The C3 protein of tobacco curly shoot virus (TbCSV), a possible evolutionary intermediate between truly monopartite begomoviruses and those requiring satellite molecules for infectivity, has been identified as a viral replication enhancer (REn). However, the mechanisms underlying this enhancement are largely unknown. In this study, we generated two mutant infectious clones of TbCSV: one with a deletion of the 3' end region of the C3 gene that does not overlap with C2 (TbCSV