Recombinant Bordetella petrii Serine hydroxymethyltransferase (glyA)

What is Bordetella petrii and why is it significant in clinical research?

Bordetella petrii is an emerging pathogen first isolated from environmental sources but subsequently found in clinical samples associated with various conditions including jaw infections, ear bone infections, cystic fibrosis, and chronic pulmonary disease . B. petrii exhibits remarkable in vivo adaptability, undergoes massive genomic rearrangements, and can establish persistent infections in humans . Unlike other Bordetella species, B. petrii possesses numerous genomic islands known as integrative and conjugative elements that contribute to its genetic plasticity . The bacterium has been documented to persist for extended periods (>1 year) in patients with chronic obstructive pulmonary disease , making it an important subject for research on bacterial adaptation and persistence mechanisms.

What is Serine hydroxymethyltransferase (glyA) and what is its role in bacterial metabolism?

Serine hydroxymethyltransferase (encoded by the glyA gene, also known as shmT in some literature) is an essential enzyme involved in one-carbon metabolism, specifically catalyzing the reversible conversion of serine to glycine . This reaction is coupled to the conversion of tetrahydrofolate to 5,10-methylenetetrahydrofolate, linking glyA activity to folate metabolism . The enzyme plays a crucial role in providing one-carbon units for various biosynthetic pathways including purine and thymidylate synthesis, making it indispensable for bacterial growth and survival. In B. petrii, as in other bacteria, glyA functions as a housekeeping gene that is essential for basic cellular metabolism.

How does B. petrii glyA differ from homologous enzymes in other Bordetella species?

While the search results don't provide direct comparison data for glyA across Bordetella species, phylogenetic analysis of other genes (risA and ompA) shows that B. petrii sequences typically share approximately 86-87% nucleotide identity with corresponding genes in B. bronchiseptica, B. parapertussis, and B. pertussis . This suggests that glyA in B. petrii likely has distinct sequence characteristics compared to other Bordetella species. The glyA gene is sufficiently conserved to be used as one of seven housekeeping genes for multilocus sequence typing (MLST) in Bordetella parapertussis, alongside adk, fumC, tyrB, icd, pepA, and pgm , indicating its utility as a phylogenetic marker while maintaining species-specific variations.

What are the optimal expression systems for producing recombinant B. petrii glyA?

For recombinant expression of B. petrii glyA, E. coli-based expression systems typically provide the highest yield and simplicity. Based on research with similar enzymes, the following expression protocol is recommended:

• Clone the glyA gene from B. petrii genomic DNA using PCR with primers designed to include appropriate restriction sites

• Insert the gene into a vector containing an inducible promoter (such as pET series vectors with T7 promoter)

• Transform into E. coli BL21(DE3) or Rosetta(DE3) strains for expression

• Induce expression with 0.5-1.0 mM IPTG at mid-log phase (OD600 = 0.6-0.8)

• Grow at reduced temperature (16-25°C) overnight to enhance soluble protein production

• Include 50 μM pyridoxal phosphate (PLP) in the growth medium as glyA is a PLP-dependent enzyme

Alternative systems including yeast (P. pastoris) or insect cell expression may be considered if E. coli produces insoluble protein or if post-translational modifications are required.

What purification strategy should be employed for recombinant B. petrii glyA?

A multi-step purification strategy is recommended for obtaining high-purity recombinant B. petrii glyA:

• Initial capture: Affinity chromatography using His-tag (if added to the recombinant construct) with Ni-NTA resin. Elute with imidazole gradient (50-300 mM).

• Intermediate purification: Ion exchange chromatography (typically Q-Sepharose) based on the theoretical pI of B. petrii glyA.

• Polishing step: Size exclusion chromatography using Superdex 200 column in buffer containing 20 mM Tris-HCl pH 7.5, 150 mM NaCl, 1 mM DTT, and 50 μM PLP.

Throughout purification, include 50 μM PLP in all buffers to maintain enzyme stability and activity. Typical yield from 1L bacterial culture can range from 10-30 mg of purified protein depending on expression conditions and specific construct design.

What assay methods are available for measuring B. petrii glyA enzymatic activity?

Several complementary methods can be used to assess the enzymatic activity of B. petrii glyA:

• Spectrophotometric coupled assay: Measure the formation of 5,10-methylenetetrahydrofolate by coupling with NADPH consumption through methylenetetrahydrofolate reductase.

• Radiochemical assay: Use 14C-labeled serine and measure the formation of [14C]glycine.

• HPLC analysis: Quantify the conversion of serine to glycine directly by derivatization and HPLC separation.

