Uracil transport inSaccharomyces cerevisiae
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Uracil transport inSaccharomyces cerevisiae is mediated by a specific “permease” which does not recognize other pyrimidines such as uridine, cytosine, thymine, 2-hydroxypyrimidine or 5-amino-uracil; hypoxanthine and 6-amino-uracil slightly inhibit the uptake of uracil in a strain lacking cytosine permease activity. Wild type cells concentrate extracellular uracil before its transformation into UMP and subsequent incorporation into nucleic acids. A strain lacking UMP pyrophosphorylase and uridine ribohydrolase (strainfur 1–8 rh, in which the endogenous production as well as the utilization of uracil are lacking) is able to concentrate14C-2 uracil from the medium. At the same time no other14C-2 labelled compound could be detected in this strain, thus suggesting that the uptake of uracil in yeast occurs by active transport which is not coupled to the UMP pyrophosphorylase. The optimal pH of uracil uptake in standard growth conditions was 4.3. It was deduced from experiments performed on strainfur 1–8 rh with3H-5 and14C-2 uracil that the intracellular pool of uracil is recycled once the steady-state has been reached. First order kinetics with similar rate constants were observed for uracil efflux in strainfur 1–8 rh (k min−1=0.75±0.08) as well as in the strain lacking uracil permease,fur 1–8 rh fur 4–6 (k min−1=0.60±0.08). The intracellular pool of14C-2 uracil can be chased in strainfur 1–8 rh by addition of3H uracil without inducing a large initial acceleration of the exit rate (the rate constant remained at 0.60). 2-4-dinitrophenol inhibits the uptake of uracil but also reduces the efflux of uracil in strainfur 1–8 rh fur 4–6. These data and the comparison with cytosine transport in the same organism support the hypothesis that, whereas uracil uptake is a “permease” mediated active transport, the efflux of uracil does not involve the uracil uptake “permease”. A coefficient of permeability of 7.4×10−7 cm sec−1 was calculated for uracil.
KeywordsCytosine Active Transport Uracil Uridine Thymine
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