Polynucleosome structure

Higher levels of organization Nucleosomes can be packed more tightly to form a polynucleosome (also called a nudeofilament). This structure assumes the shape of a coil, often referred to as a 30-nm fiber. The fiber is organized into loops that are anchored by... 

Poirer, G.G., de Murcia, G., Jongstra-Bilen, J., Niedergang, C. and Mandel, P. (1982) Poly(ADP-ribosyl)ation of polynucleosomes causes relaxation of chromatin structure. Proc. Natl. Acad. Sci. USA, 79, 3423-3427. 

Figure 3-8. A polynucleosome. (Adapted from Olins DE and Olins AL Nucleosomes The structural quantum in chromosomes. American Scientist 66 (1978), p 708.)...

Repeating units of nucleosome plus linker DNA constitute the polynucleosome, which in turn is involved in higher orders of chromatin structure 50, 63,116,117,123,194). It has been postulated, based on electron micrographs, that polynucleosomes coil into a solenoid with 100 A hole through the central axis 50, 63, 116, 117, 123, 194) one turn of a helix includes 7 to 9 nucleosomes 171). 

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. 

Poirier GG, De Murda G, Jongstra-Bilen J, Niedergang C, Mandel P (1982) Poly(ADP-ribosyla-tion) of polynucleosomes causes relaxation of chromatin structure. Proc Natl Acad Sd USA 79 3423-3427... 

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). 

Postsynthetic modifications of histones, i.e., phosphorylation, acetylation, and poly(ADP-ribosylation), have been suggested to be involved in the modulation of the structure and the function of chromatin [7, 8]. A central role has been attributed to histone HI in forming and stabilizing the nucleosomal structure and the higher order folding of the polynucleosomal chain into a solenoid conformation [8-10]. 

Fig. 2A,B. Time course of the structural changes of polynucleosomes induced by ADP-ribosylation. ADP-ribosylated polynucleosomes were fixed in buffer containing 20 mM NaCl and examined in bright field [20]. A 200 yM NAD B 20 yM NAD. The incubation times are as indicated in Fig. 1 T corresponds to the zero time. Bar indicates 100 nm...

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. 

A major problem with using isolated polynucleosomes to study chromatin organization in vitro is the uncertainty whether such preparations retain their native arrangement of chromosomal proteins and whether structural alterations in a selected chromatin preparation are representative of the situation in intact cells. The goal of the present study was to develop a technique which would allow us to determine the consequences of chromatin-associated ADP-ribosylation on chromatin organization of intact cells which undergo DNA excision repair. 

Modulation of chromatin structure by poly(ADP-ribosyl)ation effect of poly(ADP-ribosyl)ation on chromatin superstructure. We have shown that poly(ADP-ribosyl)ation of native chromatin in vitro from endogenous and exogenous activity of the enzyme led to decondensation of the 30 nm chromatin fiber (3, 5, 10). This decondensation was associated with h5 rmodified forms of histone HI at high NAD concentrations. We have thus studied the stmctural effect of poly(ADP-ribosyl)ation at a high NAD level on HI-depleted chromatin where modification of core histones was observed (7). This specific poly(ADP-ribosyl)ation prevented the recondensation of polynucleosomes reconstituted with native histone HI (Fig. 1). 

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