Conservation of type III secretion system genes in Bradyrhizobium isolated from soybean

The distribution of rhcRST genes encoding the type III secretion system (T3SS) in a collection of Bradyrhizobium strains was characterized by PCR and Southern blot hybridization. The polymorphism of the corresponding sequences amplified by PCR was characterized by RFLP and sequencing together with those available in the databank. Genomic group I is characterized by the presence of Bradyrhizobium elkanii strains and group II by the presence of B. japonicum and B. liaoningense strains. Highly conserved T3SS-like genes were detected by PCR in all Bradyrhizobium strains isolated from soybean belonging to genomic group II, and in none of the strains belonging to genomic group I. These data were confirmed by Southern blot hybridization that further indicated the presence of sequences showing similarity to the rhcRST sequence in B. elkanii strains. The high level of conservation of rhcRST among Bradyrhizobia of genomic group II and sharing the same host-plant suggests that T3SS-like genes might have undergone horizontal genetic transfer within this genomic group. When considering the three Rhizobiaceae genera, a clear congruence was recorded between the rhcRST, rRNA gene and ITS sequences in bacteria harbouring sequences encoding T3SS, suggesting a relatively ancient emergence of the T3SS in these genera.