Tgt is the bacterial tRNA-guanine transglycosylase of Pseudomonas putida KT2440. As a zinc-dependent homodimer, it exchanges guanine 34 for the bacterially synthesized base preQ1 in tRNAs with GUN anticodons, generating preQ1-tRNA for subsequent QueA and QueG reactions. Unlike eukaryotic TGT, bacterial Tgt inserts preQ1, not free queuine.
Definition: Catalysis of the exchange of guanine at position 34 in a target tRNA with 7-aminomethyl-7-deazaguanine (preQ1), producing preQ1-modified tRNA and free guanine.
Justification: Bacterial Tgt performs RHEA:24104 with preQ1, whereas the current GO:0008479 definition specifies the distinct free-queuine reaction RHEA:16633. A dedicated term would preserve the bacterial substrate and distinguish this activity from eukaryotic TGT.
Parent term: pentosyltransferase activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002099 tRNA wobble guanine modification | IEA GO_REF:0000120 | ACCEPT | Summary: Tgt directly replaces wobble guanine 34 with preQ1 in target bacterial tRNAs. Reason: UniProt records base exchange at anticodon position 34 in tRNA(Asp), tRNA(Asn), tRNA(His), and tRNA(Tyr). Cached genetic evidence also states that an E. coli tgt mutant fails to insert preQ1 in tRNA. Supporting Evidence: PMID:40703034 the mutant does not insert preQ1 in tRNA |
| GO:0005737 cytoplasm | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: Cytoplasm is compatible with Tgt but redundant with the more specific cytosol annotation. Reason: Retain the valid broader location while using GO:0005829 in the core function. Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt GO; GO:0005829; C:cytosol |
| GO:0005829 cytosol | IEA GO_REF:0000118 | ACCEPT | Summary: Cytosol is the more specific soluble compartment consistent with Tgt function. Reason: TreeGrafter assigns cytosol, consistent with a soluble tRNA-modifying enzyme lacking targeting or membrane features. Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt GO; GO:0005829; C:cytosol |
| GO:0006400 tRNA modification | IEA GO_REF:0000120 | MODIFY | Summary: Generic tRNA modification should be narrowed to wobble guanine modification. Reason: The target residue and position are known: Tgt exchanges guanine 34 for preQ1. GO:0002099 captures the direct substrate-level process. Proposed replacements: tRNA wobble guanine modification Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt at position 34 |
| GO:0008479 tRNA-guanosine(34) queuine transglycosylase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is the GOC-maintained molecular-function mapping for bacterial Tgt, although its formal definition describes free-queuine insertion. Reason: InterPro2GO maps the bacterial IPR004803 family to GO:0008479, and UniRule UR000079182 pairs the same GO term with EC 2.4.2.29 and the preQ1 exchange reaction RHEA:24104. Retain that established mapping while recording the conflict with the term's free-queuine definition and requesting a dedicated bacterial term. Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt queuine precursor 7-aminomethyl-7-deazaguanine (PreQ1) at position 34 file:rules/arba/_interpro2go.txt InterPro:IPR004803 tRNA-guanine transglycosylase > GO:tRNA-guanosine(34) queuine transglycosylase activity ; GO:0008479 |
| GO:0008616 tRNA queuosine(34) biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Tgt catalyzes the first tRNA-bound step of bacterial queuosine synthesis. Reason: Insertion of preQ1 at guanine 34 creates the substrate for QueA and QueG, so Tgt directly participates in tRNA queuosine(34) biosynthesis even though it does not insert queuine itself. Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt queuine precursor 7-aminomethyl-7-deazaguanine (PreQ1) at position 34 |
| GO:0008270 zinc ion binding | IC file:PSEPK/tgt/tgt-uniprot.txt | NEW | Summary: Tgt binds one zinc ion per subunit as part of its conserved catalytic architecture. Reason: The reviewed HAMAP record explicitly assigns the zinc cofactor and its coordinating residues. Its UniProt record also carries the broader keyword-derived metal ion binding term, while seeded GOA lacks a metal-binding row. Supporting Evidence: file:PSEPK/tgt/tgt-uniprot.txt Note=Binds 1 zinc ion per subunit. |
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Download this section (compressed HTML)Q: Should GO add a bacterial tRNA-guanosine(34) preQ1 transglycosylase activity term distinct from GO:0008479?
Experiment: Reconstitute base exchange with purified Tgt and target tRNAs, quantify preQ1 incorporation by LC-MS, and test active-site and zinc-binding mutants.
Hypothesis: Q88PL7 exchanges guanine 34 for preQ1 in KT2440 GUN-anticodon tRNAs.
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