Phage-encoded dihydrofolate reductase (DHFR) that catalyzes the NADPH-dependent reduction of dihydrofolate (DHF) to tetrahydrofolate (THF). DfrP is an auxiliary metabolic gene (AMG) encoded by Bacillus phage phiNIT1, a Bastilleviridae phage. The phage carries both dfrP and thyA (thymidylate synthase) genes to form a self-sufficient thymidine synthesis cycle during infection. This ensures robust nucleotide production for viral DNA replication. DfrP belongs to the trimethoprim-resistant DfrA family of DHFRs, which can confer antibiotic resistance to the infected host cell.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0004146
dihydrofolate reductase activity
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: Core enzymatic function. DfrP contains a conserved DHFR domain (IPR001796) and catalyzes the NADPH-dependent reduction of DHF to THF (EC 1.5.1.3). Well-supported by sequence homology and domain architecture.
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
DfrP is a dihydrofolate reductase...catalyzes the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF) using NADPH as a cofactor
|
|
GO:0006730
one-carbon metabolic process
|
IEA
GO_REF:0000043 |
ACCEPT |
Summary: DHFR activity is central to one-carbon metabolism by regenerating THF, which carries one-carbon units for nucleotide and amino acid biosynthesis. Appropriate annotation.
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
the primary function of DfrP is to sustain the one-carbon folate cycle by regenerating THF
|
|
GO:0016491
oxidoreductase activity
|
IEA
GO_REF:0000043 |
REMOVE |
Summary: True but too general. The more specific term GO:0004146 (dihydrofolate reductase activity) is already annotated and should be preferred.
Reason: Redundant with GO:0004146 which provides the specific oxidoreductase activity. The general oxidoreductase term adds no additional information.
|
|
GO:0046452
dihydrofolate metabolic process
|
IEA
GO_REF:0000118 |
ACCEPT |
Summary: Appropriate biological process annotation. DHFR directly metabolizes dihydrofolate by reducing it to tetrahydrofolate.
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
DfrP catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate
|
|
GO:0046654
tetrahydrofolate biosynthetic process
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: Core biological process. DfrP directly catalyzes the final step in THF biosynthesis - the reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. This is not merely modulation of a host process; the phage-encoded enzyme directly performs the biosynthetic reaction. THF is essential for thymidylate synthesis (DNA precursor), purine synthesis, and amino acid biosynthesis. The phage carries this gene as an auxiliary metabolic gene to ensure robust THF production during viral DNA replication, forming a complete folate cycle together with its thymidylate synthase (thyA) gene.
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
This reaction is essential for regenerating THF, the active form of folate, which is required for one-carbon transfer reactions in the cell
file:9CAUD/dfrP/dfrP-deep-research-openai.md
the reaction catalyzed by DfrP is needed to produce THF for the de novo synthesis of glycine and purines, and for the continuous production of DNA precursors like thymidine (via the thymidylate synthase pathway)
file:9CAUD/dfrP/dfrP-deep-research-openai.md
By providing its own DHFR and TS, phiNIT1 ensures that the infected host cell can efficiently produce dTMP and other nucleotides, even if host pathways are downregulated or if folate pools become limiting
file:9CAUD/dfrP/dfrP-deep-research-openai.md
The combined presence of TS and DHFR in phiNIT1 means the phage can form a complete folate cycle independent of host regulation, securing a robust supply of DNA building blocks for phage genome synthesis
|
|
GO:0046655
folic acid metabolic process
|
IEA
GO_REF:0000118 |
KEEP AS NON CORE |
Summary: Appropriate broader process annotation. DHFR is a key enzyme in folate metabolism, converting oxidized folate (DHF) back to the reduced active form (THF).
Reason: While correct, this is a broader parental process. The more specific GO:0046452 (dihydrofolate metabolic process) and GO:0046654 (tetrahydrofolate biosynthetic process) better capture the direct function.
|
|
GO:0050661
NADP binding
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: DHFR uses NADPH as the electron donor/cofactor. The domain contains conserved NADP-binding motifs. Appropriate MF annotation.
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
It binds the cofactor NADPH (evidenced by conserved NADP-binding motifs in DHFR enzymes)
|
|
GO:0030430
host cell cytoplasm
|
ISS
GO_REF:0000120 |
NEW |
Summary: DfrP is predicted to localize to the host cell cytoplasm where folate metabolism occurs. As a viral enzyme expressed during phage infection, GO guidelines specify using 'host cell cytoplasm' (GO:0030430) rather than generic 'cytoplasm' for viral proteins. Cytoplasmic localization is expected by analogy to other DHFR enzymes (no signal peptide or transmembrane domains).
