Recombinant Salmonella paratyphi B Fumarate reductase subunit C (frdC)

What is the structure and function of Fumarate reductase subunit C (frdC) in Salmonella paratyphi B?

Fumarate reductase subunit C (frdC) in Salmonella paratyphi B is a 131-amino acid membrane protein that serves as one of the anchoring components of the fumarate reductase complex . The amino acid sequence is: "MTTKRKPYVRPMTSTWWKKLPFYRFYMLREGTAVPAVWFSIELIFGLFALKHGAESWMGFVGFLQNPVVVILNLITLAAALLHTKTWFELAPKAANIIVKDEKMGPEPIIKGLWVVTAVVTVVILYVALFW" . This hydrophobic protein contains multiple transmembrane domains that integrate the enzyme complex into the bacterial cell membrane.

Functionally, frdC works alongside subunit D to anchor the catalytic subunits (A and B) to the membrane and facilitates quinone interactions during anaerobic respiration . The protein is essential for electron transfer from menaquinol to the iron-sulfur clusters in the B subunit, which ultimately enables the reduction of fumarate to succinate at the catalytic site in subunit A . Without the C subunit, the fumarate reductase complex loses its quinone reductase activity while maintaining its ability to reduce fumarate with artificial electron donors like phenazine methosulfate .

How does frdC contribute to the pathogenicity of Salmonella paratyphi B?

The frdC subunit plays a crucial role in pathogenicity by enabling Salmonella paratyphi B to survive in the anaerobic environments encountered during infection. As part of the fumarate reductase complex, frdC facilitates anaerobic respiration using fumarate as a terminal electron acceptor when oxygen is limited . This metabolic versatility is particularly important in:

• The oxygen-limited intestinal environment where S. paratyphi B initially colonizes

• The intracellular environment within host macrophages

• The anaerobic conditions of deep tissue sites during systemic infection

The ability to use alternative electron acceptors like fumarate provides S. paratyphi B with a significant survival advantage during infection, allowing it to generate energy under conditions where obligate aerobes would be metabolically inactive. This metabolic adaptation contributes to the bacterium's ability to cause enteric fever (in sensu stricto strains) or gastroenteritis (in Java strains) .

What are the genomic differences in frdC between Salmonella paratyphi B sensu stricto and Paratyphi B Java strains?

Genomic comparisons between Salmonella paratyphi B sensu stricto (causing enteric fever) and Paratyphi B Java (causing gastroenteritis) have identified multiple loci that differ between these strains . While specific differences in the frdC gene were not explicitly detailed in the available research, comparative genomics of 38 enteric fever-associated strains from Chile revealed that each serovar could be distinguished based on core genome analysis .

The majority of the differentiating loci between sensu stricto and Java strains were annotated as hypothetical or phage-related, making them suboptimal vaccine candidates . This suggests that metabolic genes like frdC might be relatively conserved between these strains despite their different disease manifestations. Functional genomic studies would be needed to determine whether subtle variations in frdC sequence or expression contribute to the different pathogenic profiles of these strains.

What are the optimal conditions for expressing recombinant Salmonella paratyphi B frdC in E. coli?

Successful expression of recombinant Salmonella paratyphi B frdC in E. coli requires careful optimization due to its hydrophobic, membrane-associated nature. Based on protocols for similar proteins, the following conditions are recommended:

• Expression System: Use of a pET-based vector with an N-terminal His-tag for purification purposes

• Host Strain: E. coli strains designed for membrane protein expression, such as C41(DE3) or C43(DE3), which have adaptations to prevent toxicity from membrane protein overexpression

• Growth Conditions:

  • Lower temperature (16-25°C) after induction

  • Reduced IPTG concentration (0.1-0.5 mM)

  • Rich media supplemented with glucose to reduce basal expression

These conditions should be empirically tested and fine-tuned for maximum protein yield while maintaining proper folding and membrane integration .

What purification methods yield the highest purity of recombinant Salmonella paratyphi B frdC?

Purification of recombinant Salmonella paratyphi B frdC requires specialized approaches for membrane proteins. The following protocol is recommended based on similar proteins:

• Membrane Isolation:

  • Harvest cells and resuspend in buffer containing protease inhibitors

  • Disrupt cells using sonication or French press

  • Remove unbroken cells and debris by low-speed centrifugation

  • Isolate membranes by ultracentrifugation (100,000 × g for 1 hour)

• Solubilization:

  • Resuspend membrane fraction in buffer containing appropriate detergent

  • Commonly used detergents include n-dodecyl-β-D-maltoside (DDM), n-octyl-β-D-glucopyranoside (OG), or digitonin

  • Incubate with gentle agitation for 1-2 hours at 4°C

• Affinity Chromatography:

  • Apply solubilized sample to Ni-NTA resin equilibrated with detergent-containing buffer

  • Wash with increasing imidazole concentrations to remove non-specific binding

  • Elute His-tagged frdC with high imidazole concentration (250-500 mM)

• Size Exclusion Chromatography:

  • Further purify by gel filtration to remove aggregates and obtain homogeneous protein

  • Typical buffers contain Tris/PBS with 6% trehalose at pH 8.0

• Storage:

  • Store at -20°C/-80°C in buffer containing 50% glycerol or lyophilize in the presence of trehalose

  • Avoid repeated freeze-thaw cycles

How can I verify the functionality of recombinant Salmonella paratyphi B frdC after expression?

Verifying the functionality of recombinant frdC requires assessing its ability to form a functional complex with the other fumarate reductase subunits. Based on studies with E. coli fumarate reductase, the following approaches are recommended:

• Reconstitution Assays:

  • Combine purified frdC with the other subunits (A, B, and D) of the fumarate reductase complex

  • Assess whether the reconstituted complex regains quinone reductase activity, which is lost when the C and D subunits are removed

• Membrane Binding Assays:

  • Evaluate the ability of frdC to integrate into liposomes or bacterial membrane vesicles

  • Measure membrane association through flotation assays or membrane fractionation

• Electron Transfer Activity:

  • Measure quinone reduction activity using appropriate electron donors and acceptors

  • Monitor fumarate reduction in the presence of menaquinol or other quinones

• Spectroscopic Analysis:

  • Circular dichroism to confirm proper secondary structure

  • Fluorescence spectroscopy to assess tertiary folding

It's important to note that individual subunits like frdC may not show enzymatic activity on their own, as demonstrated by studies showing that neither peptide C nor peptide D alone permits quinone reduction . Therefore, functional verification typically requires reconstitution of the complete ABCD complex.

How does the amino acid sequence of frdC differ between Salmonella paratyphi B and other Salmonella serovars?

Comparison of frdC amino acid sequences between different Salmonella serovars reveals a high degree of conservation, particularly among closely related pathogenic strains. The sequences for Salmonella paratyphi B and Salmonella paratyphi A frdC are identical based on available data: