Stress-Induced PARP Activation Mediates Recruitment of Drosophila Mi-2 to Promote Heat Shock Gene Expression

Eukaryotic cells respond to genomic and environmental stresses, such as DNA damage and heat shock (HS), with the synthesis of poly-[ADP-ribose] (PAR) at specific chromatin regions, such as DNA breaks or HS genes, by PAR polymerases (PARP). Little is known about the role of this modification during cellular stress responses. We show here that the nucleosome remodeler dMi-2 is recruited to active HS genes in a PARP–dependent manner. dMi-2 binds PAR suggesting that this physical interaction is important for recruitment. Indeed, a dMi-2 mutant unable to bind PAR does not localise to active HS loci in vivo. We have identified several dMi-2 regions which bind PAR independently in vitro, including the chromodomains and regions near the N-terminus containing motifs rich in K and R residues. Moreover, upon HS gene activation, dMi-2 associates with nascent HS gene transcripts, and its catalytic activity is required for efficient transcription and co-transcriptional RNA processing. RNA and PAR compete for dMi-2 binding in vitro, suggesting a two step process for dMi-2 association with active HS genes: initial recruitment to the locus via PAR interaction, followed by binding to nascent RNA transcripts. We suggest that stress-induced chromatin PARylation serves to rapidly attract factors that are required for an efficient and timely transcriptional response.真核细胞（Eukaryotic cells）可通过多聚ADP-核糖聚合酶（PAR polymerases, PARP），在DNA断裂位点或热激基因等特定染色质区域合成多聚[ADP-核糖]（poly-[ADP-ribose], PAR），以应对基因组压力与环境压力，例如DNA损伤与热激（heat shock, HS）。目前学界对该修饰在细胞应激反应中的作用知之甚少。本研究证实，核小体重塑因子dMi-2会以PARP依赖的方式被招募至活化的热激基因位点。dMi-2可结合PAR，提示这种物理相互作用对招募过程至关重要。事实上，无法结合PAR的dMi-2突变体无法在体内定位到活化的热激基因座。我们已鉴定出数个在体外可独立结合PAR的dMi-2结构域，包括染色质结构域（chromodomains）以及N端附近富含赖氨酸（K）与精氨酸（R）残基基序的区域。此外，在热激基因激活后，dMi-2会与新生热激基因转录本结合，且其催化活性对于高效转录与共转录RNA加工过程不可或缺。体外实验显示RNA与PAR会竞争性结合dMi-2，这提示dMi-2与活化热激基因位点结合存在两步过程：首先通过与PAR的相互作用被招募至基因座，随后结合新生RNA转录本。我们推测，应激诱导的染色质多聚ADP核糖基化修饰可快速募集高效且及时的转录反应所需的相关因子。