Attachment of acetyl groups to promotes transcription.

Attachment of acetyl groups to histone promotes transcription.

This mechanism is known as histone acetylation and it is a part of gene expression. The lysine residues inside the N-terminal tail from the histone core of the nucleosome are the substrate for enzymes-histone acetyltransferase which transfer an acetyl functional group from another molecule. Acetylation removes the positive charge on the histones and thus decrease the interaction negatively charged phosphate groups of DNA. As a result, chromatin becomes more relaxed structure and transcription is promoted.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.