Just as lysine acetylation dilutes the positive charge of lysine and leads to histone loosening, what actually drives for transcriptional activation or repression when lysine within histone is methylated at any particular site. Their is definitely not any charge alteration but as I have read the molecular mass of the protein (Histone) increases. So, is their any significant relation between increase in molecular mass and histone DNA interaction.
The methyl groups on lysines such as H3K4 allow for binding of reader proteins, such as chromodomain, PHD fingers, Bromodomain, etc. which help determine chromatin structure and recruitment of transcription factors. Also, it is thought that H3K4 methylation prevents Polycomb mediated repression, thus keeping regions of chromatin accessible or poised for transcription.
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