DNA Polymerase, Triphosphates and Deoxyribonuclease in a Systemof Synchronized L Cells

Abstract

In a system of synchronously dividing mouse fibroblasts (L cells), the activity of several enzymes was determined in relation to initiation of DNA synthesis. Synchrony of DNA synthesis and cell division was obtained by selective mechanical detachment of cells in mitosis, thereby avoiding the use of blocking agents. The activity of DNa polymerase under assay condition spermitting terminal or replicative activity of DNA polymerase under assay conditions permitting terminal or replicative activity to occur shows characterstic fluctuations throughout the life cycle of the cell: whereas the terminal enzyme activity exhibits a peak at about 12 h, the replicative form of theenzyme has its maximum 16 h after mitosis. DNA synthesis starts at about 9 h and reaches its peak at 16 h after mitosis. The two enzyme fractions examined (particular and supernatant) prefer native DNA as primer. The supernatant enzyme exhibits exclusively terminal activity with heat‐denatured DNA as primer. The degradation of TTP by TTPase activity of the particular enzymes fraction in the S‐phase (phase of DNA synthesis) may contribute to a preservation of the TTP pool, thus acting as an additional regulatory emchanism. DNase activity nearly constant throughout the mitotic cycle.