Curation Notes for fur1 (VPARA_18200, A0A0H2M8T9) in Variovorax paradoxus TBEA6

1. Gene & Protein Overview

• UniProtKB Accession: A0A0H2M8T9 (A0A0H2M8T9_VARPD, unreviewed, 139 aa, 15,839 Da)
• Gene Symbol / Locus Tag: fur1 (synonym fur) / VPARA_18200 (EMBL:KLN57112.1 on genomic contig JZWI01000008, RefSeq:WP_021005176.1)
• Organism: Variovorax paradoxus strain TBEA6 (NCBITaxon:34073)
• Domain Architecture & Metal-Coordination Sites (Verified against SQ block):
• Homodimeric metalloregulator (RuleBase:RU364037) of the Fur family (Pfam:PF01475, InterPro:IPR002481, NCBIfam:NF006999, PANTHER:PTHR33202:SF2 FERRIC UPTAKE REGULATION PROTEIN), comprising an N-terminal winged-helix DNA-binding domain (Gene3D:1.10.10.10, InterPro:IPR036388) and a C-terminal histidine/cysteine-rich dimerization and metal-binding domain (InterPro:IPR043135, 85-THHDHLICTSCGKVEEFYDAEIERRQQMISKDKGWILQDHAMSLYGLCGDCVGKR-139).
• Regulatory $\text{Fe}^{2+}$/$\text{Mn}^{2+}$-sensing site (PIRSR:PIRSR602481-2, 1 $\text{Fe}^{2+}$ or $\text{Mn}^{2+}$ ion per subunit): Coordinated by His86, Asp88 (within 85-THHDH-89), Glu107 (within 104-FYDAEIE-110), and His124 (within 122-QDHAM-126) (FT BINDING 86, 88, 107, 124).
• Structural $\text{Zn}^{2+}$ site (PIRSR:PIRSR602481-1, 1 $\text{Zn}^{2+}$ ion per subunit): Coordinated by the four conserved cysteines Cys92, Cys95 (92-CTSC-95), Cys132, and Cys135 (132-CGDC-135) (FT BINDING 92, 95, 132, 135).

2. Literature & Comparative Evidence

• PMID:8522528(https://pubmed.ncbi.nlm.nih.gov/8522528/) (Ochsner, Vasil & Vasil 1995, J Bacteriol 177:7194-7201): Purified Pseudomonas aeruginosa Fur and demonstrated by gel mobility shift and DNase I footprinting that holo-Fur-$\text{Fe(II)}$ specifically binds conserved 19-bp "Fur box" operator sequences (GATAATGATAATCATTATC) overlapping iron-regulated siderophore promoters (pvdS, pchR, fepA-fes) to repress transcription under iron-replete conditions.
• PMID:12581348(https://pubmed.ncbi.nlm.nih.gov/12581348/) (Pohl et al. 2003, Mol Microbiol 47:903-915): Solved the 1.8-Å crystal structure and X-ray absorption spectroscopy of Fur, characterizing the distinct structural $\text{Zn}^{2+}$ site and regulatory $\text{Fe}^{2+}$-sensing site that couple intracellular iron occupancy to operator DNA binding.
• PMID:20562273(https://pubmed.ncbi.nlm.nih.gov/20562273/) (Chen, Bleam & Hickey 2010, Appl Environ Microbiol 76:5620-5630): Demonstrated in Pseudomonas putida KT2440 that a $\Delta fur$ deletion mutant shows altered expression of iron-homeostasis genes and increased $\text{H}_2\text{O}_2$ sensitivity.

