challenges currently limit comprehensive research on S. schwarzengrund frdD:

• Membrane protein complexity: As a hydrophobic membrane protein, frdD presents technical challenges for structural studies, purification in active form, and functional reconstitution.

• Functional redundancy: Bacteria often possess alternative metabolic pathways that can compensate for the loss of specific functions, making it difficult to isolate the precise role of frdD.

• In vivo relevance: Connecting in vitro observations to in vivo significance remains challenging, particularly in understanding how frdD contributes to bacterial fitness during infection.

• Limited specific antibodies: The availability of high-quality antibodies specifically recognizing S. schwarzengrund frdD is limited, complicating certain immunological studies.

• Model system limitations: Current model systems may not fully replicate the conditions encountered by S. schwarzengrund in natural environments or during infection.

What emerging technologies could advance research on frdD?

Several cutting-edge technologies hold promise for advancing our understanding of frdD:

• Cryo-electron microscopy: This technique could provide detailed structural information about frdD in the context of the complete fumarate reductase complex.

• Single-cell technologies: Methods to study gene expression and protein function at the single-cell level could reveal population heterogeneity in frdD expression and function.

• Advanced genetic tools: CRISPR-Cas systems adapted for use in Salmonella could facilitate more precise genetic manipulation to study frdD function.

• Metabolomics approaches: Comprehensive metabolite profiling could help elucidate the impact of frdD on bacterial metabolism under different conditions.

• Microfluidic systems: These could enable high-throughput screening of conditions affecting frdD expression or function, or for evaluating potential inhibitors.

These technologies, independently or in combination, could address many of the current limitations in studying S. schwarzengrund frdD and potentially lead to new insights with implications for food safety and public health.