Use of rare‐earth elements in the phyllosphere colonizer Methylobacterium extorquens PA1

Abstract

Until recently, rare-earth elements (REEs) had been thought to be biologically inactive. This view changed with the discovery of the methanol dehydrogenase XoxF that strictly relies on REEs for its activity. Some methylotrophs only contain xoxF, while others, including the model phyllosphere colonizer Methylobacterium extorquens PA1, harbor this gene in addition to mxaFI encoding a Ca 2+ -dependent enzyme. Here we found that REEs induce the expression of xoxF in M. extorquens PA1, while repressing mxaFI, suggesting that XoxF is the preferred methanol dehydrogenase in the presence of sufficient amounts of REE. Using reporter assays and a suppressor screen, we found that lanthanum (La 3+ ) is sensed both in a XoxF-dependent and independent manner. Furthermore, we investigated the role of REEs during Arabidopsis thaliana colonization. Element analysis of the phyllosphere revealed the presence of several REEs at concentrations up to 10 μg per g dry weight. Complementary proteome analyses of M. extorquens PA1 identified XoxF as a top induced protein in planta and a core set of La 3+regulated proteins under defined artificial media conditions. Among these was a REE-binding protein that is encoded next to a gene for a TonB-dependent transporter.The latter was essential for REE-dependent growth on methanol indicating chelator-assisted uptake of REEs.

Biochemical and Structural Characterization of XoxG and XoxJ and Their Roles in Lanthanide‐Dependent Methanol Dehydrogenase Activity

“…As one example, whereas M. fumariolicum XoxF is tolerant of lanthanides up to Gd in vivo, M. extorquens cannot operate efficiently on lanthanides beyond Nd. It is possible that one aspect of these selectivity differences is dictated by differences in the cellular uptake machinery: M. fumariolicum is cultured at pH 2.7, at which all lanthanides are far more soluble than in culture conditions for M. extorquens (pH 6.75). As a result, secretion of a chelator highly selective for La–Nd uptake, as our studies in M. extorquens indicated, might be unnecessary for M. fumariolicum .…”

“…Recently, we demonstrated, using a genetically encoded fluorescent sensor for lanthanides, that only light lanthanides, La–Nd, are efficiently imported into the bacterial cytosol, a process likely facilitated by a secreted metallophore . Therefore, it appears that on multiple levels—including, at least, regulation, uptake, and trafficking,— M. extorquens is fine‐tuned to use solely light lanthanides.…”

Electrocatalysis of a Europium‐Dependent Bacterial Methanol Dehydrogenase with Its Physiological Electron‐Acceptor Cytochrome cGJ

“…[7a, 10] Among XoxF-MDHs, in the present study we were particularly interested in the Eu 3 + dependent MDH from Methylacidiphilum fumariolicum SolV.V ery recentlyt he crystal structures of this MDH wered etermined in the presence of Eu 3 + and Ce 3 + . [14] It is believed that the Lewis acidityo ft he coordinated trivalent lanthanide ion enhances the electrophilicity of C5 on the PQQ cofactor (Figure1)w hich is proposedt oa ct as ah ydride acceptord uring methanolo xidation. [12] The field of lanthanide-dependent bacterial metabolism is quicklyg rowing.…”

“…[13] Further,V orholt et al reported that in the uptake of Ln by M. extorquens aT onB related transporter is likely to be involved. [14] It is believed that the Lewis acidityo ft he coordinated trivalent lanthanide ion enhances the electrophilicity of C5 on the PQQ cofactor (Figure1)w hich is proposedt oa ct as ah ydride acceptord uring methanolo xidation. [15] The reduced( inactive) form of MDH (PQQH 2 )i sr eactivated by two sequential singleelectron transfers to the physiological c-type cytochrome electron acceptor, and PQQH 2 is oxidised back to the quinone via a semiquinone.…”

Lanthanide-dependent alcohol dehydrogenases require an essential aspartate residue for metal coordination and function

“…Lighter versions of Ln 3+ , ranging from lanthanum (La 3+ ) to europium (Eu 3+ ) (atomic numbers [57][58][59][60][61][62][63], excluding promethium (Pm 3+ ), have been shown to function with PQQ as essential cofactors in XoxF-type and ExaF-type ADHs. Ln-ADHs can be distinguished from MxaFI-type MDHs and ExaA-type EDHs that bind PQQ and coordinate Ca 2+ (26,27).…”