Figure 4.

Interaction of H4 tail peptides with DNA reconstitutes a minimal determinant for ISWI response. (A) ISWI ATPase assays in the presence of various substrates. B, buffer; D, DNA; N, nucleosomes; H3 or H4 tail peptides in the indicated amounts (5–500 pmol) in the absence (–) or presence (+) of DNA. The ATPase activity is expressed as the percentage of ATP hydrolyzed during the assay. The ATPase activity level elicited by free DNA is extended through the figure as a gray background, for reference. This result has been obtained consistently in a number of independent experiments under different conditions. (B) R17/R19 within the histone H4 tail are crucial for ISWI ATPase activity. A 25mer H4 tail peptide TGRGKGGKGLGKGGAKRHRKVLRDC induces the ATPase of ISWI as well as the 20mer used in (A). A variant 25mer H4 tail peptide in which R17 and R19 were substituted by alanines does not trigger ATPase acivity. All reactions contained DNA. (C) ISWI ATPase activity is sensitive to site-specific acetylation of histone H4 tail. The data are presented as in (A).