Tobacco Mosaic Virus (TMV) -
TMV has an RNA genome, which also serves as mRNA. Its genome has 4 genes of which the coat protein (cp) gene seems to be nonessential and can be the site of integration of a transgene.

The use of RNA viruses as vectors consists of two steps:

(1) use of cDNA copy of the viral genome for cloning in E. coli and for manipulation of the trans gene into the viral genome and
(2) in vitro transcription of the recombinant viral genome cDNA to produce infectious RNA copies to be used for plant infection. TMV CP is produced in large amounts; hence its promoter is suitable for an efficient expression of transgenes.