Supporting Evidence:
file:9CAUD/dfrP/dfrP-deep-research-openai.md
the DfrP protein is expressed in the bacterial host's cytoplasm, which is the site of folate metabolism and nucleotide synthesis
|
The dfrP gene of Bacillus phage phiNIT1 encodes a dihydrofolate reductase (DHFR) enzyme (UniProt D0VXF2). This identification is supported by sequence annotations showing a conserved DHFR domain (InterPro IPR001796) and the enzyme commission number EC 1.5.1.3, consistent with known DHFR enzymes. Importantly, phiNIT1 is a large Bacillus subtilis-infecting bacteriophage classified in the Bastillevirinae subfamily (genus Nitunavirus) (pmc.ncbi.nlm.nih.gov). Members of this phage group uniquely carry their own thymidylate synthase (thyA) and DHFR genes (pmc.ncbi.nlm.nih.gov). The dfrP gene (also reported as โorf168โ in the genome) is thus unambiguously identified as a phage-derived DHFR. There is no evidence of alternative gene meanings in this context, so all literature reviewed refers to the phiNIT1 phage DHFR enzyme described here.
DfrP is a dihydrofolate reductase, an enzyme that catalyzes the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF) using NADPH as a cofactor (go.drugbank.com) (pmc.ncbi.nlm.nih.gov). This reaction is essential for regenerating THF, the active form of folate, which is required for one-carbon transfer reactions in the cell. DHFRโs activity is a linchpin in folate metabolism, supplying critical cofactors for nucleotide and amino acid biosynthesis (go.drugbank.com). In particular, the reaction catalyzed by DfrP is needed to produce THF for the de novo synthesis of glycine and purines, and for the continuous production of DNA precursors like thymidine (via the thymidylate synthase pathway) (go.drugbank.com) (pmc.ncbi.nlm.nih.gov). By converting DHF back to THF, DfrP maintains the folate pool used to methylate deoxyuridine monophosphate (dUMP) into thymidine monophosphate (dTMP) and to support purine synthesis (pmc.ncbi.nlm.nih.gov). This enzymeโs function is highly conserved and is analogous to bacterial FolA (the canonical DHFR in bacteria).
Substrate specificity: DfrP is expected to specifically reduce 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. It binds the cofactor NADPH (evidenced by conserved NADP-binding motifs in DHFR enzymes) (go.drugbank.com) and likely follows classic Michaelis-Menten kinetics observed for DHFR, as demonstrated by enzyme assays of phage-encoded DfrA homologs showing NADPH-dependent DHF turnover (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). There is no indication that this enzyme has alternate substrates; its activity is specific to folate derivatives, as with other DHFR family enzymes. The product THF is a central cofactor that carries single-carbon units in metabolic reactions. Thus, the primary function of DfrP is to sustain the one-carbon folate cycle by regenerating THF, enabling continuous nucleotide biosynthesis in the host cell.
By virtue of this enzymatic activity, DfrP plays a direct role in the folate-mediated one-carbon metabolism pathway. This pathway impacts several downstream processes:
- Thymidylate (dTMP) synthesis: DfrP works in tandem with thymidylate synthase (TS) to ensure a supply of dTMP for DNA replication (pmc.ncbi.nlm.nih.gov). TS uses 5,10-methylene-THF to convert dUMP to dTMP, producing DHF as a byproduct; DfrP then reduces DHF back to THF, allowing the cycle to continue. (pmc.ncbi.nlm.nih.gov) This coupling is crucial for DNA synthesis, especially under rapid replication conditions.
- Purine and glycine biosynthesis: THF is also required for purine base formation and for the interconversion of serine and glycine. By maintaining THF levels, DfrP indirectly supports de novo purine synthesis and glycine production (go.drugbank.com). These processes are part of general nucleotide and amino acid biosynthetic pathways.
- One-carbon metabolic process: More broadly, DfrP ensures the supply of one-carbon units in various methylation and biosynthetic reactions (e.g. methionine synthesis). Gene ontology annotations link DHFR activity to the one-carbon metabolic process and tetrahydrofolate biosynthetic process (go.drugbank.com). In summary, DfrP is a key enzyme that keeps folate coenzymes in their reduced, active state, thereby sustaining multiple essential biosynthetic pathways in the cell.
Given these roles, it is well established that dihydrofolate reductase is indispensable for cell growth. In bacteria and other organisms, inhibition or loss of DHFR function causes depletion of thymidine and certain amino acids, leading to stalled DNA replication and cell death (pmc.ncbi.nlm.nih.gov). This is exactly why DHFR is the target of antimicrobial and chemotherapeutic agents (e.g. trimethoprim in bacteria, methotrexate in humans). In the context of phiNIT1 phage infection, the presence of a phage-encoded DHFR suggests the phage is augmenting these vital pathways during infection.
