The molecular phenomena controlling gene expression is coordinated in the genome level where the availability of DNA sequences is determined by the structure called chromatin

The molecular phenomena controlling gene expression is coordinated in the genome level where the availability of DNA sequences is determined by the structure called chromatin (van Dijk, Ding et al. 2010). Chromatin is a protein–DNA fibre containing a series of repeating nucleosome units. Each nucleosome is an octamer, containig two copies of each of the four core histone proteins (H2A, H2B, H3 and H4) with about 150 base pairs of DNA wrapped around it. This structure serves not just to bound or confine the DNA into the tiny space of the nucleus; it has also been incorporated to control gene expression by virtue of its capability to specifically make available or hide DNA sequences from DNA-binding proteins, which directly control gene expression (Deal and Henikoff 2010).
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 directly to nucleosomes is conveyed through the SWR1 complex in plants that is made up of proteins encoded by ACTIN-RELATED PROTEIN 6 (ARP6), SWC6 and PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1) (Tasset, Yadav et al. 2018). massive rearrangement of transcription-associated with cell variation during development includes switch on and off of many genes (March-Diaz, Garcia-Dominguez et al. 2007). In plants, H2A.Z is associated to the response to turbulent temperature, the phosphate starvation response, osmotic pressure, the immune response, floral induction, female meiosis, recombination, thalianol metabolism, and the modulation of microRNA abundance (Qin, Zhao et al. 2014, Xu, Leichty et al. 2018). This task requires enormous rearrangement in chromatin assembly as it has been evidenced by the recognition of chromatin-remodeling factors whose mutation impairs regular development at multiple and different levels (March-Diaz, Garcia-Dominguez et al. 2007). Three most important biochemical methods or mechanisms have been described to alter chromatin configuration and assembly. The first requires the posttranslational covalent alteration of the amino- and carboxy-terminal ends of histones. The model of chemical alteration 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 redirection of interactions between DNA and histones, which encompasses the distortion to the nucleosome assembly. The third medium of chromatin remodelling resides in the substitution of canonical histones of the octamer by histone variants, which confers evenness to the nucleosome (Mizuguchi, Shen et al. 2004, Kamakaka and Biggins 2005, March-Diaz, Garcia-Dominguez et al. 2007)