Fig. 1

Active transcription is associated with hyperacetylation of histones, when an acetyl group (Ac) is added to specific lysines residing within the N terminal region of histones. The affinity between histone and DNA is reduced, leading to an open chromatin conformation that allows transcription factors (TF) and RNA polymerase to access the promoter of the target genes. Additionally, methylation of certain lysines on histones (H3K4, H3K36, and H3K79) located at promoter regions also leads to active transcription. In contrast, DNA methylation and hypoacetylation and/or methylation of certain other lysines on histones (H3K9, H3K27, and H4K20 residues) are associated with gene repression from formation of relatively condensed/inactive chromatin. These modifications are catalyzed by a number of chromatin-modifying enzymes including DNA methyltransferases (DNMTs), histone acetyltransferases (HATs)/histone deacetylases (HDACs), histone methyltransferases (HMTs), and histone demethylases (HDMs). The expression of transcriptionally inactive genes can be upregulated by exposure to DNMT-inhibitors and HDAC inhibitors