Deoxyribonucleic Acid Polymerases of BHK-21/C13 cells RELATIONSHIP TO THE PHYSIOLOGICAL STATE OF THE CELLS, AND TO SYNCHRONOUS INDUCTION OF SYNTHESIS OF DEOXYRIBONUCLEIC ACID By ROGER K. CRAIG,* PATRICIA A. COSTELLO and HAMISH

BHK-21/C13 cells were grown in culture under conditions that provided exponentially growing cells and quiescent cells, by modifying the concentration of serum in the growth medium. The high-molecular-weight DNA polymerase (DNA polymerase I) from exponentially growing cells accounted for 90% of the total polymerase activity; the low-molecular-weight DNA polymerase (DNA polymerase II) accounted for the remaining 10 %. In quiescent cells, DNA polymerase I contributed only 39% of the total polymerase activity andDNA polymerase II 61 %. The total amount ofDNA polymerase I in exponentially growing cells was 11.3-fold greater than that in quiescent cells, whereas the amount ofDNA polymerase II appeared to be relatively independent of the physiological state of the cells. In an extension of these experiments, cells in a quiescent state (Go cells) were stimulated by the 'serum-step-up' method ofBurk (1970) to grow and to entera synchronous wave ofDNA synthesis (S-phase cells), 87% ofthe cells synthesizing DNA at 20h after the 'serum step-up'. During the synchrony experiment, the total cytoplasmic and total nuclear DNA polymerase activities each increased about 4-fold in parallel with the increase in the rate of DNA synthesis. Cytoplasmic polymerase activity was always greater than nuclear polymerase activity. The increases observed were maximal at 20h after 'serum step-up'. By 26h, there was a decrease in enzyme activity (8% for cytoplasmic polymerase and 16% for nuclear polymerase, both relative to the maximum at 20h), but the rate of DNA synthesis had declined by 37% relative to the maximum at 20h. In Go cells, DNA polymerase II (mol.wt. 46000±4000) was the predominant species, there being twice as much of it as of the total DNA polymerase I. In these cells there was little DNA polymerase IC and ID; the amounts of IA (mol.wt. 900x 103-llOOx 103) and IB (mol.wt. 460x 103-560x103) were about equal but small.