| Abstract: | It has been demonstrated recently by Poirier et al. (Poirier, G. G., de Murcia, G., Jongstra-Bilen, J., Niedergang, C., and Mandel, P. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 3423-3427) that poly(ADP-ribosyl)ation of pancreatic nucleosomes causes relaxation of the chromatin superstructure through H1 modification. The in vitro effect of poly(ADP-ribose) synthesis and degradation on calf thymus chromatin was investigated by the time course incorporation of ADP-ribose, electron microscopy, analytical ultracentrifugation, and autoradiography of the protein acceptors. Purified calf thymus poly(ADP-ribose) polymerase and partially purified bull testis poly(ADP-ribose) glycohydrolase were used. Degradation of ADP-ribose units on hyper(ADP-ribosyl)ated H1 by poly(ADP-ribose) glycohydrolase restores the native condensed chromatin superstructure. This reversible conformational change induced by poly(ADP-ribosyl)ation on nucleosomal arrangement could be one of the mechanisms by which the accessibility of DNA polymerases and/or excision-repair enzymes is favored, the native structure being fully restorable. |
