Lanthanide-Dependent Methanol and Formaldehyde Oxidation in Methylobacterium aquaticum Strain 22A

Lanthanides (Ln) are an essential cofactor for XoxF-type methanol dehydrogenases (MDHs) in Gram-negative methylotrophs. The Ln³⁺ dependency of XoxF has expanded knowledge and raised new questions in methylotrophy, including the differences in characteristics of XoxF-type MDHs, their regulation, and the methylotrophic metabolism including formaldehyde oxidation. In this study, we genetically identified one set of Ln³⁺- and Ca²⁺-dependent MDHs (XoxF1 and MxaFI), that are involved in methylotrophy, and an ExaF-type Ln³⁺-dependent ethanol dehydrogenase, among six MDH-like genes in Methylobacterium aquaticum strain 22A. We also identified the causative mutations in MxbD, a sensor kinase necessary for mxaF expression and xoxF1 repression, for suppressive phenotypes in xoxF1 mutants defective in methanol growth even in the absence of Ln³⁺. Furthermore, we examined the phenotypes of a series of formaldehyde oxidation-pathway mutants (fae1, fae2, mch in the tetrahydromethanopterin (H₄MPT) pathway and hgd in the glutathione-dependent formaldehyde dehydrogenase (GSH) pathway). We found that MxaF produces formaldehyde to a toxic level in the absence of the formaldehyde oxidation pathways and that either XoxF1 or ExaF can oxidize formaldehyde to alleviate formaldehyde toxicity in vivo. Furthermore, the GSH pathway has a supportive role for the net formaldehyde oxidation in addition to the H₄MPT pathway that has primary importance. Studies on methylotrophy in Methylobacterium species have a long history, and this study provides further insights into genetic and physiological diversity and the differences in methylotrophy within the plant-colonizing methylotrophs.