Bacteriophage phiNIT1 encodes DfrP as an โauxiliary metabolic geneโ (AMG) to boost nucleotide biosynthesis during infection. Many large bacteriophages carry auxiliary genes to redirect or enhance the hostโs metabolism in favor of phage reproduction (www.frontiersin.org). In the case of phiNIT1 (a member of the Bastillevirinae phages), comparative genomics revealed that dfrP (DHFR) and a thyA (thymidylate synthase) gene are signature features of this phage lineage (pmc.ncbi.nlm.nih.gov). By providing its own DHFR and TS, phiNIT1 ensures that the infected host cell can efficiently produce dTMP and other nucleotides, even if host pathways are downregulated or if folate pools become limiting. This strategy likely overcomes any metabolic bottleneck during the burst of viral DNA replication. Indeed, viruses use such AMGs to alter host rate-limiting processes (like dTMP synthesis) and thereby promote successful virus proliferation (www.frontiersin.org) (microbiomejournal.biomedcentral.com). The combined presence of TS and DHFR in phiNIT1 means the phage can form a complete folate cycle independent of host regulation, securing a robust supply of DNA building blocks for phage genome synthesis.
Experimental evidence from related phages supports the idea that phage-encoded DHFR confers a fitness advantage. A recent large-scale survey (2025) identified ~1,953 dihydrofolate reductase homologs (dfrA-like genes) across ~1,944 lytic phage genomes, spanning diverse bacterial hosts (pmc.ncbi.nlm.nih.gov). Notably, these phage DHFR genes (including phiNIT1โs dfrP) tend to be trimethoprim-resistant variants. They are distinct from typical bacterial FolA, falling into the DfrA family (length ~150โ250 amino acids) which retain DHFR activity but are insensitive to the antibiotic trimethoprim (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Wang et al. (Science Advances, 2025) showed that expressing a phage-borne dfr gene in E. coli can indeed confer high-level resistance to trimethoprim (pmc.ncbi.nlm.nih.gov). Moreover, under antibiotic pressure, phages carrying dfr genes had enhanced replication: phage production was higher in the presence of trimethoprim when the phage encoded its own DHFR, and the infected bacteria also grew better than they otherwise would (pmc.ncbi.nlm.nih.gov). This finding suggests that phiNIT1โs DfrP could protect and bolster the hostโs folate cycle if the host is exposed to antifolate stress (e.g. antibiotics or folate starvation), thereby indirectly benefiting phage propagation.
Importantly, even in the absence of antibiotics, phage dfr genes likely serve a mutualistic function. By ensuring the host has ample THF and nucleotides, the phage keeps the host metabolic state optimal for phage DNA replication. Expert analysis has described this as an โevolutionary mutualismโ between lytic phages and their hosts (pmc.ncbi.nlm.nih.gov). The phage provides a metabolic function that supports host survival (at least until phage assembly is complete), and in return the phage achieves maximal replication. Bacillus phage phiNIT1 fits this paradigm: it carries metabolic genes (DfrP and possibly others like ribonucleotide reductases or DNA metabolism enzymes) often found in lytic phages to overcome host limitations (pmc.ncbi.nlm.nih.gov). In fact, surveys indicate dfr (DHFR) and thyA (TS) genes are significantly enriched in lytic phages โ occurring about 10โ13 times more often in lytic phages than in temperate phages (pmc.ncbi.nlm.nih.gov). This enrichment underscores their adaptive value for viruses that rely on rapid, lytic replication cycles.
From a genomic standpoint, phiNIT1โs dfrP gene is located adjacent to a transposase, hinting at its acquisition via horizontal gene transfer. A comparative genome analysis between phiNIT1 and a related phage (Bacillus phage BSP9) showed that the transposase gene next to dfrP in phiNIT1 has clear signs of recent horizontal transfer, whereas the dfrP gene itself appears not to have spread to bacterial genomes (pmc.ncbi.nlm.nih.gov). In other words, phiNIT1 seems to have picked up the dfrP gene in its genome, but that gene has not (at least in detectable instances) been laterally transferred into host chromosomes or plasmids in nature (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is an interesting nuance: the phage retains the DHFR gene for its own lifecycle benefit without necessarily donating it to hosts long-term. It suggests that phage-borne dfr genes can persist as part of the viral genome repertoire (possibly acquired from some bacterial source in the past) primarily to serve phage fitness, rather than as mobile resistance elements in bacterial populations. Indeed, comprehensive analyses found that among ~1,953 phage dfr genes, only a single case was detected where a phage dfr had a near-identical counterpart on a bacterial plasmid (pmc.ncbi.nlm.nih.gov), indicating extremely limited dissemination.
