Transcriptional activator elements for curtovirus C1 expression reside in the 3' coding region of ORF C1

Beet curly top virus (BCTV) and Beet severe curly top virus (BSCTV), members of curtoviruses, encode seven open reading frames (ORFs) within a approximately 3 kb genome. One of these viral ORFs, C1, is known to play an important role in the early stage of viral infection in plants during initiation of viral DNA replication. We used promoter:: reporter (beta-glucuronidase) gene fusions in transgenic Ara-bidopsis to identify the putative promoter region of BCTV ORF C1. Unlike other geminiviruses, the intergenic region of BCTV was not sufficient to promote C1 expression in transgenic plants. When sequences extending into the coding region of C1 were tested, strong expression of the reporter protein was observed in vascular tissues of transgenic plants. This expression was not dependent on the presence of the intergenic regions or proximal 5' portions of the C1 coding region. Transgenic plants expressing a reporter gene under control of the putative complete C1 promoter were inoculated with virus to determine if any viral transcript affected C1 expression. Virus inoculated plants did not show any altered pattern or change in of reporter gene expression level. These results suggest that (1) important transcriptional activator elements for C1 expression reside in the 3' portion of C1 coding area itself, (2) C1 protein does not auto-regulate its own expression and (3) C1 expression of two curtoviruses is controlled differently compared to other geminiviruses.