The organization of chromatin has a crutial role for the control of gene expression (Zhu, Dong et al.
2013) in different biological processes, including genome stability, recombination, developmental reprogramming, and reaction to external impulses. Alteration in histone variants, histone modifications and DNA methylation are usually regarded as epigenetic regulation. However, these changes may or may not possibly be truly epigenetic in nature considering common epigenetics definition encompass mitotic or meiotic heritability (Chinnusamy and Zhu 2009).
H2A.Z is a conserved variant of histone H2A that is involved in many biological processes, such as transcriptional regulation, telomeric silencing, genome stability, cell cycle progression, DNA repair, and recombination (Sura, Kabza et al. 2017). Histone modification and ATP-dependent chromatin remodelling direct chromatin structure to harmonise chromatin packaging and transcriptional access (Qin, Zhao et al.
2014). H2A.Z influences many processes in fungi, plants and animals, including gene expression, recombination, and DNA repair (Xu, Leichty et al. 2018) H2A.Z is greatly enriched at the transcription start site (TSS) of a considerable set of genes across cell types, compatible with a role in the control or regulation of transcription, Genome-wide studies in yeast have revealed that H2A.Z enrichment at promoter-distal nucleosomes is needed for initiation or start of transcription, while being oppositely correlated with transcript levels (Sura, Kabza et al.
2017). Eukaryotic genomes possess several histone variants, and all of them bestow different properties to the nucleosome, which in turn influence many biological processes, most commonly and importantly transcription. Histones may also be altered post translationally and successively affect transcription (Dai, Bai et al. 2017). The incorporation of H2A.
Z on to nucleosomes is mediated through the SWR1 complex in Arabidopsis that consists of proteins encoded by ACTIN-RELATED PROTEIN 6 (ARP6), SWC6 and PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1) (Tasset, Yadav et al. 2018). Massive reprogramming of transcription-associated with cell differentiation during development involves activation and silencing of hundreds of genes (March-Diaz, Garcia-Dominguez et al. 2007).
In plants, H2A.Z has been implicated in the response to high temperature, the phosphate starvation response, osmotic stress, the immune response, floral induction, female meiosis, recombination, thalianol metabolism, and the regulation of microRNA abundance (Qin, Zhao et al. 2014, Xu, Leichty et al. 2018). This process requires extensive changes in chromatin structure as it has been evidenced by the identification of chromatin-remodeling factors whose mutation impairs normal development at many different levels (March-Diaz, Garcia-Dominguez et al. 2007). Three main biochemical mechanisms have been reported to alter chromatin structure.
The first involves the posttranslational covalent modification of the amino- and carboxy-terminal tails of histones. The pattern of chemical modifications of histones within a nucleosome (acetylation, methylation, phosphorylation, ubiquitination, and SUMOylation) seems to constitute a code that can be interpreted by other nuclear machinery (March-Diaz, Garcia-Dominguez et al. 2007). Second is the ATP-dependent reorientation of interactions between DNA and histones, which provokes the distortion of the nucleosome structure.
The third mechanism of chromatin remodelling remains in the replacement of canonical histones of the octamer by histone variants, which confers stability and interactions to the nucleosome (Mizuguchi, Shen et al. 2004, Kamakaka and Biggins 2005, March-Diaz, Garcia-Dominguez et al. 2007).