During phiNIT1 infection, the DfrP protein is expressed in the bacterial hostโs cytoplasm, which is the site of folate metabolism and nucleotide synthesis. Like canonical DHFR enzymes, DfrP lacks any signal peptide or membrane-spanning region, so it remains in the cytosol to interact with soluble folate substrates and NADPH. While specific localization studies on phiNIT1 DfrP have not been reported, by analogy to E. coli DHFR (a cytosolic monomeric enzyme ~18 kDa) (go.drugbank.com), we expect DfrP to function in the cytoplasm where it can readily access DHF produced during thymidylate synthesis cycles. In essence, DfrP supplements the host cellโs own DHFR activity. If the host DHFR (FolA) is inhibited or working at capacity, the phage-encoded DfrP provides additional reductive capacity in the same cellular compartment. This ensures that THF regeneration keeps pace with the heightened demand for nucleotide precursors during phage DNA replication.
The timing of expression for dfrP would be during the phage lytic cycleโs replication phase. Auxiliary metabolic genes in phages are often expressed early or middle in infection to prepare the host metabolism for phage DNA synthesis (www.frontiersin.org). Although phiNIT1โs transcriptional program isnโt fully characterized in literature, it is reasonable that dfrP is expressed prior to or during viral DNA replication, ensuring folate cycle enzymes (TS and DHFR) are active when viral genome amplification begins. There is no secretion of DfrP outside the cell; its role is strictly intracellular. After the phage life cycle completes and the host cell lyses, DfrP would be released and degraded in the environment unless a new host is immediately infected.
Dihydrofolate reductase is a well-known drug target, and the presence of a DHFR gene in phage phiNIT1 has both theoretical and practical implications. In medicine, trimethoprim (often used in combination as co-trimoxazole) inhibits bacterial DHFR to block folate recycling, thereby killing bacteria by halting DNA synthesis (pmc.ncbi.nlm.nih.gov). Phage phiNIT1โs DfrP, however, is predicted to be a trimethoprim-resistant DHFR variant (a member of the DfrA family) (pmc.ncbi.nlm.nih.gov). This means if Bacillus bacteria (or other susceptible species) are infected by phiNIT1, they could transiently exhibit resistance to trimethoprim due to the phage-produced enzyme. In experimental models, phage-borne dfr genes have been shown to raise the trimethoprim minimum inhibitory concentration (MIC) of infected bacteria dramatically, rescuing bacterial growth even at high drug concentrations (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). While phiNIT1 specifically has not been tested in this way, its close relativesโ Dfr enzymes conferred resistance when expressed in E. coli (pmc.ncbi.nlm.nih.gov).
From a phage therapy and antibiotic resistance perspective, this raises caution. Phages like phiNIT1 that carry antibiotic resistance functions are generally not ideal for therapeutic use, since they might inadvertently protect pathogenic bacteria during treatment. However, it is reassuring that phiNIT1โs dfrP has not been found to permanently transfer into bacterial genomes at any appreciable frequency (pmc.ncbi.nlm.nih.gov). This indicates the risk of spreading trimethoprim resistance via this phage is low, as the gene seems to stay within the phage lineage. Still, regulators and researchers note the importance of screening therapeutic phages for genes like dfrP or other resistance factors (pmc.ncbi.nlm.nih.gov). In the environment, the presence of phage-encoded DHFR contributes to the reservoir of resistance in a transient sense โ it can make infections harder to treat with antifolates while the phage infection persists (pmc.ncbi.nlm.nih.gov). Conversely, from a biotechnology angle, phage DHFR genes (such as dfrP) have been used as selectable markers in research. For example, trimethoprim-resistant DHFR genes serve as selection genes in cloning vectors for Gram-positive bacteria, allowing growth in the presence of trimethoprim. The specific phiNIT1 DfrP could potentially be exploited similarly, given its likely strong resistance profile, although common lab usage currently relies on well-characterized variants from staphylococcal or E. coli sources.
Itโs also worth noting that dfrP and its associated TS might be interesting targets for novel antivirals or antibacterials. Since phage phiNIT1 relies on this enzyme for optimal replication, molecules that selectively inhibit the phage-encoded DHFR (without affecting the hostโs enzyme) could, in theory, suppress phage propagation. This concept is analogous to antivirals targeting virus-specific metabolic enzymes. No such inhibitor is known yet for phage-specific DHFR, but structurally DfrP is highly similar to bacterial DHFR, making selectivity a challenge.
