DNA topoisomerase I Activity in the nuclear extract of yeast N180 cells was determined by conversion of the supercoiled plasmid to various levels of topoisomers in agarose gel electrophoresis. The topoisomerase I activity was inhibited by NAD, the substrate of poly (ADP-ribose) polymerase. This NAD- induced inhibition of topoisomerase I activity, however, was restored to a normal level by increasing the concentration of 3-aminobenzamide (3AB) in the reaction mixtures. The formation of nucleosome by N180 nuclear extract and plasmid DNA was also studied by the retarded migration in agarose electrophoresis upon formation of nucleosome as compared with naked plasmid DNA. Nucleosome formation was markedly accelerated by treatment with NAD, and normal levels of nucleosome were achieved when the concentration of 3AB equals to that of NAD. These results suggest that the formation of higher ordered structure of chromatin might be regulated by poly ADP-ribosylation of nuclear proteins such as histones. The presumed poly (ADP-ribose) polymerase activity was more directly analysed by determining the radioactivities of ³H-NAD incorporated in permeabilized S. cerevisiae cells that had been treated with MMS. The degree of incorporation of radio-labelled NAD increased in MMS dose- dependent manner, which was also reproduced in permeable HeLa cell system. Furthermore, the average size of colonies treated with benzamide was significantly reduced as compared with that of control, and the cell survival of MMS- treated group was remarkably decreased by the post-incubation with benzamide. All these results suggest that yeast cells, like mammalian cells, possess the poly (ADP-ribose) polymerase activity which is sensitive to DNA strand breaks.