REPLICATION SPECIFICITY ELEMENTS OF THE WORLAND STRAIN OF BEET CURLY TOP VIRUS ARE COMPATIBLE WITH THOSE OF THE CFH STRAIN BUT NOT THOSE OFTHE CAL LOGAN STRAIN/

Cloned genomes of the CFH, Worland, and Cal/Logan strains of beet curl y top virus (BCTV) served as helper viruses to trans-replicate defecti ve (D) DNAs that are incapable of self-replication due to deletions wi thin the C1 open reading frame encoding the replication initiator (Rep ) protein. The Logan Rep protein could trans-replicate a Logan-derived D DNA in a transient replication assay conducted in Nicotiana bentham iana leaf disks. However, the Logan Rep protein was unable to trans-re plicate D DNAs derived from the CFH or Worland strains. In contrast, t he Rep proteins of the CFH and Worland strains could trans-replicate C FH or Worland D DNAs, but not a Logan D DNA. These results indicate th at the cis- and trans-acting replication specificity elements of the C FH and Worland strains are compatible and that the three strains of BC TV may be divided into two groupings based upon replication specificit y determinants. A comparison of amino acid sequences of the Rep protei n for the three BCTV strains suggests that the trans acting replicatio n specificity element may reside in one or more of 12 amino acid resid ues that are identical; in two amino acid residues that are chemically similar among the CFH and Worland Rep proteins, yet are different in the Logan Rep protein; or in both. Properties including replication sp ecificity, nucleotide sequence identity, and symptom expression were u sed as criteria to propose separate species designations for each of t he three BCTV strains. In this proposal, the Cal/ Logan strain retains the name BCTV, CFH and the closely related Iranian isolate are design ated beet severe curly top virus, and Worland is designated beet mild curly top virus.