| Feature | Evidence type | What is supported | Key quantitative data | Source |
|---|---|---|---|---|
| **mllH / META1p4137 / UniProt C5B1I9** | Bioinformatic prediction | Gene-specific support that the protein is a **putative GNAT-family acetyltransferase** within the methylolanthanin (**mll**) locus; proposed role is acetylation of a **(homo)spermidine-derived linker** in methylolanthanin biosynthesis, by analogy to rhodopetrobactin-like pathways; **no direct enzymatic assay, substrate specificity, phenotype, or localization** for MllH alone was reported in the available evidence. | No gene-specific quantitative activity data available. | Zytnick et al., **2022**, **bioRxiv**. https://doi.org/10.1101/2022.01.19.476857 ; DOI: 10.1101/2022.01.19.476857 (pqac-00000000, pqac-00000001) |
| **mll locus (META1p4129–META1p4138)** | Experimental, cluster-level | The locus encodes production/handling of the lanthanide-binding metallophore **methylolanthanin (MLL)**; supports pathway assignment of **lanthanophore biosynthesis and uptake/homeostasis** in *Methylorubrum extorquens* AM1. This evidence is **not specific to mllH**, but provides the strongest functional context for the gene. | Average **~32-fold upregulation** with poorly soluble **Nd2O3** versus soluble **NdCl3**; cluster overexpression increased growth on poorly bioavailable Nd2O3 to **0.026 h⁻¹**; overexpression increased intracellular Nd accumulation by **~3.5-fold**; ΔmxaFΔmll mutant showed **~30% decreased** lanthanide bioaccumulation and **~1.8-fold lower** intracellular Nd under NdCl3 in one comparison. | Zytnick et al., **2022**, **bioRxiv**. https://doi.org/10.1101/2022.01.19.476857 ; DOI: 10.1101/2022.01.19.476857 (pqac-00000001, pqac-00000002, pqac-00000003) |
| **mll-targeted deletion/overexpression strains (META1p4132–META1p4138)** | Experimental, cluster-level | Deletion of the biosynthetic region abolished detectable MLL production; overexpression restored/enhanced MLL-associated phenotypes. This demonstrates that genes in the region are required for MLL biosynthesis, but still does **not resolve the individual catalytic reaction of mllH**. | Major MLL feature detected at **m/z 799.4232** (positive) / **797.4092** (negative); exogenous purified MLL at **50 nM** increased growth yield of biosynthesis mutants on NdCl3. | Zytnick et al., **2022**, **bioRxiv**. https://doi.org/10.1101/2022.01.19.476857 ; DOI: 10.1101/2022.01.19.476857 (pqac-00000003, pqac-00000004) |
| **Methylolanthanin structure inferred from cluster output** | Experimental product characterization with gene-function inference | MLL contains a **central citrate** linked to two **4-hydroxybenzoate** moieties through **acetylated homospermidine** residues. This product structure is consistent with, and indirectly supports, the prediction that **MllH performs an acetylation step**, likely on a polyamine-derived intermediate; however, attribution to MllH remains inferential. | Structural characterization by UPLC-MS/MS and NMR; no kinetic constants or substrate panel for MllH reported. | Zytnick et al., **2022**, **bioRxiv**. https://doi.org/10.1101/2022.01.19.476857 ; DOI: 10.1101/2022.01.19.476857 (pqac-00000004) |
| **mllH localization / subcellular site of action** | Inference from absence of targeting evidence | No direct localization data were reported for MllH. Given its annotation as a small-molecule GNAT biosynthetic enzyme in a metabolite assembly pathway, the most defensible current annotation is **unknown; likely intracellular/cytosolic**, but **unverified**. | No localization measurements available. | No direct localization experiment in the available Zytnick et al. evidence set (pqac-00000000, pqac-00000001) |


*Table: This table separates direct gene-specific evidence for mllH from stronger but broader cluster-level evidence for the methylolanthanin biosynthetic locus in *Methylorubrum extorquens* AM1. It is useful for showing what is experimentally supported versus what remains a bioinformatic inference for UniProt C5B1I9.*