title: Geminiviruses: a tale of a plasmid becoming a virus.
abstract: BACKGROUND
  Geminiviruses (family Geminiviridae) are small single-stranded (ss) DNA viruses infecting plants. Their virion morphology is unique in the known viral world - two incomplete T = 1 icosahedra are joined together to form twinned particles. Geminiviruses utilize a rolling-circle mode to replicate their genomes. A limited sequence similarity between the three conserved motifs of the rolling-circle replication initiation proteins (RCR Reps) of geminiviruses and plasmids of Gram-positive bacteria allowed Koonin and Ilyina to propose that geminiviruses descend from bacterial replicons.
  
  
  RESULTS
  Phylogenetic and clustering analyses of various RCR Reps suggest that Rep proteins of geminiviruses share a most recent common ancestor with Reps encoded on plasmids of phytoplasmas, parasitic wall-less bacteria replicating both in plant and insect cells and therefore occupying a common ecological niche with geminiviruses. Capsid protein of Satellite tobacco necrosis virus was found to be the best template for homology-based structural modeling of the geminiviral capsid protein. Good stereochemical quality of the generated models indicates that the geminiviral capsid protein shares the same structural fold, the viral jelly-roll, with the vast majority of icosahedral plant-infecting ssRNA viruses.
  
  
  CONCLUSIONS
  We propose a plasmid-to-virus transition scenario, where a phytoplasmal plasmid acquired a capsid-coding gene from a plant RNA virus to give rise to the ancestor of geminiviruses.
authors.affiliation: Department of Biological and Environmental Sciences and Institute of Biotechnology, Biocenter 2, PO Box 56 (Viikinkaari 5), FIN-00014 University of Helsinki, Helsinki, Finland. mart.krupovic@helsinki.fi
authors.forename: Mart
authors.initials: M
authors.lastname: Krupovic
authors.forename: Janne J
authors.initials: JJ
authors.lastname: Ravantti
authors.forename: Dennis H
authors.initials: DH
authors.lastname: Bamford
mesh_terms: D000595:Amino Acid Sequence; D001499:Bayes Theorem; D036022:Capsid Proteins / Q000235:genetics*; D016000:Cluster Analysis; D019143:Evolution, Molecular; D017791:Geminiviridae / Q000235:genetics*; D005814:Genes, Viral; D016679:Genome, Viral; D016013:Likelihood Functions; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D045582:Phytoplasma / Q000235:genetics; D010957:Plasmids; D017433:Protein Structure, Secondary; D012093:Replicon; D016415:Sequence Alignment
chemical_list: D036022:Capsid Proteins