Recent research underscores the significance of phage-encoded DHFR in microbial ecology and evolution. In a comprehensive 2025 study (Wang et al., Sci. Advances, Sep 2025), scientists reported that about 18.6% of analyzed lytic phage genomes carry putative antibiotic resistance genes, with dihydrofolate reductases (dfr) being one of the most prevalent types (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This challenges the earlier notion that lytic phages rarely contribute to the antibiotic resistome. The authors describe a form of positive selection for phage-borne dfrA genes, driven not by a need to spread resistance, but by the benefit these genes provide to phage fitness (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). They coined this a mutualism: the phage helps the host withstand folate-targeting antibiotics (or folate shortages), and in return the host can produce more phages. This study provided experimental data, showing that when E. coli was infected with a dfr-carrying phage, the bacteria-phage system thrived in otherwise inhibitory trimethoprim concentrations (pmc.ncbi.nlm.nih.gov). The research also performed phylogenetic analyses of hundreds of phage DHFR sequences, finding that phage DHFRs form distinct clades separate from chromosomal DHFRs, and that Bacillus phage DHFRs (like DfrP) cluster together, suggesting a common evolutionary origin or host-specific adaptation (pmc.ncbi.nlm.nih.gov). Experts commenting on these findings note that phage metabolic genes such as dfrP blur the line between purely โviralโ and โbacterialโ functions, indicating that phages actively shape metabolic capacities during infection (Sun et al., 2023, commentary in Curr. Biol.).
Another line of investigation is the origin of these phage folate genes. A 2015 comparative genomic analysis by Asare et al. proposed that the Bastille-like phages (which include phiNIT1) acquired thyA and dfr genes to form a self-sufficient thymidine synthesis cycle (pmc.ncbi.nlm.nih.gov). These genes were absent in many other phage groups, making them useful โmolecular markersโ for identifying new Bastilleviruses (pmc.ncbi.nlm.nih.gov). The authors also pointed out that all such phages carried a beta-lactamase gene and a SpoIIIE-like DNA translocase (pmc.ncbi.nlm.nih.gov). The presence of a beta-lactamase (antibiotic resistance to ฮฒ-lactams) alongside DHFR (resistance to antifolates) in these phages suggests that they evolved in environments where inhibiting host cell wall synthesis or folate metabolism could threaten phage replication. In other words, these viruses appear to be equipped to counter multiple antibacterial strategies of the host or human intervention. Expert virologists have therefore suggested that phages with auxiliary genes like dfrP can be seen as โmetabolic engineersโ, temporarily rewiring host metabolism to favor virus production (Klumpp and Loessner, 2015) (pmc.ncbi.nlm.nih.gov). This perspective shifts our understanding of phages from purely lytic agents to participants in metabolic regulation during infection.
In conclusion, dfrP (phiNIT1) encodes a functional dihydrofolate reductase that enhances folate and nucleotide metabolism during phage infection. It plays a specific, crucial role in enabling the phage to hijack the hostโs biosynthetic machinery for viral DNA production. Current research portrays this enzyme as part of a broader strategy by which lytic phages improve host fitness (temporarily) to maximize their own replication (pmc.ncbi.nlm.nih.gov). This gene exemplifies how phages contribute to metabolic and antibiotic resistance dynamics in microbial ecosystems, while also highlighting an impressive co-evolution of phage and host biochemistry. All these insights are grounded in up-to-date experimental evidence and comparative genomics (2015โ2025), painting a detailed picture of DfrPโs function and significance in both virology and microbiology (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
References: The information above is drawn from recent primary research and authoritative reviews. Key sources include Wang et al. 2025 (Science Advances) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), which provided experimental analysis of phage DHFR genes; Asare et al. 2015 (BMC Genomics) (pmc.ncbi.nlm.nih.gov), which identified DHFR as a signature gene of phiNIT1-like phages; and drug metabolism databases (DrugBank) for fundamental DHFR function (go.drugbank.com). These and other cited works offer a current and evidence-backed understanding of the dfrP gene product. Each citation above includes a URL and publication details for further reading.
Exported on March 22, 2026 at 02:34 AM
Organism: 9CAUD
Sequence:
MHVALIAAMGLNLEIGADNRLLWDIKEDMNWFRQKTKNKVVVMGRKTYESIGKPLKGRVNVVLTHNKEYDPHPDVLVRHNLEDIFFEFRNELEIMVIGGETIYNQFFPYANRIYLTQVEKTYEEADAFFPEFDCSIWNRYFYQDGSEDIGIKYNFSVYKKRLKLEGED
I begin with the InterPro architecture. Three concordant signatures cover essentially the full polypeptide: IPR012259 (Dihydrofolate reductase family, residues 1โ161) establishes enzyme identity and evolutionary family membership; IPR001796 (Dihydrofolate reductase domain, residues 2โ160) delineates the catalytic core that binds dihydrofolate and NADPH; and IPR024072 (Dihydrofolate reductase-like domain superfamily, residues 1โ167) situates the fold within the broader DHFR-like structural superfamily. The near-complete coverage and tight agreement among family, domain, and superfamily entries indicate a soluble oxidoreductase that uses a conserved DHFR fold to catalyze hydride transfer from NADPH to dihydrofolate.
