Identification of a DNA methylation-independent imprinting control region at the Arabidopsis MEDEA locus.

Genomic imprinting is exclusive to mammals and seed plants and refers to parent-of-origin-dependent, differential transcription. As previously shown in mammals, studies in Arabidopsis have implicated DNA methylation as an important hallmark of imprinting. The current model suggests that maternally expressed imprinted genes, such as MEDEA (MEA), are activated by the DNA glycosylase DEMETER (DME), which removes DNA methylation established by the DNA methyltransferase MET1. We report the systematic functional dissection of the MEA cis-regulatory region, resulting in the identification of a 200-bp fragment that is necessary and sufficient to mediate MEA activation and imprinted expression, thus containing the imprinting control region (ICR). Notably, imprinted MEA expression mediated by this ICR is independent of DME and MET1, consistent with the lack of any significant DNA methylation in this region. This is the first example of an ICR without differential DNA methylation, suggesting that factors other than DME and MET1 are required for imprinting at the MEA locus.