NRPS-independent siderophore (NIS) synthetase-like adenylate-forming ligase (mllBC fusion) of the methylolanthanin biosynthetic pathway. The mllBC fusion combines asbD and asbE homolog functions in a single polypeptide and most likely catalyzes one or more ATP-dependent acyl-adenylation and amide bond-forming steps required to assemble methylolanthanin, a citrate-based lanthanophore that binds lanthanides (La/Nd/Lu) and promotes lanthanide uptake/bioaccumulation in M. extorquens AM1. The protein lies in the mll gene cluster (META1p4129-4138; locus META1p4133), which is strongly induced (~32-fold) under poorly soluble lanthanide conditions. Functional assignment is cluster- and homology-supported (homologous to petrobactin asb/IucA-IucC NIS pathways); no purified single-enzyme assay of C5B1I5 has been reported.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008756 o-succinylbenzoate-CoA ligase activity | IEA GO_REF:0000003 | REMOVE | Summary: Incorrect - this EC assignment (6.2.1.26) is wrong. META1p4133 is not involved in menaquinone biosynthesis (which requires o-succinylbenzoate-CoA ligase). The misprediction derives from generic AMP-binding/adenylate-forming sequence similarity. The protein instead functions as an NRPS-independent siderophore (NIS) synthetase-like adenylate-forming ligase in methylolanthanin (lanthanophore) biosynthesis. Reason: The EC:6.2.1.26 o-succinylbenzoate-CoA ligase reaction belongs to menaquinone biosynthesis and is unrelated to the methylolanthanin/lanthanide-acquisition pathway that META1p4133 participates in. The falcon report and UniProt domain architecture (IucA/IucC + AMP-binding) instead place C5B1I5 in the NIS-synthetase class. Supporting Evidence: file:METEA/mllBC/mllBC-deep-research-falcon.md The mll locus was detected/annotated using an antiSMASH rule requiring **IucA/IucC** and **AMP-binding** signatures characteristic of NIS siderophore synthetases. |
| GO:0016874 ligase activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: Correct but general - the mllBC fusion protein is an ATP-dependent ligase. Within the methylolanthanin pathway it most plausibly catalyzes acyl-adenylate formation and amide bond formation (NIS-type chemistry), consistent with its IucA/IucC and AMP-binding domains. The more specific GO:0016881 (acid-amino acid ligase activity) better captures the inferred carbon-nitrogen ligase chemistry. Reason: GO:0016874 is the generic ligase parent term. It is consistent with the domain architecture and the inferred NIS adenylate-forming mechanism, but the specific MF GO:0016881 (acid-amino acid ligase activity) captures the core carbon-nitrogen ligase function. The generic parent is therefore retained as non-core rather than as a core function. Supporting Evidence: file:METEA/mllBC/mllBC-deep-research-falcon.md an **ATP-dependent acyl-adenylate-forming ligase** that catalyzes **amide bond formation** during MLL biosynthesis |
| GO:0016881 acid-amino acid ligase activity | IEA GO_REF:0000117 | ACCEPT | Summary: Correct and appropriately specific - NIS synthetases catalyze formation of an amide (carbon-nitrogen) bond between a carboxylate (acid) substrate and an amine substrate via an ATP-dependent acyl-adenylate intermediate. For methylolanthanin this corresponds to joining 4-hydroxybenzoate/citrate-derived carboxylates to amine-bearing homospermidine linkers. This term captures the inferred core molecular function of the mllBC fusion. Reason: The acid-amino acid (carbon-nitrogen) ligase chemistry matches the NIS mechanism and the methylolanthanin structure (citrate core, 4-hydroxybenzoate moieties, homospermidine linkers). This is the best-fitting available MF term; no purified C5B1I5 assay exists so it remains a homology/pathway-supported inference. Supporting Evidence: file:METEA/mllBC/mllBC-deep-research-falcon.md NIS synthetases are ATP-dependent ligases that form **amide bonds** between a **carboxylate substrate** and an **amine substrate** file:METEA/mllBC/mllBC-deep-research-falcon.md Methylolanthanin was structurally elucidated as a **citrate core** linked to **two 4-hydroxybenzoate (4-HB) moieties** through **acetylated homospermidine residues** |
| GO:0019290 siderophore biosynthetic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Analogous but not specific - mllBC synthesizes methylolanthanin, a LANTHANIDE-chelating metallophore (lanthanophore), not an iron-chelating siderophore. The biosynthetic logic and enzyme families are conserved (homologous to petrobactin asb/IucA-IucC NIS pathways), but the product binds lanthanides (La/Nd/Lu) rather than Fe(III). GO has no metallophore/lanthanophore-specific biosynthetic process term, so this siderophore term is the nearest existing process. Kept as non-core; the mll cluster shows ~32-fold induction under poorly soluble lanthanide. Reason: The siderophore biosynthetic process term is defined for Fe(III)-chelating substances; methylolanthanin is a lanthanide chelator. The annotation is a useful analogy capturing the NIS biosynthetic role but is not species/process-precise, so it is retained as non-core pending a more specific (lanthanophore/metallophore) term. Supporting Evidence: file:METEA/mllBC/mllBC-deep-research-falcon.md The mll biosynthetic gene cluster (META1p4129–4138) is highly transcriptionally induced when AM1 is grown with **poorly soluble Nd2O3**, with an average ~**32-fold** upregulation compared with growth in soluble NdCl3. file:METEA/mllBC/mllBC-deep-research-falcon.md Purified methylolanthanin forms detectable complexes with La, Nd, and Lu |
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