chromatin is a protein–DNA fibre containing repeating units of a nucleosome. Each nucleosome is an octamer containing two copies of all of the four histone proteins h2a h2b h3 and h4 which consist of about 140 – 150 bp of DNA enclosed around it.
Incorporation of h2a.z variant into nucleosomes is mediated through the swr1 complex. It is an evolutionarily conserved ATP-dependent chromatin remodelling complex which catalyzes the substitution of h2a by the h2a.z histone variant in the nucleosome. swr1 chromatin remodelling complex contains about fourteen different 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 which are non-essential 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.
