Fig. 2

Relationship between acetylation and methylation levels for the switch from heterochromatin to euchromatin. Histone methylation as the tri-methylated of the 9th lysine residue of histone 3 tail, via histone methyltransferases (HMT), is globally involved in gene silencing. These modifications tend to recruit histone binding proteins (such as heterochromatin protein 1 (HP1)) that avoid chromatin relaxation, thus preventing transcription factors from accessing DNA. This methylation state is reversible and mediated by histone demethylases (HDM) [12, 22, 23]. However, histone methylation is not always associated with gene silencing. Trimethylation on the 4th lysine of histone 3 (H3K4me3) is involved in gene expression [17,18,19]. When a histone tail is acetylated by histone acetyltransferases (HAT), this tends to neutralize the lysine positive charge interacting with the negative phosphate groups of DNA and pushes away histone cores therefore “opening” the chromatin. That allows transcription factors (TF) to recognize and bind to promoters and RNA polymerase II (RNA pol II) to initiate transcription. This acetylation level is reversible and mediated by histone deacetylases (HDAC)