MAPPING REGIONS OF THE CAULIFLOWER MOSAIC-VIRUS ORF-III PRODUCT REQUIRED FOR INFECTIVITY

The open reading frame (ORF) III product (Pill) of the pararetrovirus cauliflower mosaic virus (CaMV) has nucleic acid-binding properties in vitro, but its biological role is not yet determined. ORF III is clos ely linked to ORF II and overlaps ORF IV out of frame in the CaMV geno me. A new CaMV-derived vector (Ca Delta) devoid of ORF III and contain ing unique restriction sites between ORFs II and IV was designed. Intr oduction of the wild-type CaMV ORF III into Ca Delta results in a clon e (Ca3) infectious in turnip plants. Truncated or point-mutated versio ns of ORF III were then inserted into Ca Delta and tested in vivo. Ino culation of the different mutants into turnip revealed that the four C -terminal amino acid residues of PIII are dispensable for infectivity as well as an internal domain (amino acids 61 to 80). Taken together t he results show that PIII possesses a functional two-domain organizati on. Moreover, the CaMV PIII function(s) cannot be replaced either by t he PIII protein of another caulimovirus, the figwort mosaic virus, or by the P2 protein of the cacao swollen shoot badnavirus, a member of t he second plant pararetrovirus group. (C) 1998 Academic Press.