CONICET | Buscador de Institutos y Recursos Humanos

Bacterial motility is an important trait for processes such as adherence to host cells, host cell invasion, protein secretion, and biofilm formation. Bradyrhizobium diazoefficiens and B. japonicum can live into plant nodules as a symbiont, or in a planktonic, free-living state in the soil, where they can swim and swarm self-propelled by their flagellar systems. B. diazoefficiens USDA 110and B. japonicum USDA 6 possess two flagellar systems: a subpolar flagellum and some lateral flagella. This characteristic is shared with unrelated species such as Vibrio parahaemoliticus, Aeromonas hydrophila, Chromobacterium violaceum, Rhodobacter shaeroides, Rhodopseudomonas palustris and Azospirillum brasilense but not with other rhizobial species, which instead have a single peritrichous flagellar system similar to the lateral one of Bradyrhizobium. The presence of two flagellar systems in the same cell is intriguing, since each one consumes a large amount of cell energy and might interfere each other in motility and chemotaxis. Although in the above-mentioned species the lateral flagella were described as only required for motility on surfaces, in Bradyrhizobium both flagellar systems are expressed in liquid medium. Therefore, elucidation of the roles of these two flagellar systems in Bradyrhizobium requires more investigation. A possible approach is a comparative study of the genomic organization of these systems. The most related species with this characteristic is R. palustris. In this work we compared the genome distribution and synteny of lateral and subpolar flagellar genes in B. diazoefficiens USDA 110, B. japonicum USDA 6 and R. palustris BisA53. Genes encoding lateral flagella are located in a single cluster in Bradyrhizobium and in R. palustris. The regions are syntenic and sequences are highly conserved: around 90 % of identity between B. diazoefficiens and B. japonicum and 75% identity between each Bradyrhizobium sp. and R. palustris. An inversion of this cluster was observed between B. diazoefficiens and B. japonicum. In the case of Sinorhizobium meliloti or Rhizobium etli, they also have a cluster enclosing all the genes for the synthesis of the flagellum. The genes that encode the subpolar flagellum of B. diazoefficiens are distributed among four regions in the genome. This pattern of dispersed genes is similar also in B. japonicum and R. palustris and they are more divergent than the lateral cluster. The above observations are in agreement with the proposal that the lateral flagellum was acquired by horizontal transfer. In this way we can think that the high conservation of the lateralcluster is due to a recent insertion in the genomes, while the subpolar (primary) system evolved together with the organism. These results will aid us to pursue the study on the roles of each flagellum in the life cycle of Bradyrhizobium sp.