TfdA is the 2-oxoglutarate-dependent, Fe(2+)-dependent dioxygenase encoded on plasmid pJP4 that initiates degradation of the herbicide 2,4-dichlorophenoxyacetate (2,4-D). It cleaves the ether side chain oxidatively, consuming 2-oxoglutarate and O2 to release 2,4-dichlorophenol, glyoxylate, succinate and CO2, and defines its own family (TfdA dioxygenase) within the Fe(2+)/2-oxoglutarate oxygenase fold. It also accepts 2-methyl-4-chlorophenoxyacetate and, with the gene transferred to other hosts, enables growth on unchlorinated phenoxyacetate. Transposon mutants lacking 2,4-D monooxygenase activity are complemented by the cloned gene, and transfer of tfdA alone confers phenoxyacetate utilization, establishing the enzyme as the pathway's committed first step. The enzyme is self-inactivating through oxidative hydroxylation of Trp-113.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Root oxidoreductase term from the TfdA-family InterPro signature, redundant with the specific dioxygenase activity. Reason: IPR003819 is the TfdA-family signature itself, so the domain match is correct, but it is mapped only to the root of the oxidoreductase branch. GO:0016491 is an ancestor of GO:0018602, which is already annotated. The InterPro-to-GO mapping is conservative here because the family's members vary in substrate; for this gene the specific activity is established and the root term adds nothing. |
| GO:0018602 2,4-dichlorophenoxyacetate alpha-ketoglutarate dioxygenase activity | IEA GO_REF:0000116 | ACCEPT | Summary: Rhea-mapped IEA to the exactly matching specific term; correct, but GOA carries no experimental annotation for this well-characterized enzyme. Reason: RHEA:48984 is an exactMatch of GO:0018602, so this automatic annotation lands on the right term. It is worth recording that the only MF annotation on tfdA is electronic: the gene has a transposon-mutant and heterologous-expression history from 1987 and detailed mechanistic enzymology, none of which is captured as IDA or IMP. The term is right and the evidence code understates what is known, which is a curation gap, not an annotation error. Supporting Evidence: PMID:3036764 Expression of 2,4-D monooxygenase in Escherichia coli containing a 1.4-kilobase subfragment was demonstrated by radioisotopic enzyme assay |
| GO:0046300 2,4-dichlorophenoxyacetic acid catabolic process | IEA GO_REF:0000041 | ACCEPT | Summary: UniPathway-mapped process term, independently supported by mutant complementation and gene transfer. Reason: Plasmid pJP4 carries the complete 2,4-D degradation gene set, tfdA was located by Tn5 mutagenesis of the 2,4-D-negative phenotype, and the cloned fragment complemented those mutants. Transfer of tfdA alone let a cured derivative grow on phenoxyacetate as sole carbon and energy source. The process annotation would merit IMP from this work. Supporting Evidence: PMID:3036764 A 3-kilobase fragment of pJP4 cloned in a broad-host-range vector could complement the 2,4-D-negative phenotype of two mutants which lacked 2,4-D monooxygenase activity. |
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Download this section (compressed HTML)Q: Is unchlorinated phenoxyacetate, or another natural aryloxyalkanoate such as a plant-derived auxin-like compound, the ancestral substrate? tfdA transfer confers growth on phenoxyacetate, which suggests the chlorinated herbicide is not required for activity.
Q: Why does GOA carry no experimental annotation for tfdA despite nearly four decades of genetics and enzymology? If Rhea-derived IEAs are the only MF evidence on well-studied xenobiotic-degradation enzymes generally, that is a systematic gap worth quantifying.
Q: Should the self-inactivating Trp-113 hydroxylation be captured as an annotation, and if so under which branch? It is a documented autocatalytic modification that limits the enzyme's functional lifetime.
Experiment: Assay purified TfdA against a panel of natural aryloxyacetates and phenoxyalkanoates, including indole-3-acetate and plant phenolic ethers, and compare catalytic efficiencies with 2,4-D.
Hypothesis: TfdA's progenitor acted on a naturally occurring aryl ether, and 2,4-D activity was recruited from that pre-existing capability rather than arising de novo.
Type: enzyme kinetics
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