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Biotin Uptake by Cold-Shocked Cells, Spheroplasts, and Repressed Cells of Saccharomyces cerevisiae: Lack of Feedback Control

¹ Department of Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45219

ABSTRACT

Cold-osmotic-shocked cells and spheroplasts of Saccharomyces cerevisiae (ATCC 9896) display a biotin uptake system similar to that observed in intact cells. 2-Mercaptoethanol was found to inhibit biotin transport. Cells repressed for biotin uptake by growth in excess biotin (25 ng/ml) possess an energy-dependent transport system that has a K_m for biotin of 6.6 x 10^–7 M and a V_max equal to 39 pmol per mg (dry weight) per min. A similar K_m (6.4 x 10^–7 M) but a considerably higher V_max (530 pmol per mg (dry weight) per min) was determined for biotin uptake by cells grown in sufficient biotin (0.25 ng/ml). The V_max rates of biotin uptake by both repressed and derepressed cells were increased approximately 35-fold in the presence of glucose. These yeast cells appear to regulate their biotin uptake by two mechanisms. An exit system provides for immediate adjustments, whereas turnover of the transport system and repression of new synthesis establishes a slower adaptation to changes in the environment. Feedback inhibition was ruled out as a mechanism of regulation of transport.