On the stability of sequences inserted into viral genomes

Willemsen, Anouk; Zwart, Mark P

Shortly after, studies showed that gene insertion-rather than gene replacement-was better suited for expressing foreign genes in ssRNA(þ) viral genomes (Dawson et al. 1989; Donson et al. 1991; Chapman, Kavanagh, and Baulcombe 1992) . The chloramphenicol acetyltransferase (CAT) gene (Dawson et al. 1989) , and the dihydrofolate reductase (DHFR) and the neomycin phosphotransferase (NPT) genes (Donson et al. 1991) were successfully expressed in plants by using Tobacco mosaic virus (TMV) as a vector. In addition, the bacterial GUS gene has shown to successfully express when inserted into the viral genome of Potato virus X (PVX) (Chapman, Kavanagh, and Baulcombe 1992) . However, in all these cases the presence of a foreign gene leads to genomic instability resulting in the partial deletion of the GUS and NPT genes and a complete deletion of CAT during systemic infection. This instability may result from the presence of the insert leading to lower accumulation levels of the genomic RNA, as well as leading to mRNA instability and/or interfering with synthesis of the viral proteins.