This architecture dictates molecular function. The canonical DHFR fold binds dihydrofolate and NADPH within a Rossmann-like active site and promotes two-electron reduction to tetrahydrofolate. Such chemistry defines an oxidoreductase that transfers hydride equivalents to a pterin substrate; thus the appropriate molecular-function class is oxidoreductase activity (GO:0016491). The absence of transmembrane features and the fully soluble catalytic domain reinforce that this is a cytosolic enzyme.
From function to process, tetrahydrofolate produced by DHFR feeds the one-carbon metabolic network that generates methylene-, methenyl-, and methyl-tetrahydrofolate pools. These pools drive thymidylate synthesis and purine biosynthesis and sustain serine/glycine interconversions. Therefore, sustaining tetrahydrofolate availability places the enzyme squarely in one-carbon metabolic process (GO:0006730), with direct consequences for nucleotide and amino acid metabolism.
Cellular location follows from solubility and pathway context. The DHFR domain lacks signal peptides or membrane anchors and operates with soluble cofactors, which positions the enzyme in the cytoplasm. This aligns with a cytoplasmic residency where folate-mediated biosynthesis and nucleotide assembly commonly occur.
Mechanistically, the enzyme likely forms transient assemblies that streamline folate flux. By producing tetrahydrofolate, it supplies and may physically couple with downstream folate-dependent enzymes such as thymidylate synthase and serine hydroxymethyltransferases, as well as with dihydropteroate synthase and upstream folate pathway components. Such proximity would channel reduced folate intermediates efficiently to DNA and RNA precursor pathways in the cytoplasm of 9CAUD cells.
A cytosolic oxidoreductase that reduces dihydrofolate to tetrahydrofolate using NADPH, thereby sustaining cellular pools of reduced folate essential for oneโcarbon transfer reactions. By maintaining tetrahydrofolate supply, it supports thymidylate and purine biosynthesis and serine/glycine interconversions within the cytoplasm of 9CAUD cells. The enzyme operates through a conserved DHFR fold that binds dihydrofolate and NADPH to catalyze hydride transfer and channel reduced folate intermediates toward nucleotide and amino acid metabolism.
Involved in one-carbon metabolism and in the de novo pathways of purine, thymidylate and serine biosynthesis.
IPR012259, family) โ residues 1-161IPR024072, homologous_superfamily) โ residues 1-167IPR001796, domain) โ residues 2-160Molecular Function: molecular_function (GO:0003674), catalytic activity (GO:0003824), oxidoreductase activity (GO:0016491), oxidoreductase activity, acting on the CH-NH group of donors (GO:0016645), oxidoreductase activity, acting on the CH-NH group of donors, NAD or NADP as acceptor (GO:0016646), dihydrofolate reductase activity (GO:0004146)
Biological Process: biological_process (GO:0008150), response to stimulus (GO:0050896), response to chemical (GO:0042221), response to xenobiotic stimulus (GO:0009410)
Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), cytoplasm (GO:0005737), cytoplasm (GO:0005622)
Generated by BioReason
Source: dfrP-bioreason-rl-predictions.md
The BioReason summary states:
A cytosolic oxidoreductase that reduces dihydrofolate to tetrahydrofolate using NADPH, thereby sustaining cellular pools of reduced folate essential for one-carbon transfer reactions. By maintaining tetrahydrofolate supply, it supports thymidylate and purine biosynthesis and serine/glycine interconversions within the cytoplasm of 9CAUD cells. The enzyme operates through a conserved DHFR fold that binds dihydrofolate and NADPH to catalyze hydride transfer and channel reduced folate intermediates toward nucleotide and amino acid metabolism.
The core enzymatic function is correctly identified: DHFR activity reducing dihydrofolate to tetrahydrofolate. The curated review confirms dihydrofolate reductase activity (GO:0004146) as the core molecular function, with tetrahydrofolate biosynthetic process (GO:0046654) and one-carbon metabolic process (GO:0006730) as core biological processes.
