Chromatin remodeling involves the effective shifting of nucleosome cores, which are composed of histone octamers, that consist of a H3(2):H4(2) tetramer and two dimers of H2A and H2B, along the length of the DNA molecule, a process known as "nucleosome sliding". Recent studies suggest that this shift may involve the actual disassembly and reassembly of the nucleosome core.
Chromatin remodeling is accomplished, at least in part, by ATPase containing complexes, referred to as the SWI / SNF family. Chromatin remodeling complexes are classed into four subfamilies based upon their associated ATPase. The four ATPases associated with chromatin remodeling complexes are SWI / SNF and Brg1; imitation switch (ISWI); Mi-2 (CHD1) and INO80.
The SWI / SNF complex is a 2 MDa multi-subunit DNA-dependent ATPase that contributes to the regulation of gene transcription by altering chromatin structure. Recent studies have revealed that the SWI / SNF complex is targeted to promoters via direct interactions with transcription activators and have provided insights into mechansims by which the complex alters nucleosome structure and contributes to the remodeling of chromatin.The ISWI complexes are divided into the nucleosome remodeling factor (NURF), chromatin accessibility complex (CHRAC), and ATP-dependent chromatin and remodeling factor (ACF) complexes. NURF is composed of ISWI and three additional subunits, NURF301, NURF55 and NURF38. The SWI/SNF family members differentiate at the level of DNA sequence and gene structure.