Identification and changes in activity of five thymidine kinase forms during the cell cycle of Physarum polycephalum.

Five forms of thymidine kinase have been identified on isoelectric focusing gels of Physarum polycephalum supernatants. Their isoelectric points are 5.9, 6.4, 6.7, 6.9 and 7.1. All are inhibited by deoxythymidine 5'-triphosphate (dTTP). The activity of the pI 7.1 form does not change significantly during the cell cycle. The other four forms change in activity. About 1 h before metaphase the activity of the four more acidic forms is first detected. Their activity peaks during telophase, and by 1 h after metaphase there is a 50% decrease in activity of the 5.9 form. By 3 h after metaphase the activity of the 6.4 form has dropped more sharply than the activity of the 6.7 form. By 6 h after metaphase only the activity of the 6.9 form is present in significant amounts in addition to the 7.1 form. The activity of these new acidic forms probably accounts for the reported increase in total thymidine kinase activity during mitosis and early S phase.