INDUCED CHANGES IN THE RATES OF URIDINE-³H UPTAKE AND INCORPORATION DURING THE G₁ AND S PERIODS OF SYNCHRONIZED CHINESE HAMSTER CELLS

The rates of uridine-5-³H incorporation into RNA and the rates of uridine uptake into the acid-soluble pool during the cell cycle of V79 Chinese hamster cells were examined. Cells cultured on Eagle's minimal essential medium supplemented with fetal calf serum, lactalbumin hydrolysate, glutamine, and trypsin displayed rates of incorporation and uptake which increased only slightly during G₁ and accelerated sharply as DNA synthesis commenced. In contrast, cells cultured on minimal essential medium supplemented only with calf serum exhibited rates of incorporation and uptake which increased linearly through both G₁ and S. The transition from one pattern to the other can be induced within 24 hr and is completely reversible. The nonlinear pattern exhibited by cells grown on the supplemented fetal calf serum medium can also be overcome with high exogenous uridine concentrations. In the presence of 200 µM uridine, these cells display a linear pattern of increase in rates of uridine incorporation and uptake. It is concluded that at lower uridine concentrations the pattern of increase in the rate of uridine incorporation into RNA during the cell cycle for a given population of cells is dependent upon the rate of uridine entry into the cell, and that this pattern is not rigidly determined but can be modified by culture conditions.