title: Whole-genome sequencing of Mesorhizobium huakuii 7653R provides molecular insights into host specificity and symbiosis island dynamics.
abstract: BACKGROUND
  Evidence based on genomic sequences is urgently needed to confirm the phylogenetic relationship between Mesorhizobium strain MAFF303099 and M. huakuii. To define underlying causes for the rather striking difference in host specificity between M. huakuii strain 7653R and MAFF303099, several probable determinants also require comparison at the genomic level. An improved understanding of mobile genetic elements that can be integrated into the main chromosomes of Mesorhizobium to form genomic islands would enrich our knowledge of how genome dynamics may contribute to Mesorhizobium evolution in general.
  
  
  RESULTS
  In this study, we sequenced the complete genome of 7653R and compared it with five other Mesorhizobium genomes. Genomes of 7653R and MAFF303099 were found to share a large set of orthologs and, most importantly, a conserved chromosomal backbone and even larger perfectly conserved synteny blocks. We also identified candidate molecular differences responsible for the different host specificities of these two strains. Finally, we reconstructed an ancestral Mesorhizobium genomic island that has evolved into diverse forms in different Mesorhizobium species.
  
  
  CONCLUSIONS
  Our ortholog and synteny analyses firmly establish MAFF303099 as a strain of M. huakuii. Differences in nodulation factors and secretion systems T3SS, T4SS, and T6SS may be responsible for the unique host specificities of 7653R and MAFF303099 strains. The plasmids of 7653R may have arisen by excision of the original genomic island from the 7653R chromosome.
authors.forename: Shanming
authors.initials: S
authors.lastname: Wang
authors.forename: Baohai
authors.initials: B
authors.lastname: Hao
authors.forename: Jiarui
authors.initials: J
authors.lastname: Li
authors.forename: Huilin
authors.initials: H
authors.lastname: Gu
authors.forename: Jieli
authors.initials: J
authors.lastname: Peng
authors.forename: Fuli
authors.initials: F
authors.lastname: Xie
authors.forename: Xinyin
authors.initials: X
authors.lastname: Zhao
authors.forename: Christian
authors.initials: C
authors.lastname: Frech
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan, Hubei, P, R, China. chenn@sfu.ca.
authors.forename: Nansheng
authors.initials: N
authors.lastname: Chen
authors.forename: Binguang
authors.initials: B
authors.lastname: Ma
authors.forename: Youguo
authors.initials: Y
authors.lastname: Li
mesh_terms: D019143:Evolution, Molecular; D016680:Genome, Bacterial; D058507:Host Specificity; D059128:Mesorhizobium / Q000145:classification / Q000235:genetics* / Q000502:physiology; D008969:Molecular Sequence Data; D010802:Phylogeny; D018521:Plant Physiological Phenomena; D010944:Plants / Q000382:microbiology; D017422:Sequence Analysis, DNA; D013559:Symbiosis