Geminiviruses: a tale of a plasmid becoming a virus
Department of Biological and Environmental Sciences and Institute of Biotechnology, Biocenter 2, P.O. Box 56 (Viikinkaari 5), FIN-00014 University of Helsinki, Helsinki, Finland
BMC Evolutionary Biology 2009, 9:112 doi:10.1186/1471-2148-9-112Published: 21 May 2009
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.
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.
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.