Figure 2. ISWI ATPase activity is not influenced by H4K16ac.

(A) Steady-state ATPase assay. ATPase activity of ISWI (100 nM) was stimulated with saturating concentrations of nucleosome arrays (600 nM) carrying unmodified (H4) or acetylated H4 (H4K16ac). ATP hydrolysis rates in presence of saturating concentrations of ATP (1 mM) were determined. Control reactions did not contain nucleosome arrays. Error bars represent standard deviations (No arrays: n = 4; H4 and H4K16ac: n = 5). (B) ISWI (350 nM) was stimulated with DNA (1.2 mg/ml salmon sperm DNA) and increasing concentrations of an unmodified or H4K16ac-carrying histone H4 N-terminal peptide (H4 tail peptide). ATP hydrolysis rates were determined as above at 1 mM ATP. A peptide with scrambled amino acid sequence of the H4 tail harbouring an acetylated lysine residue served as control. Data were fit to lines to extract slopes (dashed lines; H4: 4.1*10³ s⁻¹ M⁻¹; H4K16ac: 3.5*10³ s⁻¹ M⁻¹). Error bars display standard deviations (n = 3–4).