In vitro transcription of infectious RNAs from full-length cDNAs of tobacco mosaic virus.

We have cloned full-length double-stranded cDNAs of tobacco mosaic virus (TMV) (tomato strain L) RNA into a transcription vector, pPM1, which facilitates the correct transcription initiation from the first nucleotide of the inserted double-stranded cDNA, corresponding to the 5' end of TMV RNA. When plasmid DNA is linearized at a unique restriction site (Mlu I) introduced just downstream of the double-stranded cDNA insert and used as a template for in vitro transcription by Escherichia coli RNA polymerase in the presence of m(7)GpppG, the transcribed RNAs are infectious for tobacco plants. A simple reconstitution procedure increases the infectivity >100 times. Unexpectedly, both the uncapped transcript and the transcript from the uncut plasmid DNA are also infectious, although their infectivities are very low. The progeny viruses multiplying in tobacco plants accurately reflect the cloned sequence. By the same method, we succeeded in the in vitro transcription of infectious RNA of attenuated strain L(11)A, which is phenotypically distinguishable from wild-type TMV on both tobacco and tomato plants.