Identification of a biosynthetic gene cluster encoding a novel lanthanide chelator in Methylorubrum extorquens AM1

ABSTRACT While lanthanide-dependent metabolism is widespread in nature and has been proven to drive one-carbon metabolism in bacteria, details about the machinery necessary to sense, sequester, and traffic lanthanides (Ln) remain unknown. This gap in knowledge is in part because nearly all bacterial growth studies with Ln to date have used soluble chloride salts, compounds that do not reflect the insoluble Ln sources common in the natural environment. Here, we describe the changes in the metabolic machinery of Methylorubrum extorquens AM1 in response to poorly soluble Nd 2 O 3 , including 4-fold increases in orphan pqqA genes and the Ln-dependent ADHs xoxF1 and exaF compared to growth with soluble NdCl 3 . We report the first description of a Ln-chelator biosynthetic gene cluster, encoded by META1p4129 through META1p4138 that we named the Lanthanide Chelation Cluster (LCC). The LCC encodes a TonB dependent receptor and NRPS biosynthetic enzymes and is predicted to produce a metal-chelating molecule. As some LCC protein sequences share similarity to biosynthetic enzymes producing the Fe-chelating siderophore aerobactin, the capacity of aerobactin for binding Ln was tested. It was found that while aerobactin can bind lanthanum (La), neodymium (Nd) and lutetium (Lu) at physiological pH, providing only exogenous aerobactin did not affect growth rate or yield. The LCC was highly upregulated when M. extorquens AM1 was grown using Nd 2 O 3 and expression in trans enabled an increase of Nd bioaccumulation by over 50%. Expression of the LCC in trans did not affect iron bioaccumulation, providing further evidence that its product is a novel Ln-chelator. Finally, expression of the LCC in trans increased Nd, dysprosium (Dy), and praseodymium (Pr) bioaccumulation from the complex Ln source NdFeB magnet swarf by over 60%, opening new strategies for sustainable recovery of these critical Rare Earth Elements.