Induced alteration in uptake properties of Tetrahymena and its association with the development of mating competency

The rate of uridine uptake in Tetrahymena declines by an order of magnitude by two hours after shiftdown to a non-nutrient buffer. This alteration in uptake properties cannot be accounted for by an increase in the intracellular pool of uridine, an increase in apparent Km for uptake or a decline in the rate in which uridine is processed intracellularly. It is argued that the decrease in uridine uptake is due to a reduction in numbers of functional transport molecules exposed at the cell surface and is a reflection of a developmentally related cell surface transformation. In addition, the putative decline in functional transport molecules cannot be entirely explained by metabolic turnover of these molecules in the absence of replacement, nor does it require the synthesis of new protein. We discuss the possibility that a shift in equilibrium between accessible and inaccessible transporters is operating. Finally, a close correlation between conditions which elicit the transport alteration and those which allow the development of mating competency suggests that the two phenomena may be coordinately regulated.