Transgene silencing: New insights into an old puzzle

The use of transgenic organisms has become routine in biology in the last decade. The technology is now being put to practice for creating grains with increased protein content, fruits and vegetables with enhanced nutritional value and flowers with exotic colours. A common means of doing so is to try and overexpress the plant's own proteins. Often however, instead of producing large quantities of proteins, the manipulation leads to exactly the opposite result: it causes suppression of the host gene(s) as well as of copies inserted as transgenes ("co-suppression") and results in a strong reduction in both host and trans gene steady-state mRNA levels. This could occur as a result of methylation of DNA sequences, particularly promoter elements, leading to a decrease in transcription. In a number of cases however, the (trans)genes are apparently transcribed at normal rates in the nucleus, indicating that the suppression is post-transcriptional. When the trans genes contain a cDNA derived from the genome of an RNA virus, plants carrying the transgene can be resistant to the virus (Lindbo et al 1993). Although the effect has been documented in several systems, the mechanism by which it occurs remains unclear