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The interaction between purified calf thymus poly(ADP‐ribose) polymerase and its activating co‐purified DNA (sDNA) was investigated by electron microscopy. We have shown that the enzyme‐DNA complex possesses a nucleosome‐like structure. The enzyme‐bound DNA (sDNA) was found to be enriched in single‐stranded regions and branched structures, presumed to be replication forks. The auto‐ribosylated polymerase as well as the branched poly(ADP‐ribose) formed were visualized by dark field electron microscopy during the auto‐ADP‐ribosylation reaction and the possible mechanism of this phenomenon is discussed.

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A slight DNA topoisomerase I activity was detected in highly purified poly(ADP-Rib)polymerase prepared from calf thymus. This copurified activity was found to be suppressed under conditions where the poly(ADP-ribosylation) reaction occurs in the presence of NAD.Purified topoisomerase I from calf thymus was shown to be ADP-ribosylated by poly(ADP-Rib) polymerase purified from the same tissue. Poly(ADP-ribosylation) of topoisomerase I produces an inhibition of the enzymatic activity in parallel to the extent of ADP-ribosylation.The fact that a slight poly(ADP-Rib) polymerase activity was also found to copurify with a topoisomerase I preparation and that toposiomerase I activity can be modified by ADP-ribosylation, may suggest a spacial and functional correlation of these two enzymes in chromatin.Several enzymes present in chromatin are involved in a coordinated manner in the expression of DNA functions. Among these enzymes, DNA topoisomerases control the conformation of DNA by concerted breaking and rejoining of phosphodiester bonds. These enzymes, present in high copy numbers in nuclei are considered to be involved in DNA replication, transcription, recombination or modification of chromatin structure in vivo [I, 21. Poly(ADP-Rib) polymerase is an enzyme found in chromatin of all eukaryotic cells tested. ADP-ribosylation catalysed by this enzyme is one of the post-translational modifications of nuclear proteins involved in several DNA functions such as DNA repair, replication and transcription (for review see [3 -51). Identification of the target proteins is of great importance for the elucidation of the roles fulfilled by ADP-ribosylation during these nuclear processes. Among ADP-ribosylated proteins, histones (particularly histone H1) are the most studied [3-51. ADP-ribosylation was recently shown to cause the relaxation of nucleosome structure presumably through its effect of modification of histone H1 [6]. Only very few nonhistone proteins have so far been identified as acceptors of poly(ADP-Rib). Among them are HMG proteins [7,8] Moreover, ADP-ribosylation of CaZ +, MgZ +-dependent endonuclease, an enzyme acting on DNA as a substrate, has been reported [12]. The functions fulfilled by these proteins might be regulated by ADP-ribosylation.In this report we present data showing that poly(ADPRib) polymerase purified from calf thymus [13] is copurified with a small amount of topoisomerase I activity and, vice versa, that topoisomerase I purified from the same tissue [14] is copurified with a minor poly(ADP-Rib) polymerase activity. Abbreviations. ADP-Rib, adenosine(5')diphospho(5')-B-~-ribose;poly(ADP-Rib), polymer of ADP-Rib; CI,AcOH, trichloroacetic acid; SDS, sodium dodecyl sulphate; sDNA, fragments of small double-stranded DNA.Enzyme. Poly(ADP-Rib) polymerase (EC 2.4.99. -).After the characterization of the copurified topoisomerase I activity in our poly(ADP-Rib) polymerase preparation we investigated the effect of ADP-ribosylation on topoisomerase I reaction. MATERIALS AND METHODS ChemicalsEscherichia col...