| Claim / annotation item | Evidence type | Key supporting details / quantitative values | Confidence | Primary citation(s) |
|---|---|---|---|---|
| **mllF identity and membership in the methylolanthanin BGC** | Direct experiment in *M. extorquens* AM1; comparative genomics | META1p4135 is explicitly annotated as **mllF** within the **mll/mlu** biosynthetic gene cluster spanning **META1p4129–META1p4138**; the cluster is described as homologous in part to the petrobactin **asb** locus, placing mllF in a metallophore-biosynthetic context. | High | (pqac-00000000, pqac-00000002) |
| **mll locus is induced under poorly soluble lanthanide conditions** | Direct experiment in *M. extorquens* AM1 (RNA-seq) | The entire **mll** locus was reported as highly upregulated, with an average of **~32-fold** higher expression during growth with **Nd2O3** versus **NdCl3**, consistent with a role in scavenging poorly bioavailable lanthanides. | High | (pqac-00000000, pqac-00000003) |
| **Pathway role: methylolanthanin biosynthesis / lanthanophore system** | Direct experiment in *M. extorquens* AM1; comparative genomics | The cluster was identified as producing **methylolanthanin**, the first reported biological lanthanide chelator; the BGC also includes predicted uptake/signaling functions such as a TonB-dependent outer membrane receptor, supporting a dedicated lanthanide acquisition pathway. | High | (pqac-00000000, pqac-00000003) |
| **Methylolanthanin chemical structure** | Direct experiment in *M. extorquens* AM1 | UPLC-MS/MS and NMR established methylolanthanin as a citrate-centered molecule linked to **two 4-hydroxybenzoate (4-HB) moieties** via homospermidine residues; this distinguishes it from rhodopetrobactin, which uses 3,4-DHBA. | High | (pqac-00000002) |
| **Methylolanthanin binds lanthanides** | Direct experiment in *M. extorquens* AM1 | Direct-injection MS showed complexes with **La(III), Nd(III), and Lu(III)**, observed as **[MLL-H+ + Ln3+]2+** species with matching isotopic patterns, demonstrating binding across lanthanides of different ionic radii. | High | (pqac-00000004) |
| **Physiological effect of deleting/overexpressing the mll cluster** | Direct experiment in *M. extorquens* AM1 | Overexpression (**ΔmxaF/pAZ1**) improved growth with poorly soluble **Nd2O3** to **0.026 hr^-1**, described as a **nearly 50% increase** over ΔmxaF under the same condition and closer to **0.037 hr^-1** observed for ΔmxaF with soluble **NdCl3**. Exogenous methylolanthanin (**50 nM**) significantly increased growth yield of both ΔmxaF and ΔmxaFΔmll cultures. | High | (pqac-00000004) |
| **Effect on intracellular neodymium accumulation** | Direct experiment in *M. extorquens* AM1 | **Δmll** reduced Nd bioaccumulation with both lanthanide sources, notably by **1.8-fold** in the **NdCl3** condition, whereas overexpression of **mll** increased Nd bioaccumulation by **3.5-fold on average**. | High | (pqac-00000004) |
| **Predicted enzymatic activity of mllF: divergent AsbF-like PF01261 enzyme** | Comparative genomics; characterized homolog inference | The study grouped **mllF** with genes resembling the petrobactin **asb** biosynthetic locus, but specifically noted that **no asbF model was used** in genome mining because **mllF had a low bitscore to Pfam PF01261**, implying it is **PF01261-related but divergent**. This supports cautious inference rather than definitive assignment. | Medium | (pqac-00000000, pqac-00000001) |
| **Reference reaction for the closest characterized homolog AsbF** | Characterized homolog | **AsbF** is a **Mn2+-dependent 3-dehydroshikimate dehydratase** that converts **3-dehydroshikimate (3-DHS)** to **3,4-dihydroxybenzoic acid (3,4-DHBA / protocatechuate)**, a precursor for petrobactin biosynthesis. | High | (pqac-00000006, pqac-00000007, pqac-00000008) |
| **Reference kinetics and structural family for AsbF** | Characterized homolog | Reported AsbF parameters at **pH 7.5**: **Km ~290 µM**, **kcat ~80 min^-1**. Structurally, AsbF adopts an **(α/β)8 TIM-barrel** fold within the **AP endonuclease 2 / xylose-isomerase-like** family and uses a divalent metal, with **Mn2+ preferred** and **Zn2+ inhibitory**. | High | (pqac-00000006, pqac-00000007) |
| **Most likely subcellular localization for mllF function** | Inference from pathway chemistry and homolog class | Because methylolanthanin is a **small-molecule biosynthetic product** and mllF is not described as a membrane or secreted protein, the most plausible localization is **intracellular (cytosolic) biosynthesis**, with the lanthanophore subsequently functioning in extracellular/periplasm-associated metal acquisition. This localization has not been directly tested for mllF alone. | Low | (pqac-00000003, pqac-00000004) |


*Table: This table summarizes the strongest available evidence for functional annotation of mllF/META1p4135 in Methylorubrum extorquens AM1, separating direct AM1 experiments from inference based on the characterized homolog AsbF. It is useful for distinguishing what is experimentally established from what remains a domain-based prediction.*