CysI is the siroheme/iron-sulfur catalytic protein implicated in assimilatory sulfite reduction to sulfide. Pseudomonad genetics support an Fpr-linked electron-supply system distinct from the enterobacterial CysJ complex. The immediate electron donor to the KT2440 enzyme remains unresolved; ferredoxin, another carrier, or direct Fpr-mediated transfer are candidate architectures.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | MODIFY | Summary: Refine generic oxidoreductase activity to sulfur-group redox catalysis. Reason: The siroheme/iron-sulfur reductase architecture and pseudomonad sulfate-assimilation evidence support sulfite reduction. GO:0016667 retains that sulfur-redox chemistry without selecting an unverified immediate donor. The donor-unspecified sulfite-reductase term GO:0016002 is obsolete (live QuickGO check). Proposed replacements: oxidoreductase activity, acting on a sulfur group of donors Supporting Evidence: file:PSEPK/cysI/cysI-uniprot.txt InterPro; IPR006067; NO2/SO3_Rdtase_4Fe4S_dom. file:PSEPK/cysI/cysI-uniprot.txt InterPro; IPR005117; NiRdtase/SiRdtase_haem-b_fer. PMID:23794620 it also remains possible that FprA transfers electrons directly to CysI. |
| GO:0020037 heme binding | IEA GO_REF:0000002 | ACCEPT | Summary: Siroheme is part of the catalytic reductase architecture. Reason: The sulfite/nitrite reductase hemoprotein signature supports heme binding as a core cofactor property. This is domain-based inference, not an isolated KT2440 cofactor assay. Supporting Evidence: file:PSEPK/cysI/cysI-uniprot.txt InterPro; IPR005117; NiRdtase/SiRdtase_haem-b_fer. |
| GO:0051536 iron-sulfur cluster binding | IEA GO_REF:0000002 | MODIFY | Summary: Refine iron-sulfur binding to [4Fe-4S] cluster binding. Reason: The conserved reductase 4Fe-4S domain supports the specific cluster class rather than an unspecified iron-sulfur cluster. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: file:PSEPK/cysI/cysI-uniprot.txt InterPro; IPR006067; NO2/SO3_Rdtase_4Fe4S_dom. |
| GO:0050311 sulfite reductase (ferredoxin) activity | ISS file:interpro/panther/PTHR32439/PTHR32439-entries.csv | REMOVE | Summary: Withdraw the earlier donor-specific NEW proposal; the immediate electron donor of KT2440 CysI is unresolved. Reason: This row is not a GOA annotation but an earlier review's NEW proposal, which chose the ferredoxin-specific term by analogy with the PTHR32439 family member M. tuberculosis Sir. PMID:23794620 explicitly leaves open whether FprA reduces CysI directly or through a carrier, and FprA prefers flavodoxin among tested partners, so GO:0050311 donor specificity is not supported for Q88KB9. The proposal is withdrawn (REMOVE) and kept retired for provenance. The sulfite-reduction chemistry itself is retained through the donor-neutral GO:0016667 core function, so this withdrawal does not deny sulfite-reductase activity. Supporting Evidence: PMID:23794620 it also remains possible that FprA transfers electrons directly to CysI. PMID:23794620 These data indicate that fprA is necessary for effective sulfate assimilation by P. putida |
| GO:0000103 sulfate assimilation | ISS file:interpro/panther/PTHR32439/PTHR32439-entries.csv | NEW | Summary: CysI performs the sulfite-reduction step of sulfate assimilation. Reason: Preserve the existing pathway proposal: conserved catalytic architecture and pseudomonad Fpr/sulfite-reduction genetics support participation. The exact KT2440 immediate carrier remains unresolved and is not required to infer the catalytic step. Supporting Evidence: PMID:23794620 These data indicate that fprA is necessary for effective sulfate assimilation by P. putida file:PSEPK/cysI/cysI-uniprot.txt InterPro; IPR006067; NO2/SO3_Rdtase_4Fe4S_dom. |
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Download this section (compressed HTML)Q: Does KT2440 CysI receive electrons through a discrete ferredoxin, directly from Fpr-I, or through more than one interchangeable carrier?
Experiment: Reconstitute Q88KB9 sulfite reduction with Q88MD5, NADPH, and candidate KT2440 ferredoxins, measuring sulfide formation and electron transfer through each candidate carrier.
Type: reconstituted electron-transfer assay
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The immediate electron-transfer route from Fpr-I to CysI in Pseudomonas putida KT2440 is unresolved, including whether it uses an intermediate ferredoxin/flavodoxin or direct transfer.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Fpr-I reduces tested Fd/Fld partners, and pseudomonad genetic complementation supports its participation in assimilatory sulfite reduction. These observations do not identify the physiological carrier.
What would resolve it: Reconstitute purified KT2440 Fpr-I/CysI with and without candidate ferredoxin and flavodoxin carriers, then test supported routes genetically.
Provenance (the field's own admissions):
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