A full-length cDNA clone of the 9786 nt plum pox virus (PPV) RNA genome has been cloned downstream from a phage T7 RNA polymerase promoter. The RNAs synthesized by in vitro run-off transcription in the presence of the 5' cap analog m7GpppG were infectious in Nicotiana clevelandii plants. No infectivity was detected when the transcriptions were carried out in the absence of the cap analog. Inoculations of the local lesion host Chenopodium foetidum indicated that the infectivity of the synthetic transcripts was about 1% of that of the native viral RNA. An extra G present at the 5' terminus of the transcripts was lost during their replication in plants, and the typical length distribution of the poly(A) tails was recovered. The viral RNA recovered from transcript-infected plants had approximately the same specific infectivity as native viral RNA. A G/A sequence heterogeneity found between different cDNA subgenomic clones was used to demonstrate that the infections were caused by the in vitro transcripts and were not the result of contamination.