ABSTRACT
A genome of single-stranded circular DNA has been shown to be a characteristic of all geminiviruses so far investigated in depth. Recent evidence concerning the leafhopper-transmitted viruses chloris striate mosaic virus (CSMV) and maize streak virus (MSV) has suggested that fundamental differences exist with respect to genome structure when compared to the whitefly-transmitted viruses. A more appropriate subdivision of the geminivirus group may well be made between members infecting either monocotyledons or dicotyledons. With respect to those members that infect monocotyledons, even a distant serological relationship has yet to be demonstrated. For those autonomous viruses with a single-stranded DNA genome other than the insect densonucleosis viruses, the primary event on invading a host cell is the synthesis of a second DNA strand complementary to the virion DNA. This function appears to be carried out by purely host-coded factors, although the mechanism varies between viruses.