The standard reaction buffer typically contains 50 mM HEPES pH 7.5, 0.5 mM tetrahydrofolate, 1-5 mM serine, and 50 μM PLP at 37°C. Kinetic parameters (Km, Vmax) should be determined under varying substrate concentrations to characterize the enzyme fully.

How does glyA contribute to B. petrii adaptation and persistence in the human host?

While direct evidence for the role of glyA in B. petrii adaptation is limited in the provided search results, parallels can be drawn from studies in other bacteria. In Staphylococcus aureus, serine hydroxymethyltransferase has been functionally identified as contributing to antimicrobial resistance, specifically against lysostaphin . Knockout studies showed that ΔshmT mutants became susceptible to lysostaphin, while complementation with the shmT gene restored resistance .

In the context of B. petrii, which is known for its remarkable adaptability and persistence in vivo, glyA could play similar roles in:

• Supporting metabolic adaptation under nutrient limitation in the host

• Contributing to antibiotic resistance mechanisms

• Maintaining one-carbon metabolism necessary for cell wall synthesis and repair

The sequential B. petrii isolates from patients exhibit differences in growth, antibiotic susceptibility, and antigenic profiles over time , suggesting dynamic adaptation processes that may involve metabolic enzymes like glyA.

Is there evidence that glyA sequence variants correlate with B. petrii virulence or persistence?

In other bacteria, sequence variations in housekeeping genes like glyA are used for multilocus sequence typing (MLST) , indicating that these genes can harbor sequence diversity even within a species. This diversity could potentially impact enzyme function or regulation in ways that affect bacterial fitness within the host.

To investigate correlations between glyA variants and virulence/persistence, researchers should:

• Sequence glyA from multiple clinical isolates with varied virulence profiles

• Perform site-directed mutagenesis to test specific variants

• Conduct infection studies in appropriate models to assess the impact of these variants

How can structural analysis of B. petrii glyA inform drug discovery efforts?

Structural characterization of B. petrii glyA through X-ray crystallography or cryo-EM could reveal unique structural features that differentiate it from human SHMT. This information would be valuable for structure-based drug design approaches targeting B. petrii infections.

Key steps in this research direction include:

• Expressing and purifying recombinant B. petrii glyA to >95% homogeneity

• Performing crystallization trials with various precipitants and additives

• Collecting X-ray diffraction data and solving the structure

• Comparing with human SHMT structures to identify unique binding pockets

• In silico docking studies with compound libraries to identify potential inhibitors

• Validating hits through biochemical and cellular assays

Given that B. petrii can establish persistent infections that are difficult to eradicate , developing specific inhibitors of glyA could provide a novel therapeutic approach, particularly if the enzyme proves essential for in vivo survival or contributes to antibiotic resistance mechanisms.

What are the technical challenges in developing conditional knockout systems for glyA in B. petrii?

Developing genetic manipulation systems for B. petrii presents several challenges:

• Genetic tools limitation: Unlike model organisms, B. petrii lacks well-established genetic tools. Researchers need to adapt systems from other Bordetella species or develop new ones.

• Essential gene targeting: Since glyA is likely essential (based on its role in one-carbon metabolism), conditional knockouts are necessary. Options include:

  • Inducible promoter systems (e.g., tetracycline-responsive)

  • Temperature-sensitive plasmids

  • CRISPR interference (CRISPRi) for partial knockdown

• Genome plasticity concerns: B. petrii undergoes massive genomic rearrangements in vitro , which could affect the stability of genetic modifications.

• Verification methods: Confirm knockdown/knockout through:

  • RT-qPCR for mRNA levels

  • Western blotting for protein levels

  • Metabolomic analysis for pathway disruption

  • Enzyme activity assays

Successful implementation would provide valuable tools for investigating glyA function in vivo, complementing biochemical studies with recombinant protein.

How do metabolic flux changes through the glyA pathway affect B. petrii antibiotic susceptibility profiles?

This advanced research question addresses potential links between one-carbon metabolism and antibiotic resistance. While direct evidence for B. petrii is not available in the search results, the finding that SHMT contributes to lysostaphin resistance in S. aureus suggests that metabolic enzymes can influence antimicrobial susceptibility.

A comprehensive investigation would include:

• Metabolic flux analysis: Use 13C-labeled serine to trace carbon flow through glyA-dependent pathways under antibiotic pressure.

• Modulation experiments:

  • Chemical inhibition of glyA with established inhibitors

  • Genetic manipulation (conditional knockdown)

  • Overexpression studies

• Susceptibility testing: Determine minimum inhibitory concentrations (MICs) for various antibiotic classes before and after glyA modulation.

