Analysis of a Tobacco Transgene Locus That Triggers Both Transcriptional and Posttranscriptional Silencing

Abstract

INTRODUCTION
Numerous studies have demonstrated that introduction of a transgene in plants can result in transgenic plants which either express the transgene or not (for recent reviews, see Flavell 1994; Matzke and Matzke 1995). Although not all silencing phenomena have been studied in detail at the molecular level, it is known that gene silencing in plants can occur either at the transcriptional level —i.e., transcription is blocked —or at the posttranscriptional level — i.e., RNA turnover is increased.

Transcriptional Silencing
Transcriptional silencing is defined as cis-inactivation when it affects transcription of either single or multiple copies of a transgene inserted at a single locus (Linn et at. 1990; Meyer et al. 1993). It is defined as dominant trans-inactivation when a transgene driven by a given promoter inhibits transcription of unlinked transgenes driven by the same promoter (Matzke et al. 1989Matzke et al. 1994; Vaucheret 1993; Park et al. 1996).

Posttranscriptional Silencing
Posttranscriptional silencing events have been described as the result of an increase of the turnover of RNA encoded either by single or multiple copies of a transgene. Although the exact step at which silencing occurs remains unknown, it has been shown that RNAs are produced in the nucleus but do not accumulate in the cytosol (Dehio and Schell 1994; Ingelbrecht et al. 1994; Elmayan and Vaucheret 1996). Posttranscriptional silencing is known as cosuppression when it simultaneously affects endogenous host genes that are homologous to the introduced transgenes (Napoli et al. 1990; van der Krol et al. 1990; van Blockland et al. 1994;...