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(2'-5')Oligoadenylic acid [(2'-5')An] polymerase activity was measured in extracts of human lymphoblastoid cells of the Namalva line cultured under different conditions. Exponentially growing cells had a relatively low polymerase activity level, whereas cells grown to limit density showed elevated levels. When fresh medium was added to growth-arrested cells, (2'-5')An polymerase activity decreased concomitantly with the initiation of active deoxyribonucleic acid synthesis. An increase in polymerase activity level was also observed after exponentially growing cells were transferred from medium containing 20% serum to fresh medium containing 0.2% serum. These cells diminished deoxyribonucleic acid synthesis and remained quiescent until 20% serum was again added. Polymerase activity level decreased as the cells entered into S phase. The addition of the inhibitor of deoxyribonucleic acid synthesis, hydroxyurea, to exponentially growing cells did not increase polymerase level, indicating that cells blocked in S phase and at the G1-S boundary maintained the basal level of this enzyme. Degradation of labeled (2'-5')An was measured in extracts of Namalva cells cultured under different conditions, but no significant differences among degradative activities were observed. Since (2'-5')An polymerase activity is one of the enzymatic activities induced by interferon, we measured interferon titers in Namalva cell medium. Less than 1 reference unit per ml was detected in cells grown under different conditions. Moreover, the increase in (2'-5')An polymerase activity level in cells transferred from 20 to 0.2% serum was not prevented by including anti-lymphoblastoid interferon antibody in the medium. These results suggest that the activity level of (2'-5')An polymerase is regulated in Namalva cells on the basis of the growth status of the cells and that this regulatory mechanism is apparently not activated by interferon.

Elevated levels of (2'-5')oligoadenylic acid polymerase activity in growth-arrested human lymphoblastoid Namalva cells.