A cDNA clone of tomato mosaic virus (ToMV) genomic RNA was fused to the cauliflower mosaic virus 35S RNA promoter and the nopaline synthase gene polyadenylation signal. The transcriptional initiation site of the 35S RNA promoter was altered by in vitro mutagenesis so that the resulting transcripts start at the first nucleotide of the ToMV sequence. In addition, 12 nucleotides were inserted in the 5' untranslated region of the ToMV genome. This plasmid, pSLN, was used to inoculate several host plants of ToMV. Among five plant species tested, only Chenopodium quinoa accumulated large amounts of viral particles. The infectivities and systemic movements of the resulting viruses were the same as those of virus preparations obtained from a ToMV infection of C. quinoa. Primer extension analyses revealed that the 5' end of the viral genomic RNA was identical to those of RNAs isolated from virus progeny of an infection with T7 transcripts analogous to pSLN. Moreover, the insertion in the 5' untranslated region of the viral genome was stably maintained through several systemic passages of the virus. Thus, inoculation of plants with a plasmid containing a cDNA clone of an RNA virus under the control of a eukaryotic promoter seems to be a convenient alternative to the generation of in vitro transcripts and should facilitate the analysis of viral mutants generated at the DNA level.