Study of the genetic organisation of a plant viral RNA genome by in vitro expression of a full-length DNA copy.

The genetic approach for elucidating functions encoded by RNA plant viruses has been hampered by the lack of methods to select desired mutants following random mutagenesis. An alternative might be to copy RNA genomes into DNA and use methods for site-directed mutagenesis to modify specific regions of the DNA copy. Transcription of the DNA copy will subsequently produce viral RNA with desired mutations. We have constructed a full-length DNA copy of the smaller of the two cowpea mosaic virus (CPMV) RNAs, referred to as M RNA. The DNA copy was positioned downstream from the promoter of bacteriophage SP6 and using SP6 RNA polymerase, this copy and two derivatives of it containing a specific deletion and insertion, respectively, have been transcribed into RNA molecules which are efficiently translated in rabbit reticulocyte lysates. The results obtained show that the subsequent in vitro transcription and translation of DNA copies may be a powerful tool to unravel the genetic properties of viral RNA genomes.