Regulation of lanthanide-dependent methanol oxidation pathway in the legume symbiotic nitrogen-fixing bacterium Bradyrhizobium sp. strain Ce-3

Plasma-Membrane-Localized Transporter NREET1 is Responsible for Rare Earth Element Uptake in Hyperaccumulator Dicranopteris linearis

Rare earth elements (REEs) are critical for numerous modern technologies, and demand is increasing globally; however, production steps are resource-intensive and environmentally damaging. Some plant species are able to hyperaccumulate REEs, and understanding the biology behind this phenomenon could play a pivotal role in developing more environmentally friendly REE recovery technologies. Here, we identified a REE transporter NRAMP REE Transporter 1 (NREET1) from the REE hyperaccumulator fern Dicranopteris linearis. Although NREET1 belongs to the natural resistance-associated macrophage protein (NRAMP) family, it shares a low similarity with other NRAMP members. When expressed in yeast, NREET1 exhibited REE transport capacity, but it could not transport divalent metals, such as zinc, nickel, manganese, or iron. NREET1 is mainly expressed in D. linearis roots and predominantly localized in the plasma membrane. Expression studies in Arabidopsis thaliana revealed that NREET1 functions as a transporter mediating REE uptake and transfer from root cell walls into the cytoplasm. Moreover, NREET1 has a higher affinity for transporting light REEs compared to heavy REEs, which is consistent to the preferential enrichment of light REEs in field-grown D. linearis. We therefore conclude that NREET1 may play an important role in the uptake and consequently hyperaccumulation of REEs in D. linearis. These findings lay the foundation for the use of synthetic biology techniques to design and produce sustainable, plant-based REE recovery systems.

Draft Genome Sequence of a Lanthanide-Responsive Bacterium, Bradyrhizobium sp. Strain Ce-3

We present the draft genome sequence of Bradyrhizobium sp. strain Ce-3, which produced exopolysaccharide (EPS) and oxidized methanol in the presence of Ce 3+ . The genome for strain Ce-3 was estimated at 7,608,996 bp and showed that the strain is closely related to Bradyrhizobium erythrophlei MT12 and Bradyrhizobium sp. strain C9.

Biosynthesis of Polyhydroxyalkanoate Terpolymer from Methanol via the Reverse β-Oxidation Pathway in the Presence of Lanthanide

Methylorubrum extorquens AM1 is the attractive platform for the production of value-added products from methanol. We previously demonstrated that M. extorquens equipped with PHA synthase with broad substrate specificity synthesized polyhydroxyalkanoates (PHAs) composed of (R)-3-hydroxybutyrate and small fraction of (R)-3-hydroxyvalerate (3HV) and (R)-3-hydroxyhexanoate (3HHx) units on methanol. This study further engineered M. extorquens for biosynthesis of PHAs with higher 3HV and 3HHx composition focusing on the EMC pathway involved in C₁ assimilation. The introduction of ethylmalonyl-CoA decarboxylase, catalyzing a backward reaction in the EMC pathway, aiming to increase intracellular propionyl/butyryl-CoA precursors did not affect PHA composition. Reverse β-oxidation pathway and subsequent (R)-specific hydration of 2-enoyl-CoA were then enhanced by heterologous expression of four genes derived from Ralstonia eutropha for the conversion of propionyl/butyryl-CoAs to the corresponding (R)-3-hydroxyacyl-CoA monomers. The resulting strains produced PHAs with higher 3HV and 3HHx compositions, while the methylotrophic growth was severely impaired. This growth impairment was interestingly restored by the addition of La³⁺ without a negative impact on PHA biosynthesis, suggesting the activation of the EMC pathway by La³⁺. The engineered M. extorquens synthesized PHA terpolymer composed of 5.4 mol% 3HV and 0.9% of 3HHx with 41% content from methanol as a sole carbon source in the presence of La³⁺.

Discovery of lanthanide-dependent methylotrophy and screening methods for lanthanide-dependent methylotrophs

The lanthanide elements (Lns) affect the physiology and growth of certain microorganisms known as "Ln-responsive microorganisms." Among them, in 2011, it was first reported that strains of Methylobacterium exhibited high methanol dehydrogenase (MDH) activity when grown in the presence of Lns; the purified Ln-inducible MDH was identified as XoxF-type MDH, whose catalytic function had previously been unknown. XoxF was the first enzyme to be identified as Ln-dependent, and its function in methylotrophy is more fundamental and important than that of the corresponding Ca²⁺-dependent MDH MxaFI. XoxF is encoded in the genomes of methylotrophic as well as non-methylotrophic bacteria. Thus, Lns are among the most fascinating and important growth factors for bacteria that potentially utilize methanol. Bacteria that require Lns for methanol growth are called "Ln-dependent methylotrophs." Recent findings indicate that these microorganisms comprise an "Ln-dependent ecosystem" that we have not been able to reconstruct under laboratory conditions without Lns. In this chapter, we summarize methods for (1) screening of Ln-responsive microorganisms, (2) purification of native XoxFs from Ln-dependent methylotrophs, and (3) screening of Ln-dependent methylotrophs from natural environments, while providing a history of the discovery of the Ln-dependent methylotrophs.