T4 bacteriophage dihydrofolate reductase (DHFR), encoded by the frd gene, catalyzes the reduction of dihydrofolate to tetrahydrofolate using NADPH as hydride donor (EC 1.5.1.3). The 193 amino acid protein (21.7 kDa monomer) functions as a homodimer and supports phage DNA precursor synthesis by regenerating reduced folate cofactors required by thymidylate synthase. The enzyme interacts with thymidylate synthase and dCMP hydroxymethyltransferase as part of a dNTP synthetase complex that coordinates nucleotide precursor synthesis during phage replication. Older studies proposed that T4 DHFR was also a virion baseplate structural component, but later quantitative immunoblot evidence argued that DHFR is present only as a minor contaminant in purified virions and should not be curated as a bona fide structural virion/baseplate protein without new evidence.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004146 dihydrofolate reductase activity | IEA GO_REF:0000120 | ACCEPT | Summary: T4 frd encodes a bona fide dihydrofolate reductase that catalyzes the reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate using NADPH as cofactor. This is the core enzymatic function of the gene product, supported by extensive biochemical characterization and structural data. Reason: DHFR activity is the primary molecular function of this enzyme. The annotation is well-supported by IDA evidence (PMID:4936128) and structural characterization. The enzyme belongs to the DHFR family with conserved domain architecture (IPR001796, IPR017925). Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md T4 DHFR catalyzes the two-electron reduction of dihydrofolate (FH2) to tetrahydrofolate (FH4), using NADPH as the hydride donor PMID:10818362 Dihydrofolate reductase (DHFR) from bacteriophage T4 is a homodimer consisting of 193-residue subunits |
| GO:0006730 one-carbon metabolic process | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: T4 DHFR regenerates tetrahydrofolate, which carries one-carbon units for biosynthetic reactions including thymidylate synthesis. The enzyme is part of a one-carbon metabolic pathway that supports DNA precursor production during phage infection. Reason: DHFR catalyzes a key reaction in folate-mediated one-carbon metabolism. The product tetrahydrofolate serves as a one-carbon carrier required for thymidylate synthesis. The deep research confirms that frd-driven FH4 regeneration sustains thymidylate synthase conversion of dUMP to dTMP during infection. This parent process is accurate but less specific than tetrahydrofolate biosynthetic process and DHFR activity, so it should be kept as non-core. Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md frd-driven FH4 regeneration sustains thymidylate synthase (td) conversion of dUMP to dTMP during infection |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: DHFR is an oxidoreductase that catalyzes the transfer of electrons from NADPH to dihydrofolate. This is a correct but general annotation. Reason: The annotation is correct - DHFR is an oxidoreductase (EC 1.5.1.3). However, the more specific term GO:0004146 (dihydrofolate reductase activity) is also present and is the preferred annotation. This general term is acceptable as a parent annotation that captures the broader enzymatic class, but it should not be treated as the core molecular function. Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md T4 DHFR catalyzes the two-electron reduction of dihydrofolate (FH2) to tetrahydrofolate (FH4), using NADPH as the hydride donor |
| GO:0031427 response to methotrexate | IEA GO_REF:0000043 | REMOVE | Summary: This annotation derives from the UniProt keyword "Methotrexate resistance" (KW-0487). T4 DHFR is competitively inhibited by methotrexate and other antifolates at the substrate binding site. However, the phage enzyme does not confer resistance to methotrexate and the phage does not "respond to" methotrexate as a biological process. Reason: This annotation represents a conflation of "being inhibited by a drug" with "responding to" that drug as a biological process. Methotrexate is a competitive inhibitor of T4 DHFR with respect to dihydrofolate. The enzyme is a TARGET of this drug, not a biological responder. The GO term "response to methotrexate" implies a coordinated cellular response pathway, which is inappropriate for: (1) a phage-encoded enzyme, and (2) an enzyme that is simply inhibited by the compound. The UniProt keyword "Methotrexate resistance" refers to the fact that DHFR is the target of methotrexate and mutations can confer resistance - but this phage enzyme is wild-type and simply subject to inhibition. Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md Competitive inhibition by methotrexate, aminopterin, and trimethoprim with respect to FH2 has been documented for T-even phage DHFRs |
| GO:0046654 tetrahydrofolate biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: T4 DHFR catalyzes the reduction of dihydrofolate to tetrahydrofolate, which is the final step in THF biosynthesis. This is a core function of the enzyme. Reason: DHFR catalyzes the conversion of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate, which provides reduced folate cofactors. UniProt explicitly states the pathway role. This is a direct annotation of the enzyme's biosynthetic role. Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md T4 DHFR catalyzes the two-electron reduction of dihydrofolate (FH2) to tetrahydrofolate (FH4) |
| GO:0046677 response to antibiotic | IEA GO_REF:0000043 | REMOVE | Summary: This annotation derives from UniProt keywords "Antibiotic resistance" (KW-0046) and "Trimethoprim resistance" (KW-0817). T4 DHFR is inhibited by the antibiotic trimethoprim (and other antifolates), but the phage enzyme does not participate in an "antibiotic response" biological process. Recent phylogenomic analysis explicitly showed that phage-encoded DHFRs do NOT confer antibiotic resistance. Reason: This is a clear example of inappropriate SPKW-based over-annotation. The annotation conflates two distinct concepts: (1) DHFR being a known target of antifolate antibiotics, and (2) organisms mounting a biological "response" to antibiotics. A 2020 phylogenomics study explicitly concluded that phage-encoded DHFRs do NOT confer trimethoprim resistance despite homology and that phage folA genes primarily serve phage nucleotide metabolism rather than resistance. The GO term "response to antibiotic" implies a coordinated biological response pathway - phages do not have antibiotic response pathways, and this particular enzyme is simply an enzymatic target that gets inhibited by certain drugs. The fact that an enzyme CAN BE INHIBITED by an antibiotic does not mean the organism "responds to" that antibiotic. Supporting Evidence: file:BPT4/frd/frd-deep-research-falcon.md phage-encoded DHFRs (found on cryptic plasmids/phages) do not confer trimethoprim resistance despite homology, and that only a small fraction of complete phage genomes carry functional antibiotic-resistance determinants. This supports the view that phage folA genes primarily serve phage nucleotide metabolism rather than resistance |
| GO:0004146 dihydrofolate reductase activity | IDA PMID:4936128 T4 bacteriophage-specific dihydrofolate reductase: purificat... | ACCEPT | Summary: Direct experimental evidence for DHFR activity. The 1971 publication by Erickson and Mathews purified T4 DHFR to homogeneity by affinity chromatography and characterized its enzymatic activity. Reason: This IDA annotation is the primary experimental evidence for DHFR activity. The enzyme was purified and its activity directly demonstrated. This is the core molecular function annotation for frd and should be retained. Supporting Evidence: PMID:4936128 T4 bacteriophage-specific dihydrofolate reductase: purification to homogeneity by affinity chromatography |
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Download this section (compressed HTML)Q: How should the historically proposed T4 DHFR virion/baseplate localization be handled in GO curation, given that later quantitative immunoblot work argues that DHFR is not a bona fide virion structural protein?
Experiment: Detailed steady-state kinetic characterization of T4 DHFR with modern methods to establish precise Km values for dihydrofolate and NADPH, and Ki values for antifolate inhibitors.
Hypothesis: While inhibition by antifolates is documented, precise kinetic parameters under modern conditions are not well-characterized in available literature.
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