A Hypothesis on the Effect on Chromatin by Acetylation of Histone Tails

The N-terminal tails of the histone proteins are protruding from each nucleosome into the surrounding space. Their location in the chromatin is not known today but this picture illustrates two hypothetical alternatives: a) the positively charged tails may be contacting negatively charged patches on neighboring nucleosomes in the 30 nm chromatin fiber, or b) the histone N-terminal tails may form electrostatic bonds to the negatively charged phosphate backbone of the DNA helix. Since there are eight N-terminal tails protruding from each nucleosome, and since each tail contains several lysines that may be either acetylated or positively charged there is ample opportunity for many different interactions and various structural effects on the chromatin fiber by variation of the site of acetylation. In any case, the histone acetylation effectuated by the enzyme histone acetyltransferase (HAT) will reduce the positive charge of the histone tails and hence, be expected to decrease histone/DNA and/or nucleosome/nucleosome interactions. This would then lead to a more open chromatin structure which is making the DNA more accessible to the transcription machinery. Conversely, the activity of histone deacetylase (HDA) will reestablish the positive charge on the lysine residues on the histone tails, which leads to increased electrostatic interactions and a tighter packing of the DNA, thus becoming less accessible for basal transcription factors.
It should be stressed that the picture shown here is hypothetical and that the structure of the histone tails in different states of acetylation and its functions remain to be determined.