We have investigated the relationship between viral DNA replication and virion sense gene expression in wheat dwarf virus (WDV), a member of the geminivirus group, by testing a series of deletion mutants in transfected Triticum monococcum (einkorn) protoplasts. Mutants contained a transcription fusion of the chloramphenicol acetyltransferase coding sequence to the virion sense promoter that replaced the viral coat protein coding sequence. The deletion analysis revealed that WDV replication and virion sense transcription can proceed independently and are controlled in part by nonoverlapping elements in the large intergenic region. These data and those from a C2 open reading frame (ORF) frameshift mutant also showed that the product of the C2 ORF (C1-C2 protein) is independently involved in both DNA replication and activation of the virion sense promoter. The amino acid sequences encoded by C2, which are highly conserved in the geminivirus group, show some homology to the DNA binding domain of the myb-related class of plant transcription factors. The possible involvement of the host in controlling the function of the C1-C2 protein and the implication of these data for the development of WDV-based gene vectors are discussed.