On the stability of sequences inserted into viral genomes

Willemsen, Anouk; Zwart, Mark P

Interestingly, as for the DNA viruses, the site and size of the insert seems to be important for ssRNA(þ) viruses. First, the positioning of the CAT gene downstream (instead of upstream) of the TMV coat protein, resulted in a poorly replicating virus that was not able to systematically infect the host plants (Dawson et al. 1989) . And second, the DHFR gene (238 bp) inserted in a TMV background appears to be maintained stably through several passages, while the 3.5Â larger NPT gene (832 bp) in the same experimental setup was unstable during systemic movement of the virus. This may also be related to the nature of the insert, where sequences with a codon usage similar to that of the viral vector may be retained longer than those that have an opposite codon usage. Interestingly, Chung, Canto, and Palukaitis (2007) generated recombinant plant viruses with inserted genes of unrelated plant viruses and observed instability and variation in the rate of partial or complete loss of the insert depending on the inserted sequence itself, the host used, or the viral vector used (Chung, Canto, and Palukaitis 2007) . Also sequences with a high toxicity for the host, are more likely to become deleted faster or to impede viral replication.