Critical role for the histone H4N terminus in nucleosome remodeling by ISWI

The ATPase ISWI can be considered the catalytic core of several multiprotei n nucleosome remodeling machines. Alone or in the context of nucleosome rem odeling factor, the chromatin accessibility complex (CHRAC), or ACF, ISWI c atalyzes a number of ATP-dependent transitions of chromatin structure that are currently best explained by its ability to induce nucleosome sliding. I n addition, ISWI can function as a nucleosome spacing factor during chromat in assembly, where it will trigger the ordering of newly assembled nucleoso mes into regular arrays. Both nucleosome remodeling and nucleosome spacing reactions are mechanistically unexplained. As a step toward defining the in teraction of ISWI with its substrate during nucleosome remodeling and chrom atin assembly we generated a set of nucleosomes lacking individual histone N termini from recombinant histones. We found the conserved N termini (the N-terminal tails) of histone H4 essential to stimulate ISWI ATPase activity , in contrast to other histone tails. Remarkably, the H4 N terminus, but no ne of the other tails, was critical for CHRAC-induced nucleosome sliding an d for the generation of regularity in nucleosomal arrays by ISWI. Direct nu cleosome binding studies did not reflect a dependence on the H4 tail for IS WI-nucleosome interactions. We conclude that the H4 tail is critically requ ired for nucleosome remodeling and spacing at a step subsequent to interact ion with the substrate.