Fig. 4.

Truncation of a single-histone H3 tail or repositioning of both H3 tails results in loss of cooperativity of substrate acetylation. (A) Initial velocity of nucleosomal arrays per enzyme containing octamers with a single H3 tail as a function of nucleosome concentration. Three independent trials with differing nucleosome concentration were performed. Shown is a representative trial. Each trial was fit to a cooperative saturation model to give an average cooperativity constant of 0.86 ± 0.13. (B) Initial velocity of nucleosomal arrays per enzyme containing 2 copies of the amino-terminal His₆-tagged H3 histones as a function of nucleosome concentration. Data were fit to a cooperative saturation model to give a cooperativity constant of 1.86 ± 0.07. (C) Relative orientation of the H3 and H4 histone tails based on mononucleosome structure 1AOI.¹¹ DNA is depicted in white, H3 and H3′ histones in blue, and H4 and H4′ histones in green. The points at which the H3 and H4 tails exit the nucleosome are labeled in yellow. The histone tails, histones H2A and histone H2B, are omitted for clarity. (Lower) The point at which the H4 tail exits the nucleosome occurs behind the DNA and is not labeled. (D) Initial velocity per enzyme of histone H3 tail-swap mononucleosome as a function of nucleosome concentration. These mononucleosomes replace the H3 histones with tailless H3 histone, and histone H4 with the H3gH4 fusion histone. Data were fit to a cooperative saturation model to give a cooperativity constant of 1.02 ± 0.07.