• Mechanistic investigations:

  • Cell wall composition analysis

  • Membrane permeability assays

  • Global transcriptomic response

Expected outcomes might include altered susceptibility to cell wall-targeting antibiotics or changes in persister cell formation rates, given B. petrii's known ability to establish persistent infections .

How does B. petrii glyA compare functionally with glyA enzymes from other bacterial pathogens?

While specific comparative data for B. petrii glyA is not provided in the search results, a research approach to address this question would involve:

• Phylogenetic analysis: Construct phylogenetic trees based on glyA sequences from diverse bacterial species, with particular focus on:

  • Other Bordetella species (B. pertussis, B. parapertussis, B. bronchiseptica)

  • Respiratory pathogens with similar ecological niches

  • Environmental bacteria related to B. petrii's original isolation source

• Enzyme kinetics comparison: Express and purify recombinant glyA from multiple species and compare:

  • Substrate affinity (Km for serine and tetrahydrofolate)

  • Catalytic efficiency (kcat/Km)

  • pH and temperature optima

  • Inhibition profiles

• Complementation studies: Test whether glyA genes from other species can complement B. petrii glyA knockouts/knockdowns.

This comparative approach would reveal whether B. petrii glyA has evolved unique functional characteristics that might contribute to its environmental adaptability and clinical persistence.

What experimental evidence exists for horizontal gene transfer affecting glyA in B. petrii?

B. petrii possesses numerous genomic islands and integrative and conjugative elements that facilitate horizontal gene transfer and genome plasticity . While the search results don't provide direct evidence for horizontal gene transfer specifically affecting glyA, the general propensity for genomic rearrangements in B. petrii suggests this possibility.

To investigate this question:

• Comparative genomic analysis: Examine the genomic context of glyA in multiple B. petrii isolates to identify any evidence of:

  • Unusual GC content compared to the core genome

  • Proximity to mobile genetic elements

  • Flanking repeat sequences indicative of integration events

• Phylogenetic incongruence testing: Compare glyA-based phylogenies with those based on other housekeeping genes to identify potential horizontal transfer events.

• Functional characterization: Express and characterize any variant glyA genes identified to determine if they confer novel phenotypes.

This research would contribute to understanding the evolution of B. petrii and potential acquisition of adaptive traits through horizontal gene transfer.

Table 1: Comparison of Key Experimental Parameters for Recombinant B. petrii glyA Expression Systems

Table 2: Methodological Comparison of glyA Activity Assays

What are the most promising approaches for targeting B. petrii glyA in persistent infections?

Given B. petrii's ability to persist in chronic infections , developing targeted approaches against glyA could offer new therapeutic strategies. The most promising approaches include:

• Structure-based inhibitor design: Using the resolved crystal structure of B. petrii glyA to design specific inhibitors that do not affect human SHMT.

• Combination therapy strategies: Testing glyA inhibitors in combination with currently used antibiotics to potentially overcome persistence mechanisms.

• Targeted delivery systems: Developing nanoparticle or liposome formulations to deliver glyA inhibitors specifically to sites of B. petrii infection, particularly in pulmonary settings where the pathogen is commonly found.

• Anti-virulence approach: If glyA contributes to virulence rather than basic survival, targeting it might reduce pathogenicity without creating strong selection pressure for resistance.

• Immunological approaches: Developing antibodies or vaccines targeting surface-exposed epitopes of glyA if the protein has any external exposure.

These approaches would need to be validated in appropriate infection models before clinical translation.

How might genome-wide CRISPR screens identify genetic interactions with glyA in B. petrii?

CRISPR-based functional genomics could reveal genetic interactions with glyA, providing insights into its role in metabolism and pathogenesis. A comprehensive approach would include:

• Development of a B. petrii-adapted CRISPR system:

  • Design sgRNA library targeting all B. petrii genes

  • Optimize Cas9 or dCas9 expression in B. petrii

  • Establish delivery methods (electroporation or conjugation)

• Genetic interaction screening approaches:

  • CRISPRi knockdown of glyA combined with genome-wide CRISPR screening

  • Chemical inhibition of glyA combined with genome-wide CRISPR screening

  • Synthetic lethality screens under various stress conditions

• Validation studies:

  • Targeted gene knockouts of identified interactors

  • Biochemical assays to confirm pathway connections

  • Metabolomic analysis to map metabolic rerouting

This systems biology approach would map the genetic network centered on glyA, potentially identifying new therapeutic targets or resistance mechanisms in B. petrii infections.