Key omission -- phage biology context: The BioReason summary treats dfrP as a generic DHFR "within the cytoplasm of 9CAUD cells," apparently not recognizing that 9CAUD is a viral taxonomy and dfrP is a phage-encoded auxiliary metabolic gene (AMG). The curated review explicitly describes dfrP as "an auxiliary metabolic gene (AMG) encoded by Bacillus phage phiNIT1, a Bastilleviridae phage." Key aspects missed:
Phage context: dfrP is expressed in a bacterial host during phage infection. The correct localization is "host cell cytoplasm" (GO:0030430), not generic cytoplasm.
thyA pairing: The phage carries both dfrP and thyA (thymidylate synthase) to form a "self-sufficient thymidine synthesis cycle during infection." This functional pairing is central to understanding dfrP's biological role.
Trimethoprim resistance: dfrP belongs to the "trimethoprim-resistant DfrA family of DHFRs, which can confer antibiotic resistance to the infected host cell." Not mentioned.
Purpose for phage fitness: The gene ensures "robust nucleotide production for viral DNA replication" even when host pathways are downregulated. The summary does not discuss this phage-centric rationale.
The biochemical description of DHFR function is accurate and well-explained.
Comparison with interpro2go:
The ai-review.yaml does not have GO_REF:0000002 annotations for dfrP specifically (the existing annotations are from GO_REF:0000120 and GO_REF:0000043). BioReason's reasoning from the DHFR domain architecture (IPR001796, IPR012259) produces results equivalent to interpro2go: DHFR activity, oxidoreductase activity, and one-carbon metabolism. The summary adds appropriate biochemical context (NADPH as electron donor, thymidylate and purine synthesis) but cannot detect the phage biology context from domain architecture alone.
The trace correctly identifies the DHFR domain family, the Rossmann-like fold, and the catalytic mechanism. The hypothesized coupling with thymidylate synthase is present ("may physically couple with downstream folate-dependent enzymes such as thymidylate synthase"), though this is framed as general cell biology rather than phage-specific gene pairing.
id: D0VXF2
gene_symbol: dfrP
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:207656
label: Bacillus phage phiNIT1
description: >-
Phage-encoded dihydrofolate reductase (DHFR) that catalyzes the NADPH-dependent reduction
of dihydrofolate (DHF) to tetrahydrofolate (THF). DfrP is an auxiliary metabolic gene (AMG)
encoded by Bacillus phage phiNIT1, a Bastilleviridae phage. The phage carries both dfrP and
thyA (thymidylate synthase) genes to form a self-sufficient thymidine synthesis cycle during
infection. This ensures robust nucleotide production for viral DNA replication. DfrP belongs
to the trimethoprim-resistant DfrA family of DHFRs, which can confer antibiotic resistance
to the infected host cell.
existing_annotations:
- term:
id: GO:0004146
label: dihydrofolate reductase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: >-
Core enzymatic function. DfrP contains a conserved DHFR domain (IPR001796) and
catalyzes the NADPH-dependent reduction of DHF to THF (EC 1.5.1.3). Well-supported
by sequence homology and domain architecture.
action: ACCEPT
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
DfrP is a dihydrofolate reductase...catalyzes the reduction of dihydrofolic acid (DHF)
to tetrahydrofolic acid (THF) using NADPH as a cofactor
- term:
id: GO:0006730
label: one-carbon metabolic process
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
DHFR activity is central to one-carbon metabolism by regenerating THF, which carries
one-carbon units for nucleotide and amino acid biosynthesis. Appropriate annotation.
action: ACCEPT
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
the primary function of DfrP is to sustain the one-carbon folate cycle by regenerating THF
- term:
id: GO:0016491
label: oxidoreductase activity
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
True but too general. The more specific term GO:0004146 (dihydrofolate reductase activity)
is already annotated and should be preferred.
action: REMOVE
reason: >-
Redundant with GO:0004146 which provides the specific oxidoreductase activity.
The general oxidoreductase term adds no additional information.
- term:
id: GO:0046452
label: dihydrofolate metabolic process
evidence_type: IEA
original_reference_id: GO_REF:0000118
review:
summary: >-
Appropriate biological process annotation. DHFR directly metabolizes dihydrofolate
by reducing it to tetrahydrofolate.
action: ACCEPT
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
DfrP catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate
- term:
id: GO:0046654
label: tetrahydrofolate biosynthetic process
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: >-
Core biological process. DfrP directly catalyzes the final step in THF biosynthesis -
the reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. This is not merely
modulation of a host process; the phage-encoded enzyme directly performs the
biosynthetic reaction. THF is essential for thymidylate synthesis (DNA precursor),
purine synthesis, and amino acid biosynthesis. The phage carries this gene as an
auxiliary metabolic gene to ensure robust THF production during viral DNA replication,
forming a complete folate cycle together with its thymidylate synthase (thyA) gene.
