PLANT-VIRUS DNA-REPLICATION PROCESSES IN AGROBACTERIUM - INSIGHT INTOTHE ORIGINS OF GEMINIVIRUSES

Agrobacterium tumefaciens, a bacterial plant pathogen, when transforme d with plasmid constructs containing greater than unit length DNA of t omato leaf curl geminivirus accumulates viral replicative form DNAs in distinguishable from those produced in infected plants. The accumulati on of the viral DNA species depends on the presence of two origins of replication in the DNA constructs and is drastically reduced by introd ucing mutations into the viral replication-associated protein (Rep or C1) ORF, indicating that an active viral replication process is occurr ing in the bacterial cell. The accumulation of these viral DNA species is not affected by mutations or deletions in the other viral open rea ding frames. The observation that geminivirus DNA replication function s are supported by the bacterial cellular machinery provides evidence for the theory that these circular single-stranded DNA viruses have ev olved from prokaryotic episomal replicons.