Novel insights into the genome organization of Rhizobiaceae: identification of linear plasmids

Members of the family <i>Rhizobiaceae</i> typically have multipartite genomes that are split between two or more replicons, including the chromosome and a variable number of extrachromosomal replicons (chromids and plasmids). Nearly all <i>Rhizobiaceae</i> replicons sequenced and described to date have a circular topology, except the linear chromid found in the genomes of most <i>Agrobacterium</i> spp. In this study, genomes of five non-pathogenic <i>Agrobacterium</i> strains and one plant tumourigenic <i>Allorhizobium</i> strain were fully sequenced. Surprisingly, genome analysis revealed that these six strains each carry an 80-kbp linear plasmid. Linear plasmids have so far not been identified in this bacterial family or other bacteria within the class <i>Alphaproteobacteria</i>. The ends of all six plasmids identified in this study have a hairpin structure with covalently closed ends. The plasmid sequences showed a high degree of homology, clearly indicating their common ancestry. Database searches led to the identification of additional linear plasmids in previously published <i>Rhizobiaceae</i> genome assemblies that were not previously recognized to have linear plasmids, suggesting that these replicons may be more widespread. Most likely, linear plasmids may be even more widely distributed than anticipated. Although the biological functions of the linear plasmids identified in this study remain unknown, they are associated with both non-pathogenic and plant tumourigenic <i>Rhizobiaceae</i> strains.