A full-length cDNA clone of an aphid non-transmissible isolate of plum pox potyvirus (PPV) was rendered biologically active when placed under the control of the cauliflower mosaic virus 35S RNA promoter and the nopaline synthase polyadenylation signal. The cDNA was constructed so that the exact 5' end of the PPV RNA was present at the transcription initiation site.Inoculation of plasmid DNA onto Nicotiana benthamiana led to systemic infection, whereas local lesions were produced in Chenopodium amaranticolor and C.quinoa, typical of an infection with PPV. Examination of infected plants revealed PPV-specific virus particles as well as viral RNA, the coat protein and the nonstructural large nuclear inclusion protein (NIb).Potyviruses belong to the largest group of plant viruses and cause serious diseases in a number of economically important crop plants. Members of the potyvirus group are flexuous rods 720 to 900 nm in length (Hollings & Brunt, 1981) and encapsidate a single positive-sense RNA molecule of about 10 kb, attached to a 5'-linked protein (VPg) and including a poly(A) tract at the 3' end. The RNA is translated into a large polyprotein which is subsequently processed by at least two virus-encoded proteases. Upon proteolytic cleavage this polyprotein yields the viral coat protein (CP) and the non-structural proteins helper component-protease, cylindrical inclusion protein and two nuclear inclusion proteins (NIa and NIb). In addition, at least two other gene products which have not yet been identified are predicted from the nucleotide sequence of all potyviruses sequenced (Allison et al., 1986;Balint et al., 1990;Domier et al., 1986;Lain et al., 1989;Maiss et al., 1989;Robaglia et al., 1989).The in vitro synthesis of biologically active RNAs from full-length cDNA clones has been reported for two potyviruses (Domier et al., 1989;Riechmann et al., 1990). Genetic manipulation of full-length cDNA together with the subsequent transcription of infectious RNA provide a powerful tool for the examination of the molecular biology of RNA viruses. Moreover, it has been shown that infectious RNA can be transcribed in vivo by unknown promoters from plasmids containing cDNA copies of RNA 3 of alfalfa mosaic virus (Dore & Pinck, 1988), tobacco necrosis satellite RNA (van Emmelo et al., 1987) and potato spindle tuber viroid (Crees et al., 1983). Recently, Mori et al. (1991) have described the infectivity of plasmids containing the 35S RNA promoter of cauliflower mosaic virus (CaMV) upstream of fulllength cDNAs of all three brome mosaic virus (BMV) RNAs. Also, Commandeur et al. (1991) have demonstrated the infectivity of clones containing the 35S RNA promoter and cDNAs of RNA 3 and RNA 4 of beet necrotic yellow vein virus when complemented with a virus isolate which contains only RNA 1 and RNA 2. In this paper, the construction of a plasmid containing a full-length cDNA of an aphid non-transmissible isolate of plum pox potyvirus (PPV-NAT;Maiss et al., 1989) linked to the 35S RNA promoter of CaMV is described. PPV host plan...