DNA methylation plays crucial roles in transcriptional regulation, genomic imprinting, X chromosome inactivation and long-term repression (3, 4).
Long-term repression in the course of replicative senescence requires the ability to maintain localized silencing through many cell divisions.
The long-term repression of these genes would have important consequences for the plant's capacity to withstand biotic stress under environmental conditions.
In the case of MeCP2, the protein is capable of binding to a single symmetrically methylated cytosine and contributing to the long-term repression of transcription [ 2].
The PcG proteins are present in all cells, but they impose long-term repression on different target genes in different lineages.
Promoter hypermethylation is a causal event for long-term repression of BVES and POPDC3, whereas EGF stimulation is an immediate repression mechanism for both genes in gastric cancer.
These factors, named after their founding member Polycomb (Lewis 1978), are required for long-term repression of HOX and other developmental regulator genes.
Genetic studies in Drosophila first identified Polycomb group (PcG) genes as regulators that are required for the long-term repression of HOX genes during development (reviewed in Ringrose and Paro, 2004).
These apparently opposite histone modifications can be found either as sharply localized peaks (for example, around promoters) or as more extended domains structuring the genome into 'open' regions of activation or competence (euchromatin), and 'close' regions of long-term repression (heterochromatin).
The presence of the methylated cytosines correlates with gene activity and suggests that very few, or even a single methylated cytosine mark, is sufficient for long-term repression of gene expression, thus adding one further example of a specific cytosine correlating with gene expression [ 28].
