Plasmid DNA Containing a Copy of RNA3 Can Substitute for RNA3 in Alfalfa Mosaic Virus RNA Inocula

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 coinoculation with RNAs 1, 2 and 4 from strain B (mix Io) 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 Io 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 Io were protected from degradation by base-pairing and were therefore active.