Abstract
Dual flagellar systems have been described in several bacterial genera, but the extent of their prevalence has not been fully explored. Bradyrhizobium diazoefficiens USDA 110T possesses two flagellar systems, the subpolar and the lateral flagella. The lateral flagellum of Bradyrhizobium displays no obvious role, since its performance is explained by cooperation with the subpolar flagellum. In contrast, the lateral flagellum is the only type of flagella present in the related Rhizobiaceae family. In this work, we have analyzed the phylogeny of the Bradyrhizobium genus by means of Genome-to-Genome Blast Distance Phylogeny (GBDP) and Average Nucleotide Identity (ANI) comparisons of 128 genomes and divided it into 13 phylogenomic groups. While all the Bradyrhizobium genomes encode the subpolar flagellum, none of them encodes only the lateral flagellum. The simultaneous presence of both flagella is exclusive of the B. japonicum phylogenomic group. Additionally, 292 Rhizobiales order genomes were analyzed and both flagellar systems are present together in only nine genera. Phylogenetic analysis of 150 representative Rhizobiales genomes revealed an uneven distribution of these flagellar systems. While genomes within and close to the Rhizobiaceae family only possess the lateral flagellum, the subpolar flagellum is exclusive of more early-diverging families, where certain genera also present both flagella.
Highlights
Many of the diverse lineages that integrate the Bacteria domain have a free-living planktonic state as an important part of their lifestyles
These 13 phylogenomic groups (PG) are in full agreement according to the Modified Rand Index (MRI = 1) with the reference partition using a distance threshold T = 0.153 within the Genome Blast Distance Phylogeny (GBDP) distances, which equals 28.1% digital DNA-DNA hybridization
The same 13 PGs with a MRI = 0.993 were obtained by clustering intergenomic distances calculated with ANIb using T = 0.15 (Supplementary Data S2), which equals an ANIb of 85%
Summary
Many of the diverse lineages that integrate the Bacteria domain have a free-living planktonic state as an important part of their lifestyles. In planktonic state, bacteria synthesize extracellular structures known as flagella that are used for chemotactic motility. Flagella are composed of a basal body that anchors the flagellar structure to the cell surface, a long filament protruding from the cell body, and a hook that serves as a connector between the basal body and the filament. The basal body contains the flagellar motor, consisting of a stator (MotAB or PomAB, according to the motive force) anchored to the cytoplasmic membrane and peptidoglycan, and a rotor that moves the extracellular substructure. This rotor is composed of the MS-ring (FliF), the C-ring (FliGMN) and the rod (FlgBCFG). A substantial amount of protein synthesis is required to maintain the flagellar system throughout the cell cycle, and considerable energy is dedicated to motor rotation
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