The Strain-Specificcis-Acting Element of Beet Curly Top Geminivirus DNA Replication Maps to the Directly Repeated Motif of theOri

AbstractStrains of beet curly top geminivirus (BCTV) possess distinctcis- andtrans-acting replication specificity elements which are not separately interchangeable among strains. Analysis of the replication competency of chimeric BCTV genomes, in which portions of the origin of DNA replication (ori) were derived from heterologous BCTV strains, have permitted identification of an essentialcis-acting element governing strain-specific replication in a subgroup II geminivirus. Our studies indicate that thecis-acting element responsible for strain-specific replication properties resides within the directly repeated motif of the BCTVori.Transient replication assays conducted in leaf disks and complementation experiments conducted in whole plants indicated that thetrans-acting replication specificity element, residing within the amino-terminal region of the C1 Rep protein, may recognize and replicate a chimeric BCTV genome containing a heterologousoriso long as all or portions of the core element of the directly repeated motif are derived from the same strain as the Rep protein. As Rep protein binding to the core element of the directly repeated motif has been demonstrated by others to be essential for replication of subgroup III geminiviruses, our results support the hypothesis that replication specificity of subgroup II viruses is governed by processes similar to that of subgroup III viruses. However, a secondcis-acting element of theori,which appears to contribute to subgroup III virus replication specificity, does not seem to be required for replication specificity among the subgroup II viruses examined. Nonetheless, a potential role for a secondcis-acting element in the BCTVoricontributing to maximal replication cannot be excluded