Complete Genome sequence of Burkholderia phymatum STM815(T), a broad host range and efficient nitrogen-fixing symbiont of Mimosa species

Abstract

Burkholderia phymatum is a soil bacterium able to develop a nitrogen-fixing symbiosis with species of the legume genus Mimosa, and is frequently found associated specifically with Mimosa pudica. The type strain of the species, STM 815(T), was isolated from a root nodule in French Guiana in 2000. The strain is an aerobic, motile, non-spore forming, Gram-negative rod, and is a highly competitive strain for nodulation compared to other Mimosa symbionts, as it also nodulates a broad range of other legume genera and species. The 8,676,562 bp genome is composed of two chromosomes (3,479,187 and 2,697,374 bp), a megaplasmid (1,904,893 bp) and a plasmid hosting the symbiotic functions (595,108 bp).

Keywords: Burkholderia; Mimosa; nitrogen fixation; rhizobia; symbiosis.

Figure 1

Transmission electron microscopy of B. phymatum STM815 (credit: Geoffrey Elliott).

Figure 2

Phylogenetic tree highlighting the position of Burkholderia phymatum strain STM815^T relative to other type strains within the genus Burkholderia. The 16S rDNA sequences from type strains were obtained from the ribosomal database project [13], aligned with muscle 3.6, and a neighbor-joining tree was built from a Kimura-2P corrected distance matrix using BioNJ on the www.phylogeny.fr server [14]. Numbers at nodes are % bootstraps from 1000 replicates (shown only if >50%). Accession numbers of 16S rDNA are indicated between parentheses for each strain. C. taiwanensis LMG19424^T was used as outgroup.

Figure 3

Circular maps of Chromosome 1 (left) and Chromosome 2 (right) of B. phymatum STM815^T. From outside to center: Genes on forward strand (color by COG categories as denoted by the IMG platform), Genes on reverse strand (color by COG categories), RNA genes (tRNAs green, sRNAs red, other RNAs black), GC content, GC skew. Replicons are not drawn to scale.

Figure 4

Circular maps of Plasmid 1 (left) and Plasmid 2 (right) of B. phymatum STM815^T. From outside to center: Genes on forward strand (color by COG categories as denoted by the IMG platform), Genes on reverse strand (color by COG categories), RNA genes (tRNAs green, sRNAs red, other RNAs black), GC content, GC skew. Replicons are not drawn to scale.

Figure 5

B. phymatum STM815^T was compared to 3 others Burkholderia strains from similar and different ecological niches: a legume symbiont (B. phenoliruptrix BR3459a, a Mimosa flocculosa nodule symbiont from Brazil [37,38]; a soil bacterium (B. xenovorans LB400) and a human opportunistic pathogen (B. cenocepacia AU1054). The core genomes of all four bacteria yielded 1,582 gene families. Each bacterium had more gene families specific to its species, (from 3,002 to 5,656 depending on strain) than shared ones (1,582 core gene families). There were 418 gene families specific to the two Mimosa symbionts (STM815 and BR3459a), including symbiosis-related genes (nod genes) and nitrogen fixation genes (nif, fix), glutamine transporters, biosynthesis genes of the phytohormone indol acetic acid (IAA), and hydrogenase genes (hup, hyp).