title: Distribution and diversity of mycoplasma plasmids: lessons from cryptic genetic elements.
abstract: BACKGROUND
  The evolution of mycoplasmas from a common ancestor with Firmicutes has been characterized not only by genome down-sizing but also by horizontal gene transfer between mycoplasma species sharing a common host. The mechanisms of these gene transfers remain unclear because our knowledge of the mycoplasma mobile genetic elements is limited. In particular, only a few plasmids have been described within the Mycoplasma genus.
  
  
  RESULTS
  We have shown that several species of ruminant mycoplasmas carry plasmids that are members of a large family of elements and replicate via a rolling-circle mechanism. All plasmids were isolated from species that either belonged or were closely related to the Mycoplasma mycoides cluster; none was from the Mycoplasma bovis-Mycoplasma agalactiae group. Twenty one plasmids were completely sequenced, named and compared with each other and with the five mycoplasma plasmids previously reported. All plasmids share similar size and genetic organization, and present a mosaic structure. A peculiar case is that of the plasmid pMyBK1 from M. yeatsii; it is larger in size and is predicted to be mobilizable. Its origin of replication and replication protein were identified. In addition, pMyBK1 derivatives were shown to replicate in various species of the M. mycoides cluster, and therefore hold considerable promise for developing gene vectors. The phylogenetic analysis of these plasmids confirms the uniqueness of pMyBK1 and indicates that the other mycoplasma plasmids cluster together, apart from the related replicons found in phytoplasmas and in species of the clade Firmicutes.
  
  
  CONCLUSIONS
  Our results unraveled a totally new picture of mycoplasma plasmids. Although they probably play a limited role in the gene exchanges that participate in mycoplasma evolution, they are abundant in some species. Evidence for the occurrence of frequent genetic recombination strongly suggests they are transmitted between species sharing a common host or niche.
authors.affiliation: University Bordeaux, UMR 1332 Biologie du Fruit et Pathologie, 71 avenue Edouard Bourlaux, F-33140, Villenave d'Ornon, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Breton
authors.forename: Florence
authors.initials: F
authors.lastname: Tardy
authors.forename: Emilie
authors.initials: E
authors.lastname: Dordet-Frisoni
authors.forename: Eveline
authors.initials: E
authors.lastname: Sagne
authors.forename: Virginie
authors.initials: V
authors.lastname: Mick
authors.forename: Joël
authors.initials: J
authors.lastname: Renaudin
authors.forename: Pascal
authors.initials: P
authors.lastname: Sirand-Pugnet
authors.forename: Christine
authors.initials: C
authors.lastname: Citti
authors.forename: Alain
authors.initials: A
authors.lastname: Blanchard
mesh_terms: D000818:Animals; D016000:Cluster Analysis; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics*; D023061:Gene Order; D022761:Gene Transfer, Horizontal; D014644:Genetic Variation; D008969:Molecular Sequence Data; D009175:Mycoplasma Infections / Q000382:microbiology / Q000662:veterinary; D009176:Mycoplasma mycoides / Q000235:genetics* / Q000302:isolation & purification; D010802:Phylogeny; D010957:Plasmids; D011995:Recombination, Genetic; D012418:Ruminants; D017422:Sequence Analysis, DNA
chemical_list: D004269:DNA, Bacterial