Effect of cytosol on DNA synthesis in isolated mammary gland nuclei from rabbits during pregnancy and early lactation.

The in vitro DNA synthesis of nuclei isolated from rabbit mammary gland was stimulated by a cytosol fraction from the same tissue. Time-course of this phenomenon was followed during rabbit pregnancy at 5-day intervals. The stimulation was shown to be physiological state-dependent in that it could be detected only in the case of cytosols prepared on day-10 of pregnancy and from day-20 or 25 of pregnancy through day-5 of lactation. Moreover, only nuclei isolated on days-15 or -30 of pregnancy responded to the exogenously added cytosol. The DNA-stimulating activity was partially characterized. It was shown to be protein-like since it was heat-labile, mostly non-dialysable and sensitive to N-ethylmaleimide and trypsin treatment. Sedimentation analysis on sucrose density gradients separated this activity into 3-4 peaks, distinct from the cytoplasmic DNA polymerase.