Histone polyADP-ribosylation

In the case of HI variants, linker histones selectively bind ADPr homopolymers over competitor DNA [223]. Furthermore, Hit displays a high degree of affinity for the ADPr subunits even in the presence of salt [223]. Interestingly, this testis specific variant interacts with DNA the least tightly, and has been implicated in fiber decondensation [25,226,227]. This result suggests that potential interactions between HI molecules and ADPr are specific and not just the bi-product of electrostatic attractions. In this regard, specificity for the ADPr subunits may facilitate removal of HI from chromatosomal DNA, and initiate an unraveling of the 30 nm fiber required for DNA activation or repair. Unfortunately, the 

From a different perspective, circumstantial evidence suggests that ADPr may have a functional role in the activation of transcription. PARP copurifies with TFnC [230] and upregulates AP-2 (Activator Protein 2) controlled transcription. However, these results need to be interpreted cautiously, as a molecular mechanism for ADP-ribosylation of targeted histones has yet to be identified. 

ADP-ribosylation has also been implicated as a proteolytic antagonist during embryonic development [231]. Following fertilization in sea urchin, sperm-specific histones are degraded by the sperm-histone-selective (SpH) protease and subsequently replaced by cleavage stage histone variants. During this process, the maternal replacement histones are protected from proteolysis by ADP-ribosylation. 

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