Regulation of Biotin Transport in Saccharomyces cerevisiae

The metabolic control of biotin transport in Saccharomyces cerevisiae was investigated. Nonproliferating cells harvested from cultures grown in excess biotin (25 ng/ml) took up small amounts of biotin, whereas cells grown in biotin-sufficient medium (0.25 ng/ml) accumulated large amounts of the vitamin. Transport was inhibited maximally in cells grown in medium containing 9 ng (or more) of biotin per ml. When avidin was added to biotin-excess cultures, the cells developed the ability to take up large amounts of biotin. Boiled avidin was without effect, as was treatment of cells with avidin in buffer. Avidin did not relieve transport inhibition when added to biotin-excess cultures treated with cycloheximide, suggesting that protein synthesis was required for cells to develop the capacity to take up biotin after removal of extracellular vitamin by avidin. Cycloheximide did not inhibit the activity of the preformed transport system in biotin-sufficient cells. The presence of high intracellular free biotin pools did not inhibit the activity of the transport system. The characteristics of transport in biotin-excess cells (absence of temperature or pH dependence, no stimulation by glucose, absence of iodoacetate inhibition, independence of uptake on cell concentration, and nonsaturation kinetics) indicated that biotin entered these cells by diffusion. The results suggest that the synthesis of the biotin transport system in S. cerevisiae may be repressed during growth in medium containing high concentrations of biotin.