| Item | Evidence summary | Primary source(s) with year and URL |
|---|---|---|
| Identity | Target protein is mllA in *Methylorubrum extorquens* AM1, corresponding to locus META1p4132 / MexAM1_META1p4132; it is part of the methylolanthanin (mll) biosynthetic gene cluster and is annotated as a siderophore synthetase component consistent with an IucA/IucC-like NRPS-independent siderophore synthetase. (pqac-00000000, pqac-00000030) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857 |
| Gene/locus mapping | Zytnick et al. explicitly list META1p4132-4135 as mllA, mllBC, mllDE, and mllF, establishing that META1p4132 maps to mllA in the mll locus. The same source places the cluster within META1p4129-4138. (pqac-00000000, pqac-00000030) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857 |
| Protein family/domains | Direct domain architecture for C5B1I4 is provided by UniProt in the user prompt, and the literature-supported family-level assignment is IucA/IucC-like / NIS synthetase. Mechanistic work on IucA/IucC-family enzymes supports interpreting mllA as an ATP-dependent amide-bond-forming carboxylate:amine ligase in siderophore/metallophore assembly. (pqac-00000001, pqac-00000021) | Mydy et al. 2020, Biochemistry, https://doi.org/10.1021/acs.biochem.0c00250; Gulick et al. 2024, Methods in Enzymology, https://doi.org/10.1016/bs.mie.2024.06.012 |
| Reaction class | IucA/IucC-family NIS synthetases catalyze ATP-dependent amide formation between a carboxylate and an amine/hydroxamate via an acyl-adenylate intermediate, releasing PPi then AMP. For homologs such as mllA, this supports annotation as an adenylating amide-bond-forming siderophore/metallophore synthetase rather than a transporter or redox enzyme. (pqac-00000016, pqac-00000021, pqac-00000022) | Mydy et al. 2020, Biochemistry, https://doi.org/10.1021/acs.biochem.0c00250; Gulick et al. 2024, Methods in Enzymology, https://doi.org/10.1016/bs.mie.2024.06.012 |
| Likely substrates | The exact mllA substrates were not directly measured in the provided snippets. By homology to IucA/IucC-family enzymes and to the mll cluster’s similarity to rhodopetrobactin/petrobactin/roseobactin pathways, mllA likely uses ATP plus a carboxylate acceptor such as citrate and an amine-containing donor in methylolanthanin assembly; the cluster is proposed to use citrate and 3,4-dihydroxybenzoate-derived chelating features. Family-wide substrate ranges include citrate, α-ketoglutarate, succinate or derivatives as carboxylates and hydroxamate/amine donors derived from lysine/ornithine or decarboxylated amines. (pqac-00000000, pqac-00000021) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Gulick et al. 2024, Methods in Enzymology, https://doi.org/10.1016/bs.mie.2024.06.012 |
| Pathway role | mllA is a biosynthetic component of the methylolanthanin pathway, which produces a lanthanide-binding metallophore (lanthanophore). The mll locus is proposed to synthesize methylolanthanin, and pathway perturbation changes lanthanide bioaccumulation and growth under low-bioavailability lanthanide conditions. (pqac-00000000, pqac-00000014, pqac-00000030) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857 |
| Cellular localization/compartment | mllA itself is a biosynthetic enzyme and is therefore most plausibly intracellular/cytosolic, but the provided snippets do not directly localize the protein. The broader lanthanide acquisition pathway spans extracellular chelation by methylolanthanin, outer-membrane uptake via TonB-dependent systems, periplasmic trafficking, and cytoplasmic storage as phosphate-containing deposits. A ferritin-like DUF4142 protein in the mll cluster is predicted to be exported to the periplasm, indicating compartmentalized pathway components. (pqac-00000000, pqac-00000007, pqac-00000009, pqac-00000014) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Roszczenko-Jasińska et al. 2020, Scientific Reports, https://doi.org/10.1038/s41598-020-69401-4 |
