The Effects of Calcium Withdrawal on the Uridine Uptake Process in a Long-day Duckweed, Lemna gibba G3

Abstract

Summary The effects of calcium withdrawal on the process of uridine uptake was examined in a long-day duckweed, Lemna gibba G3. A short term treatment in sodium citrate reduced the rate of uridine uptake. It changed the value of Vmax for uridine uptake more than that of Km. CaCl2 was the most efficient compound in offsetting the citrate reduction. Sodium citrate did not, however, change the pattern of intracellular conversion of uridine absorbed into the duckweed cells. The lowered uptake rate recovered completely after 12 h when the duckweeds were cultured in the normal culture medium. The recovery process was inhibited by compounds which normally inhibit protein synthesis, although they did not change the rate of leucine transport into the duckweed or the activity of uridine phosphorylation. We concluded from these experiments that the protein(s) necessary for the uridine transporting system were released from the duckweed by treatment with sodium citrate and that calcium may be necessary for the molecular connection of the uridine transporting protein(s) to the cell membrane.