The outflow of uracil from the yeast Saccharomyces cerevisiae is known to be relatively fast in certain circumstances, to be retarded by proton conductors and to occur in strains lacking a uracil proton symport. In the present work, it was shown that uracil exit from washed yeast cells is an active process, creating a uracil gradient of the order of -80 mV relative to the surrounding medium. Glucose accelerated uracil exit, while retarding its entry. DNP or sodium azide each lowered the gradient to about -30 mV, simultaneously increasing the rate of uracil entry. They also lowered cellular ATP content. Manipulation of the external ionic conditions governing Δμ;H+ at the plasma membrane had no detectable effect on uracil transport in yeast preparations thoroughly depleted of ATP. It was concluded that uracil exit is probably not driven by the proton gradient but may utilize ATP directly. It is known that thymine is not normally absorbed by yeast. However, thymine expulsion was here observed during deamination of the substrate 5-methylcytosine in the presence of glucose. In the absence of glucose, or following ATP depletion, thymine uptake from the medium only occurred when Δμ;H+ was dissipated, either by DNP or azide, or by manipulation of the external ionic environment. The yeast expelled absorbed thymine when Δμ;H+ was restored to the physiological range. The properties of the system corresponded to those of an H+/thymine antiport that is distinct from the mechanism expelling uracil.
BrethèsD.,
ChirioM.,
NapiasC,
ChevallierM.,
LavieJ.L.,
ChevallierJ.1992; In vivo and in vitro studies of the purine-cytosine permease of Saccharomyces cerevisiae.. Eur J Biochem 204:699–704
EddyA.A.,
HopkinsP.1996; Cytosine accumulation as a measure of the proton electrochemical gradient acting on the overexpressed cytosine permease of Saccharomyces cerevisiae.. Microbiology 142:449–457
ForetM.,
SchmidtR.,
ReichertU.1978; On the mechanism of substrate binding to the purine-transport system of Saccharomyces cerevisiae.. Eur J Biochem 82:33–43
GrensonM.1969; The utilization of exogenous pyrimidines and the recycling of uridine-5̓-phosphate derivatives in Saccharo-myces cerevisiae, as studied by means of mutants affected in pyrimidine uptake and metabolism.. Eur J Biochem 11:249–260
HopkinsP.,
ChevallierN.R.,
JundR.,
EddyA.A.1988; Use of plasmid vectors to show that the uracil and cytosine permeases of the yeast Saccharomyces cerevisiae are electrogenic proton symports.. FEMS Microbiol Lett 49:173–177
HopkinsP.,
ShawR.,
AcikL.,
OliverS.,
EddyA.A.1992; Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease.. Yeast 8:1053–1064
van der RestM.E.,
KammingaA.H.,
NakanoA.,
AnrakuY.,
PoolmanB.,
KoningsW.N.1995; The plasma membrane of Saccharomyces cerevisiae: structure, function, and biogenesis.. Microbiol Rev 59:304–322
SeastonA.,
CarrG.,
EddyA.A.1976; The concentration of glycine by preparations of the yeast Saccharomyces cerevisiae depleted of adenosine triphosphate. Effects of proton gradients and uncoupling agents.. Biochem J 154:669–676
SerranoR.1991; Transport across yeast vacuolar and plasma membranes.. In The Molecular and Cellular Biology of the Yeast Saccharomyces1JonesE.W.,
PringleJ.R.,
BroachJ.R.
Edited by pp. 523–585 Cold Spring Harbor, NY :: Cold Spring Harbor Laboratory.;
StanleyP.E.,
WilliamsS.G.1969; Use of the liquid scintillation spectrometer for determining adenosine triphosphate with luciferase.. Anal Biochem 29:381–392