Studies on changes in DNA polymerase activity during the cell cycle in synchronized KB cells

We have demonstrated the presence of two DNA polymerase in KB cells and studied the variation of their activities in a synchronous cell population. During the cell cycle we observed in nuclei, only one DNA dependent DNA polymerase, the 3.4 S or minipolymerase, and similarly in the cytoplasm only one enzyme, the 8.3 S or maxipolymerase. The former shows preference for native DNA and the latter for denatured DNA. Their Mg ++ and K + requirements are different and their pH optima are 8.5 and 7 for nuclear polymerase and cytoplasmic polymerase respectively. The cytoplasmic polymerase activity remains stable from one cell cycle to the other with each cell reconstituting its stock at the start of the following cycle (G 1 and early S phases). On the contrary nuclear activity decreases in G 2, M and early G 1, then increases to a maximum in the middle of the S phase. This fluctuation in enzyme activity could be due to degradation, transfer to the cytoplasm or the association of the enzyme with the chromatin and/or the nuclear membrane after completion of DNA synthesis. Our results do not permit us to choose between these three hypotheses. However their significance is discussed in the light of the results obtained by some authors who, on the contrary, have tended to minimise the role of the minipolymerase in DNA duplication, whereas we, from our findings, ascribe a preponderant role to this enzyme. The cytoplasmic maxipolymerase (8.3 S) may simply be a storage form of the enzyme from which minipolymerase can be formed as needed. Les DNA polymérases et leurs variations d'activité durant le cycle cellulaire sont étudiées dans des populations synchrones de cellules KB. A tout moment du cycle, le noyau et le cytoplasme ne contiennent qu'une DNA polymérase DNA dépendante : une minipolymérase 3,4 S dans le noyau ; une maxipolymérase 8,3 S dans le cytoplasme. La première préfère le DNA natif, la seconde le DNA dénaturé. Les deux diffèrent aussi pour leurs exigences en Mg ++ et K + et ont un pH optimum respectivement de 8,5 et 7. Au cours du cycle cellulaire, l'activité cytoplasmique reste stable : tout se passe comme si, après la mitose, chaque cellule reconstitue (en G 1 et début de S) le stock caractéristique des phases S et G 2 : en revanche, l'activité nucléaire diminue en G 2, pendant la mitose et en début de phase G 1, pour retrouver un optimum au milieu de la phase S. Une telle évolution implique une dégradation, un transfert vers le cytoplasme ou une fixation acc