Fig. 1. Biphasic activation of PARP by UVB. (A) Time course of PARP activation in UVB-irradiated cells. Cells were exposed to 1.6 kJ/m2 UVB and pADPr was visualized by indirect immunofluorescence using monoclonal 10H antibody. Data represent one of five experiments with identical results. (B) Quantification of pADPr signal in UVB-exposed nuclei. Cells were exposed to UVB, as described for A and relative fluorescent intensity was measured in at least 150 nuclei per time point from different fields. Data are expressed as mean±s.d. of relative fluorescence units. (C) Polymer immunoblotting of cells exposed to UVB. Extracts of cells treated with 1.6 kJ/m2 UVB were western blotted for pADPr with polyclonal LP96-10 or for actin as a loading control. One of six replicates is shown.