action: ACCEPT
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
This reaction is essential for regenerating THF, the active form of folate, which
is required for one-carbon transfer reactions in the cell
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
the reaction catalyzed by DfrP is needed to produce THF for the de novo synthesis
of glycine and purines, and for the continuous production of DNA precursors like
thymidine (via the thymidylate synthase pathway)
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
By providing its own DHFR and TS, phiNIT1 ensures that the infected host cell can
efficiently produce dTMP and other nucleotides, even if host pathways are
downregulated or if folate pools become limiting
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
The combined presence of TS and DHFR in phiNIT1 means the phage can form a
complete folate cycle independent of host regulation, securing a robust supply
of DNA building blocks for phage genome synthesis
- term:
id: GO:0046655
label: folic acid metabolic process
evidence_type: IEA
original_reference_id: GO_REF:0000118
review:
summary: >-
Appropriate broader process annotation. DHFR is a key enzyme in folate metabolism,
converting oxidized folate (DHF) back to the reduced active form (THF).
action: KEEP_AS_NON_CORE
reason: >-
While correct, this is a broader parental process. The more specific GO:0046452
(dihydrofolate metabolic process) and GO:0046654 (tetrahydrofolate biosynthetic process)
better capture the direct function.
- term:
id: GO:0050661
label: NADP binding
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: >-
DHFR uses NADPH as the electron donor/cofactor. The domain contains conserved
NADP-binding motifs. Appropriate MF annotation.
action: ACCEPT
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
It binds the cofactor NADPH (evidenced by conserved NADP-binding motifs in DHFR enzymes)
- term:
id: GO:0030430
label: host cell cytoplasm
evidence_type: ISS
original_reference_id: GO_REF:0000120
review:
summary: >-
DfrP is predicted to localize to the host cell cytoplasm where folate metabolism occurs.
As a viral enzyme expressed during phage infection, GO guidelines specify using
'host cell cytoplasm' (GO:0030430) rather than generic 'cytoplasm' for viral proteins.
Cytoplasmic localization is expected by analogy to other DHFR enzymes (no signal
peptide or transmembrane domains).
action: NEW
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
the DfrP protein is expressed in the bacterial host's cytoplasm, which is the site of
folate metabolism and nucleotide synthesis
references:
- id: GO_REF:0000043
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
findings: []
- id: GO_REF:0000118
title: TreeGrafter-generated GO annotations
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods.
findings: []
- id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
title: Deep research report on dfrP gene function
findings:
- statement: >-
DfrP is a dihydrofolate reductase enzyme that catalyzes the NADPH-dependent reduction
of dihydrofolate to tetrahydrofolate.
supporting_text: >-
DfrP is a dihydrofolate reductase...catalyzes the reduction of dihydrofolic acid (DHF)
to tetrahydrofolic acid (THF) using NADPH as a cofactor
- statement: >-
Phage DHFR genes (dfrA family) are widespread in lytic phages and enhance phage fitness
by improving nucleotide biosynthesis during infection.
supporting_text: >-
phiNIT1 encodes DfrP as an auxiliary metabolic gene (AMG) to boost nucleotide biosynthesis
during infection
- statement: >-
PhiNIT1 and related Bastillevirinae phages uniquely carry both thyA and DHFR genes
to form a self-sufficient thymidine synthesis cycle.
supporting_text: >-
By providing its own DHFR and TS, phiNIT1 ensures that the infected host cell can
efficiently produce dTMP and other nucleotides
- statement: >-
DfrP belongs to the trimethoprim-resistant DfrA family of DHFRs.
supporting_text: >-
These phage DHFR genes...tend to be trimethoprim-resistant variants...distinct from
typical bacterial FolA, falling into the DfrA family
core_functions:
- description: >-
Catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate,
regenerating the active folate cofactor needed for nucleotide biosynthesis during
phage DNA replication.
molecular_function:
id: GO:0004146
label: dihydrofolate reductase activity
directly_involved_in:
- id: GO:0046654
label: tetrahydrofolate biosynthetic process
- id: GO:0006730
label: one-carbon metabolic process
locations:
- id: GO:0030430
label: host cell cytoplasm
substrates:
- id: CHEBI:57451
label: 7,8-dihydrofolate(2-)
- id: CHEBI:58349
label: NADPH(4-)
supported_by:
- reference_id: file:9CAUD/dfrP/dfrP-deep-research-openai.md
supporting_text: >-
DfrP is a dihydrofolate reductase, an enzyme that catalyzes the reduction of
dihydrofolic acid (DHF) to tetrahydrofolic acid (THF) using NADPH as a cofactor