Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 28

Figure 1.8 Schematic of enzymatic mechanism of demethylation in pollen diploid veg-
etative cell (VC), ovary diploid central cell (CC), and triploid endosperm. Demethylation
triggers the synthesis of small RNAs (sRNAs), which move to the sperm cell (SC) of the
pollen and embryo cell (EC) of the ovary. These sRNAs are involved in the process of
RNA-directed DNA methylation (RdDM) in cells of the developing plant embryo.
cells), and mediates genome-wide demethylation in the triploid endosperm.
Parental alleles of both maternally and paternally expressed imprinted genes
are particularly important targets of DME ( Hsieh et al., 2011 ). DME has
activity similar to ROS1, which was initially identified in screens for trans-
gene silencing ( Gong et al., 2002; Ishitani et al., 1997 ). Inactivating muta-
tions in the ros1 gene lead to methylation and transcriptional silencing of
plant transgenes ( Gong et al., 2002 ) and endogenous genes ( Zhu et al.,
2007 ). ROS1 is a bifunctional protein that both removes 5-mC (glycosylase
activity) and cleaves the sugar-phosphate backbone at the abasic site (AP