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On the Mechanism of Nucleosome Assembly by Histone Chaperone NAP1*

The process of mononucleosome assembly mediated by histone chaperone NAP1 was investigated using DNA fragments 146 and 207 bp in length containing the Lytechinus variegatus 5 S rDNA nucleosome positioning sequence. A quantitative description was derived using gel electrophoresis and fluorescent anisotropy data. First, NAP1-bound H3·H4 was released forming a DNA-histone tetramer complex with a time constant of k1 = (2.5 ± 0.7) · 104 m–1 s–1. The tetrasome was converted quickly (k2 = (4.1 ± 3.5) · 105 m–1 s–1), by the addition of a single H2A·H2B dimer, into a “hexasome,” i.e. a nucleosome lacking one H2A·H2B dimer. From this intermediate a nucleosome was formed by the addition of a second H2A·H2B dimer with an average rate constant k3 = (6.6 ± 1.4) · 103 m–1 s–1. For the back-reaction, significant differences were observed between the 146- and 207-bp DNA upon substitution of the canonical H2A histone with H2A.Z. The distinct nucleosome/hexasome ratios were reflected in the corresponding equilibrium dissociation constants and revealed some differences in nucleosome stability. In a fourth reaction, NAP1 mediated the binding of linker histone H1 to the nucleosome, completing the chromatosome structure with k4 = (7.7 ± 3.7) · 103 m–1 s–1. The activity of the chromatin remodeling complex ACF did not increase the kinetics of the mononucleosome assembly process.