Poly (ADP-ribose) is not involved in the rejoining of DNA breaks accumulated to high levels in u.v.-irradiated HeLa cells

U.v. damage to the DNA of HeLa cells induces the polymer-isation of ADP-ribose, but only if repair synthesis is inhibited so that incomplete repair sites (i.e., DNA breaks) accumulate to abnormally high levels. 3-Aminobenzamide greatly reduces the ADP-ribose polymerisation response. However, 3-amino-benzamide does not reduce the rate of rejoining of the ac-cumulated breaks when the inhibition of repair synthesis is reversed. Therefore, rejoining of these DNA breaks (in con-trast to the rejoining of other kinds of break) appears not to depend on activation of polynucleotide ligase by ADP-ribosylation