Methylotrophic Methylobacterium Bacteria Nodulate and Fix Nitrogen in Symbiosis with Legumes

Abstract

Rhizobia described so far belong to three distinct phylogenetic branches within the α-2 subclass ofProteobacteria. Here we report the discovery of a fourth rhizobial branch involving bacteria of theMethylobacterium genus. Rhizobia isolated fromCrotalaria legumes were assigned to a new species, “Methylobacterium nodulans,” within theMethylobacterium genus on the basis of 16S ribosomal DNA analyses. We demonstrated that these rhizobia facultatively grow on methanol, which is a characteristic ofMethylobacterium spp. but a unique feature among rhizobia. Genes encoding two key enzymes of methylotrophy and nodulation, the mxaF gene, encoding the α subunit of the methanol dehydrogenase, and the nodA gene, encoding an acyltransferase involved in Nod factor biosynthesis, were sequenced for the type strain, ORS2060. Plant tests and nodAamplification assays showed that “M. nodulans” is the only nodulating Methylobacterium sp. identified so far. Phylogenetic sequence analysis showed that “M. nodulans” NodA is closely related to BradyrhizobiumNodA, suggesting that this gene was acquired by horizontal gene transfer.