The infectivity of a DNA copy of RNA3 from strain S of alfalfa mosaic virus inserted in a pT7-1 plasmid vector (clone 3pT7-1) was tested as circular DNA by co-inoculation with RNAs 1, 2 and 4 from strain B (mix I) on young tobacco leaves. Virus multiplication was monitored by serological detection of the coat protein. Progeny RNA3 derived from circular plasmid DNA 3pT7-1 was of the same length as naturally occurring RNA3-S. Analysis by primer extension demonstrated that it had the 5′-terminal sequence of RNA3-S and not that of RNA3-B. Mix I also became infectious when mixed with M13 single-stranded DNA carrying RNA3-S inserted in the (-) strand orientation, probably because traces of RNA3-B present in mix I were protected from degradation by base-pairing and were therefore active.


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