Physarum Thymidine Kinase A Step or a Peak Enzyme Depending upon Temperature of Growth

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Abstract
The variations of thymidine kinase or ATP:thymidine 5''-phosphotransferase (EC 2.7.1.21) during the cell cycle of P. polycephalum plasmodia were studied at 2 extreme physiological temperatures: 22.degree. C and 32.degree. C. At 22.degree. C the enzyme activity increases near mitosis and stays constant during late S and G2 phases, exhibiting the typical pattern of a step enzyme. But at 32.degree. C thymidine kinase activity goes through a maximum 1 h 30 min after mitosis and decreases during the subsequent phases as expected for a peak enzyme. The rate of enzyme degradation and/or inactivation, measured in the presence of metabolic poisons (cycloheximide or dinitrophenol), appears to follow a simple exponential function with a half-life of approximately 3 h and 1 h at 22.degree. C and 32.degree. C, respectively. The effect of growth temperature on the decrease of thymidine kinase activity can account entirely for the differences in the pattern of enzyme activity at the 2 extreme temperatures. Tentative calculations indicate that the rate of enzyme synthesis is nearly constant during the cell cycle except near mitosis, where it is temporarily increased. Apparently there exists a regulatory mechanism able to modulate the rate of synthesis of thymidine kinase during the cell cycle.