Question

Describe a mechanism for how heterochromatin can spread via histone methylation.

Answer 1

Methylation of histon protein plays important role in spreading heterochromatin region. Heterochromatin region is highly packed DNA region which reduced gene expression. The nucleosome assembly is made up of histon proteins (H2a, H2b, H3 and H4). The tail end of histon is made up of arginine and lysine amino acids. Methylation of histon is a process of addition of methy group to the amino acid of protein by methyl transferase enzyme. Lysine residue of histon 3 and 4 are common site for methylation. Methylation of histon covering heterochromatin region does reduces gene expression. Overall structure of heterochromatin is depends on binding of HP1 (heterochromatin protein 1) and H3K9me3 (histon 3, 9^th lysine and methylated 3 times). Methylation of lysine provides binding of these two factors for heterochromatin formation. Lysine may be mono-, di- or tri-methylated which results in spreading heterochromatin.

The classical example of histon methylation is inactivated X chromosome in female. One X chromosome of X is inactivated by methylation of lysine reduce of histon. Inactivated X chromosome is tightly packed heterochromatin DNA which decreases expression of this DNA.