# Heat Stress Affects H3K9 me3 Level at Human Alpha Satellite DNA Repeats
## 1 Introduction  
Satellite DNAs are tandemly repeated sequences assembled within constitutive heterochromatin at the (peri)centromeric and subtelemeric regions [1]. However, some satellite DNAs are not only clustered within heterochromatin in the form of long arrays but are dispersed as short arrays within euchromatin, in the intronic and intergenic regions [2; 3; 4; 5; 6]. Although satellite DNAs are preferentially embedded in constitutive heterochromatin which is considered transcriptionally inert, their transcription was reported in vertebrates, invertebrates, and plants [7]. Constitutive heterochromatin is sensitive to temperature fluctuations and is dynamically regulated in response to environmental stimuli [8; 9]. Temperature-mediated heterochromatin disruption seems to be an epigenetic event which includes stress-response transcription factors involved in heterochromatin assembly. In human cells, heat stress activates heat shock transcription factor 1 (HSF1) which recruits major cellular acetyltransferases to pericentric heterochromatin leading to targeted hyperacetylation [10], facilitating particularly the transcription of satellite III DNA [11; 12]. In _Drosophila_, under standard conditions, transcriptionfactor dATF-2, which regulates expression of stress response genes, recruits heterochromatin protein 1 (HP1) to pericentromeric heterochromatin, while under heat stress, activated MAP kinases such as p38 phosphorylate dATF-2 which is released from heterochromatin, leading to abolishment of HP1 and disruption of heterochromatin [13]. In vivo studies on the insect _Tribolium castaneum_ revealed heat-stress induced destabilization of heterochromatin coupled with overexpression of a major heterochromatin satellite DNA TCAST1, followed by processing of satellite transcripts into siRNAs [14]. Satellite DNA-derived small interfering RNAs are, through RNA interference mechanism (RNAi), involved in the epigenetic process of heterochromatin formation in different organisms [15; 16; 17]. In _T. castaneum_, heat-stress induced transcription of TCAST1 satellite DNA correlates with an increased level of repressive heterochromatin marks H3K9 me2/3 not only on satellite repeats in constitutive heterochromatin but also on dispersed TCAST1 satellite elements and their proximal regions within euchromatin, affecting expression of nearby genes [18]. Alpha satellite DNA comprises up to 10% of the human genome and is located in the centromeric and pericentromeric regions of all chromosomes [19]. The fundamental unit of human alpha satellite DNA is based on diverged 171 bp monomers which are often organized in complex higher order repeats [20]. In addition to their (peri)centromeric location, a bioinformatic search of the human genome revealed the presence of 133 blocks of alpha satellite located \(>\) 5 Mb from the centromere [21]. Alpha satellite DNA is actively transcribed and transcripts contribute to essential chromosomal functions such as centromere and kinetochore assembly and heterochromatin formation [19], while interaction between SUV39H1 histone methyltransferase and pericentromeric alpha satellite DNA transcripts is necessary for proper heterochromatin silencing in humans [22]. In this paper, using different human cell lines as a model system, we analyze transcription of alpha satellite DNA upon heat stress. In addition, we followed epigenetic changes of histones, in particular the \"silent\" H3K9 me3 and \"active\" H3K4 me2/3 marks at alpha repeats dispersed within euchromatin, in introns and at intergenic regions as well as tandemly arranged alpha satellite which are characteristic for heterochromatin. The results reveal an increase of H3K9 me3 level at alpha repeats and spreading of H3K9 me3 to the flanking regions up to 1-2 kb from the insertion sites of the euchromatic repeats. The dynamics of H3K9 me3 enrichment at alpha repeats correlates with the dynamics of alpha satellite DNA transcription activation upon heat stress, being the most prominent immediately after heat stress.