Polynucleosomes sedimentation velocity

When rat pancreatic polynucleosomes were poly(ADP-ribosylated) with purified calf thymus poly(ADPR) polymerase and examined by electron microscopy a relaxation of their native zigzag structure was observed, even at high ionic strengths they showed a close resemblance to chromatin depleted of histones HI. The relaxed state of poly(ADP-ribosylated) polynucleosomes was also confirmed by sedimentation velocity analysis [19, 20]. Locally relaxed regions can also be generated within poly-nucleosome chains by the activity of their intrinsic poly(ADPR) polymerase and appeared to be correlated with the formation of hyper(ADP-ribosylated) forms of histone HI and an increase of DNA polymerase activity [21]. The posttranslational transitory modifications of histones are potential modulators of chromatin stmcture. This may be involved in DNA transcription, replication, and repair. 

Concomitantly, structures resulting from the poly(ADP-ribosyl)ation of native chromatin, chromatin-Hl and core particles were examined by electron microscopy. We found, as described earlier for pancreatic chromatin [6, 7], that calf thymus chromatin adopts a more relaxed conformation upon poly(ADP-ribosyl)ation by purified calf thymus poly(ADP-ribose) polymerase free of its DNA (Fig. 3a,d). It was also found that this chromatin exhibited a lower sedimentation velocity as compared to control chromatin [6]. And recently, it has been shown that DNA polymerase a activity is more than twofold higher in the presence of pancreatic polynucleosomes ADP-ribosylated as compared to control polynucleosomes [16]. In striking contrast, no ultrastructural effect was observed when chromatin depleted of histone HI was poly(ADP-ribosyl)ated (Fig. 3b, e). 

Fig. 3. Sedimentation velocity of ADP-ribosylated polynucleosomes. At various time intervals aliquots of ADP-ribosylated nucleosomes were taken, dialyzed, and the sedimentation coefficients were determined in 20 mM NaCl[211...

In order to investigate the modification of DNA accessibility in chromatin structures relaxed by ADP-ribosylation, the DNA polymerase a activity has been determined [22]. As shown in Table 1, this activity is more than twofold higher in polynucleosomes ADP-ribosylated for 25 min than in control polynucleosomes. Moreover, this increase in template capacity is correlated with the ADP-ribosylation induced relaxation time course as demonstrated by electron microscopy and sedimentation velocity analysis. 

---

See also in sourсe #XX -- [ ]

---