Cultured cells were grown in various concentrations of amino acids for periods up to 3 days and the characteristics of the glycine transport system measured under fixed experimental conditions. During this time, the effect of enucleation, using cytochalasin B, and the effects of protein synthesis inhibitors (cycloheximide and actinomycin D) were investigated. 2. Glycine influx is regulated by the prior growth concentration of similarly transported amino acids. 3. The modification in transport involves primarily a change in Vmax (but also a change in Km in HeLa cells) and is effected within 2-10 hr after media change. Increased transport activity is calculated to be sufficient to compensate for the reduction in extracellular amino acid concentration, so that nearly normal influx values from media are maintained. Regulation over the range of concentrations studied is shown to be very accurate. 4. The nucleus is essential for the regulatory mechanism to function. It seems probable that mRNA synthesis is required for acquisition of increased transport activity and mRNA translation required for maintenance of normal activity. 5. The controlling factor in the regulatory mechanism appears unlikely to be intracellular pool size. Other possible signals are discussed.
