Chromatin is a protein–DNA fibre comprising repeating nucleosome units.
Each of the nucleosomes is an octamer, comprised of two copies of each of the four core histone proteins (H2A, H2B, H3 and H4) which contain about 140 to 150 base pairs of DNA wrapped around it. The incorporation of H2A.Z histone variant on to nucleosomes is mediated through the SWR1 chromatin remodelling complex. It is an evolutionarily conserved ATP-dependent chromatin remodelling complex that catalyzes the substitution of H2A by the H2A.Z histone variant in the nucleosome. SWR1 chromatin remodelling complex contains about fourteen distinct subunits or components, eight of which—Swr1, Swc2 (also known as Vps72), Swc3, Swc5 (also known as Aor1), Swc6 (also known as Vps71), Swc7, Arp6, Yaf9 and Bdf1—are encoded by genes that are nonessential for cell viability. Other subunits or components of the complex—Act1 (actin), Arp4, Swc4 (also known as God1), Rvb1 and Rvb2—are essential. Some of these have functions other than the SWR1 complex or are shared subunits of the INO80 chromatin remodelling complex.
The SWR1 complex changes nucleosome composition and positioning, which leads to regulation of DNA accessibility through chromatin compactness. Due to its unique physical and chemical properties, H2A.Z histone variant affects nucleosome stability as well as chromatin structure, to regulate gene expression. Incorporation of H2A.Z Histone variant into the chromatin and its physical and chemical properties has made it a key regulator in transcriptional regulation underlying developmental and environmental responses. ARP6 aids and promotes binding between other subunits, such as Swc2, and the ATPase domain of SWR1, Swc2 directly binds to and is essential for transfer of H2AZ histone variant. Swc6 a homologue of SEF in Arabidopsis and Arp6 is required for the association of Swc2 and also for nucleosome binding, other subunits such as Swc5 and Yaf9, are necessary for H2AZ transfer but neither H2AZ nor nucleosome binding.
SWR1 chromatin remodelling complex plays a central role in Plant developmental processes such as flower development, stress tolerance, plant immunity and DNA damage repair.