DNA Methylation of Flower Color Transgenes in Petunia hybrida

Abstract

Inactivation of transgene constructs that have been introduced into plants is a frequently reported phenomenon. Many such silencing events are associated with DNA methylation, although it is still a matter of debate whether de novo methylation is the cause or the consequence of gene inactivation.

A correlation between DNA methylation and gene inactivation is not limited to transgenes but is also observed for transposable elements (Chandler and Walbot 1986; Schwarz and Dennis 1986) and for some (Spena et al. 1983), but not all (Nick et al. 1986), endogenous genes. Interestingly, changes in methylation patterns that correlate with changes in tissue-specific expression have been found in distant upstream regions of some genes (Langdale et al. 1986), indicating that methylation-mediated control of gene expression does not exclusively affect promoter regions.

We can assign at least two distinct molecular functions to hyper-methylated sequences. First, it is conceivable that methylation patterns influence the binding of transcription factors to regulatory promoter elements. For a number of viral or human cellular promoters, position-specific methylation has been shown to inhibit or decrease promoter activity (Muiznieks and Doerfler 1994), supporting the assumption of a direct regulatory role for DNA methylation in transcription. Second, DNA methylation within a larger area may alter the local chromatin structure. This would also prevent transcription factors from accessing promoters within such regions due to the inaccessibility of the genomic region harboring the promoter, rather than the modification of a specific binding. Strong evidence for this indirect role of de novo methylation in the...