mllG

Uncharacterized DUF2218-family protein (mllG, locus MexAM1_META1p4136) encoded within the methylolanthanin (mll) biosynthetic gene cluster (META1p4129-4138) of Methylorubrum extorquens AM1. The mll cluster directs biosynthesis and uptake of methylolanthanin, the first characterized biological lanthanide chelator (lanthanophore), which solubilizes poorly bioavailable lanthanides (rare earth elements) that are essential cofactors for XoxF-type lanthanide-dependent methanol dehydrogenases. The mll locus is the most highly induced region (~32-fold) when cells are grown with poorly soluble Nd2O3 rather than soluble NdCl3. The UniProt name "2,4-dihydroxyhept-2-ene-1,7-dioic acid aldolase" is a ProtNLM (Google) machine-generated prediction (ECO:0008006; PE 4 Predicted) that is NOT supported by any experimental enzymology or by the methylolanthanin literature; no aldolase reaction, substrate, or kinetics has been demonstrated for mllG. Instead, mllG belongs to DUF2218 (Pfam PF09981), is also present in the homologous rhodopetrobactin biosynthetic locus, and its contextual homolog Vibrio cholerae VCA0233 neighbors iron-uptake regulation and xeno-siderophore uptake genes, leading the discoverers to propose that DUF2218 proteins act in regulation or transport rather than in biosynthesis. The precise molecular function and subcellular localization of mllG remain undetermined; no specific GO molecular function is asserted here because none is supported by the available evidence.