Involvement of deoxyribonuclease activity in the differential sedimentation rates of nucleoids from non-transformed and transformed mouse embryo fibroblasts.

Lysis of non-transformed confluent C3H10T1/2C18 mouse embryo fibroblasts in the presence of detergents, high concentrations of salt and EDTA on top of neutral sucrose gradients revealed a reduced sedimentation rate of the resulting nucleoids from these cells compared to those from exponentially growing non-transformed cells or from transformed cells. Exposure of confluent cells to 1000 rads of X-ray had no effect on this rate of nucleoid sedimentation; and ethidium bromide titration and alkaline sucrose analysis suggested the presence of discontinuities in the DNA. An endonucleolytic activity could be extracted from nuclei of these cells with 0.5 M NaCl, indicating a very tight association with the chromatin. Such an enzyme in non-transformed confluent cells may account for the differences in nucleoid structure and may be related to changes in cell function with normal arrest of cell growth. There was no growth-phase effect on the properties of nucleoids from transformed cells.