One of the best-conserved ATPase families involved in chromatin remodelling is the imitation switch (ISWI) family ISWI is a conditional ATPase that hydrolyzes ATP maximally only in the presence of nucleosomes, its natural substrate. In contrast, free histones do not affect the low basal level of ATP hydrolysis while free DNA triggers ATPase activity only modestly All ISWI proteins contain a conserved ATPase domain that belongs to the superfamily of DEAD?H (Asp-Glu-Ala-Asp ? His)-helicases [3,4], located in the N-terminal half of the proteins. In the C-terminal part, a HAND domain, a Swi3 Ada2 N-CoR TFIIIB (SANT) domain and a juxtaposed SANT-like ISWI (SLIDE) domain are present. ISWI chromatin remodellers not only play a role during chromatin (dis)assembly, as is the case in DNA repair or replication, but they can also switch nucleosome positions to block or clear promoters. This can directly change the expression level of the corresponding gene.
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