Gardening the genome: DNA methylation in Arabidopsis thaliana

DNA methylation is a conserved epigenetic modification of the genome that serves the dual roles of gene regulation and control of repetitive elements, such as transposons.The genome of Arabidopsis thaliana contains extensive DNA methylation and encodes four classes of cytosine methyltransferase (three of which are found in mammals). Genetic and genomic approaches in this species are providing the means to analyse the control of DNA methylation patterns.Cytosine methylation can occur in symmetric (CG) or non-symmetric (CNG or CHH) contexts. The establishment and maintenance of methylation in these contexts has different characteristics and uses different genetic pathways.Recent work has identified RNA as guiding much of DNA methylation. Components of the RNA-interference pathway produce siRNAs, which are able to target cytosine methyltransferases to homologous sequences.Histone modification is also a key process involved in maintaining patterns of DNA methylation.A new class of enzymes that function in a demethylation pathway has also been characterized, and has roles in gene silencing and imprinting.