General Genetics/Regulation of Gene Expression

Chromatin remodeling allows transcription to occur

How can the bulky transcription complex initiate transcription if eukaryotic DNA is usually tightly condensed into chromatin?

As mentioned previously, histones have numerous positive amino acid residues in order to bind to DNA. The interactions between DNA and the histones can be weakened by neutralizing positively-charged lysine residues. Histone acetyl transferase has a high affinity for activator proteins and acetylate the lysine residues at the enhancer site, loosening the chromatin structure. The chromatin becomes euchromatic: loosely-wound and active.

ATP-Dependent Nucleosome (Chromatin) Remodeling Complexes (CRCs) are large protein complexes capable of binding to and repositioning the nucleosome. This exposes binding sites for TATA-box-binding protein (TBP) and the transcriptional activating protein (TAP) which allow the final conformational changes necessary for transcription.

After transcription, histone deacetylase reverses the lysine residue acetylation, allowing the chromatin to revert to a heterochromatic (inactive and tightly-wound) state.