3. Review of Existing GOA Annotations (13 IEA rows)

Following the curated comparator reviews genes/PSEAE/fur/fur-ai-review.yaml and genes/PSEPK/fur/fur-ai-review.yaml:
1. GO:0000976 (transcription cis-regulatory region binding, enables, IEA GO_REF:0000120 from ARBA:ARBA00028970, PANTHER:PTN002848968) -> ACCEPT:
- Core molecular function; holo-Fur binds conserved Fur-box operator sequences in cis-regulatory promoter regions of iron-regulated genes (PMID:8522528).
2. GO:0001216 (DNA-binding transcription activator activity, enables, IEA GO_REF:0000117 from ARBA:ARBA00027660) -> MODIFY to GO:0001217 (DNA-binding transcription repressor activity):
- ARBA (GO_REF:0000117) assigned GO:0001216 (activator), whereas Fur (RuleBase:RU364037, KW Repressor) acts primarily as an $\text{Fe(II)}$-dependent transcriptional repressor when bound to Fur-box operators overlapping target promoters (PMID:8522528; positive effects on some iron-storage genes are typically indirect via repression of small RNAs such as RyhB/PrrF). Refined laterally to GO:0001217 (DNA-binding transcription repressor activity), matching PSEAE/fur.
3. GO:0003677 (DNA binding, enables, IEA GO_REF:0000104 from UniRule:UR000481764) -> TOO_GENERAL:
- True but uninformative parent term already entailed by the accepted specific child term GO:0000976 (transcription cis-regulatory region binding).
4. GO:0003700 (DNA-binding transcription factor activity, enables, IEA GO_REF:0000120 from InterPro:IPR002481, PANTHER:PTN002848968, UniRule:UR000481764) -> MODIFY to GO:0001217 (DNA-binding transcription repressor activity):
- Unsigned parent transcription factor term; refined to the signed child term GO:0001217 (DNA-binding transcription repressor activity), matching PSEAE/fur.
5. GO:0005737 (cytoplasm, located_in, IEA GO_REF:0000044 from UniProtKB-SubCell:SL-0086) -> TOO_GENERAL:
- True but uninformative parent cellular component already entailed by the accepted specific child term GO:0005829 (cytosol).
6. GO:0005829 (cytosol, located_in, IEA GO_REF:0000118 from PANTHER:PTN002848968) -> ACCEPT:
- Core subcellular localization of the soluble 139-aa cytoplasmic repressor (RuleBase:RU364037).
7. GO:0006355 (regulation of DNA-templated transcription, involved_in, IEA GO_REF:0000002 from InterPro:IPR002481) -> TOO_GENERAL:
- True but uninformative unsigned parent process already entailed by the accepted specific child term GO:0045892 (negative regulation of DNA-templated transcription).
8. GO:0008270 (zinc ion binding, enables, IEA GO_REF:0000118 from PANTHER:PTN002848968) -> KEEP_AS_NON_CORE:
- Unlike P. putida Fur (which lacks the 4-Cys motif), V. paradoxus TBEA6 Fur1 retains the canonical four-cysteine structural $\text{Zn}^{2+}$-binding motif at Cys92, Cys95 (92-CTSC-95), Cys132, and Cys135 (132-CGDC-135), annotated by PIRSR:PIRSR602481-1 (FT BINDING 92, 95, 132, 135). As in PSEAE/fur, structural $\text{Zn}^{2+}$ binding is genuine but non-core relative to regulatory $\text{Fe}^{2+}$ sensing, so GO:0008270 is retained as KEEP_AS_NON_CORE while GO:0008198 (ferrous iron binding) is added as NEW.
9. GO:0032993 (protein-DNA complex, part_of, IEA GO_REF:0000117 from ARBA:ARBA00028896) -> KEEP_AS_NON_CORE:
- Matching PSEAE/fur, holo-Fur forms a transient operator-bound protein-DNA complex when repressing target promoters (PMID:8522528), which is true but adds little beyond GO:0000976 and is kept as KEEP_AS_NON_CORE.
10. GO:0045892 (negative regulation of DNA-templated transcription, involved_in, IEA GO_REF:0000118 from PANTHER:PTN002848968) -> ACCEPT:
- Core biological process of Fur (RuleBase:RU364037, KW Repressor; PMID:8522528).
11. GO:0045893 (positive regulation of DNA-templated transcription, involved_in, IEA GO_REF:0000108 from GO:0001216) -> MODIFY to GO:0045892 (negative regulation of DNA-templated transcription):
- Automatically inferred via GO_REF:0000108 from the ARBA GO:0001216 row; refined laterally to GO:0045892 (negative regulation of DNA-templated transcription) to match the modification of GO:0001216 $\rightarrow$ GO:0001217.
12. GO:1900376 (regulation of secondary metabolite biosynthetic process, involved_in, IEA GO_REF:0000118 from PANTHER:PTN002848968) -> TOO_GENERAL:
- True but uninformative parent process already entailed by the accepted specific child term GO:1900705 (negative regulation of siderophore biosynthetic process, since GO:1900705 is_a GO:1900376).
13. GO:1900705 (negative regulation of siderophore biosynthetic process, involved_in, IEA GO_REF:0000118 from PANTHER:PTN002848968) -> ACCEPT:
- Core biological process in siderophore-producing bacteria such as Variovorax paradoxus (which synthesizes vacidobactin and variobactin siderophores, PMID:26412281, and carries the exbD5-exbB4-tonB-bfrD1 ferric-siderophore uptake locus).

4. Proposed New Annotations

• GO:0008198 (ferrous iron binding, enables, IC from file:VARPD/fur1/fur1-uniprot.txt and PMID:12581348) -> NEW:
• Captures the physiological iron-sensing cofactor activity of Fur1 at its conserved regulatory site (His86, Asp88, Glu107, His124; PIRSR:PIRSR602481-2, CC -!- COFACTOR: Name=Fe(2+); Xref=ChEBI:CHEBI:29033; Evidence={ECO:0000256|PIRSR:PIRSR602481-2}).