Cell density-dependent increase in chromatin-associated ADP-ribosyltransferase activity in simian virus 40-transformed cells.

ADP-ribosyltransferase activity associated with chromatin is two- to tenfold higher in simian virus 40 (SV40)-transformed cells than in untransformed cells. When confluent transformed cells were subcultured, their specific enzyme activity first decreased two- to fourfold and the rapidly increased during the logarithmic phase of growth. This increase ceased or slowed down when the cells entered the stationary phase. In contrast, the activity in the untransformed cells remained low throughout the growth cycle. In SV40tsA-transformed cells (ts = temperature sensitive), this density-dependent increase in the enzyme activity was observed when the cells were cultivated at the permissive temperature, whereas the activity remained low at the restrictive temperature. The enzyme activity did not increase during induction of cellular DNA synthesis in quiescent cells either by addition of fresh medium or by infection with SV40. The chromatin-associated enzyme activity extracted with 1 m NaCl was eluted together with almost all the DNA-binding proteins from a phosphocellulose column with 0.6 m NaCl. The enzyme activity in this fraction from transformed cells, measured with or without added DNA and histones, was higher than that in a similar fraction from untransformed cells, reflecting the difference in the original activities present in the nuclei of these cells. The chain lengths of poly(ADP-ribose) formed by chromatin from SV40-transformed and untransformed cells were not significantly different. These results suggest that the number of initiation sites for ADP-ribosylation is increased in the chromatin of SV40-transformed cells compared to that of untransformed cells.