STRAIN-SPECIFIC DETERMINANTS OF BEET CURLY TOP GEMINIVIRUS DNA-REPLICATION

The Logan and CFH strains of the geminivirus beet curly top virus (BCT V) possess cis- and trans-DNA replication factors which exhibit specif icity and are not functionally interchangeable. We demonstrate that th e cis-acting replication specificity element is entirely contained wit hin a 82- to 97-bp fragment which includes most of the Viral DNA origi n of replication. We also demonstrate that the strain-specific trans-a cting replication determinant is located within amino acid residues 3- 89 of the BCN C1 replication protein. Transient replication assays ind icated that chimeric BCTV genomes containing reciprocally exchanged re gions of the CFH and Logan genomes were replication competent when the cis- and trans-replication specificity elements were derived from the same strain. Two reciprocal chimeric viral genomes with heterologous cis- and trans-replication elements were incapable of self-replication , yet could trans-replicate one another in a coinoculation experiment. Only chimeric genomes possessing the Logan trans-replication element were capable of mobilizing and amplifying a transgenic Logan derived D I-DNA. DI-DNA mobilization and amplification occurred in transient rep lication assays even when the helper virus genome was incapable of sel f-replication, providing that the trans-replication element was derive d from the Logan strain. These results genetically define specific reg ions of the BCTV C1 replication protein determining viral DNA replicat ion origin recognition and provide clear evidence that strains of BCN have evolved specific cis- and trans-replication factors which functio nally define the Logan and CFH strains as distinct Viral agents. (C) 1 995 Academic Press, Inc.