THE STRAIN-SPECIFIC CIS-ACTING ELEMENT OF BEET CURLY TOP GEMINIVIRUS DNA-REPLICATION MAPS TO THE DIRECTLY REPEATED MOTIF OF THE ORI

Strains of beet curly top geminivirus (BCTV) possess distinct cis- and trans-acting replication specificity elements which are not separatel y interchangeable among strains. Analysis of the replication competenc y of chimeric BCTV genomes, in which portions of the origin of DNA rep lication (ori) were derived from heterologous BCTV strains, have permi tted identification of an essential cis-acting element governing strai n-specific replication in a subgroup II geminivirus. Our studies indic ate that the cis-acting element responsible for strain-specific replic ation properties resides within the directly repeated motif of the BCT V ori. Transient replication assays conducted in leaf disks and comple mentation experiments conducted in whole plants indicated that the tra ns-acting replication specificity element, residing within the amino-t erminal region of the C1 Rep protein, may recognize and replicate a ch imeric BCTV genome containing a heterologous ori so long as all or por tions of the core element of the directly repeated motif are derived f rom the same strain as the Rep protein. As Rep protein binding to the care element of the directly repeated motif has been demonstrated by o thers to be essential for replication of subgroup III geminiviruses, o ur results support the hypothesis that replication specificity of subg roup II viruses is governed by processes similar to that of subgroup I II viruses. However, a second cis-acting element of the ori, which app ears to contribute to subgroup III virus replication specificity, does not seem to be required far replication specificity among the subgrou p II viruses examined. Nonetheless, a potential role for a second cis- acting element in the BCTV ori contributing to maximal replication can not be excluded. (C) 1996 Academic Press, Inc.