| Genomic context/neighbor genes | The mll biosynthetic region includes nearby uptake/regulatory genes META1p4129-4131 encoding a TonB-dependent outer-membrane receptor, anti-sigma factor, and sigma factor, followed by biosynthetic genes including mllA (META1p4132), mllBC, mllDE, mllF, mllG, mllH, and mllJ. This organization supports coupling of biosynthesis to export/import and transcriptional control. (pqac-00000000, pqac-00000002) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857 |
| Regulation/induction conditions | The mll locus is induced when lanthanide bioavailability is poor: expression is ~32-fold higher with poorly soluble Nd2O3 than with soluble NdCl3. In the same low-solubility condition, xoxF1 is upregulated 5-fold, linking lanthanophore production to lanthanide-dependent methanol oxidation. More generally, lanthanide uptake genes in the lut system are repressed by excess lanthanides. (pqac-00000000, pqac-00000030, pqac-00000007) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Roszczenko-Jasińska et al. 2020, Scientific Reports, https://doi.org/10.1038/s41598-020-69401-4 |
| Key experimental evidence/phenotypes | Evidence for the pathway containing mllA is strong at the cluster/product level: overexpression of MLL biosynthetic genes increases growth and lanthanide bioaccumulation; deletion causes severe defects; purified methylolanthanin binds lanthanides; exogenous MLL rescues growth of a biosynthesis mutant. Analogous evidence from *Methylobacterium aquaticum* 22A shows that IucA/IucC-containing siderophore clusters can solubilize lanthanide oxide and are required for methanol growth in specific genetic backgrounds, supporting the plausibility of mllA as a lanthanide-mobilizing metallophore synthetase. (pqac-00000000, pqac-00000023, pqac-00000026, pqac-00000030) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Juma et al. 2022, Frontiers in Microbiology, https://doi.org/10.3389/fmicb.2022.921635 |
| Key quantitative stats | mll/locus response: ~32-fold higher expression under Nd2O3 vs NdCl3; xoxF1 upregulated 5-fold under low-solubility lanthanide conditions. IucA-family benchmark kinetics: for hvKP IucA with ahLys, apparent KM = 0.79 ± 0.02 mM, kcat = 51.2 ± 0.5 min⁻¹, kcat/KM = 1,100 M⁻¹ s⁻¹; wild-type IucA shows ~100-fold preference for ahLys over N6-acetyllysine and a ~4-fold activity decrease with tricarballylic acid relative to citrate. Ecological prevalence/statistics from weathered rock metagenomes: ~1,900 BGCs identified across 136 genomes; 168 NRPS/PKS BGCs; metallophore-predictive TonB transporters co-occurred with 8 NRPS/PKS BGCs; three Acidobacteria had co-localized XoxF3-plus-putative metallophore systems and three more had non-colocalized combinations; no system similar to the AM1 lanthanophore cluster was observed. (pqac-00000004, pqac-00000019, pqac-00000022, pqac-00000030) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Mydy et al. 2020, Biochemistry, https://doi.org/10.1021/acs.biochem.0c00250; Voutsinos et al. 2024, BMC Biology, https://doi.org/10.1186/s12915-024-01841-0 |
| Current interpretation / confidence | mllA can be annotated with moderate-to-high confidence as an IucA/IucC-family NRPS-independent siderophore/metallophore synthetase component in methylolanthanin biosynthesis, but the exact reaction step and substrate specificity of the mllA protein itself have not been directly biochemically demonstrated in the provided evidence. Thus, pathway membership is experimentally supported, while enzyme-level chemistry remains inferred from family homology and related systems. (pqac-00000000, pqac-00000001, pqac-00000021) | Zytnick et al. 2022, bioRxiv, https://doi.org/10.1101/2022.01.19.476857; Mydy et al. 2020, Biochemistry, https://doi.org/10.1021/acs.biochem.0c00250; Gulick et al. 2024, Methods in Enzymology, https://doi.org/10.1016/bs.mie.2024.06.012 |


*Table: This table summarizes the evidence-supported functional annotation of mllA (META1p4132; UniProt C5B1I4) in Methylorubrum extorquens AM1, integrating direct cluster-level evidence with mechanistic inference from IucA/IucC-family enzymes. It is useful for distinguishing experimentally supported conclusions from homology-based inferences.*