EXPULSION OF URACIL AND THYMINE FROM THE YEAST SACCHAROMYCES-CEREVISIAE - CONTRASTING RESPONSES TO CHANGES IN THE PROTON ELECTROCHEMICAL GRADIENT

The outflow of uracil from the yeast Saccharomyces cerevisiae is known to be relatively fast in certain circumstances, to be retarded by pro ton 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 o f -80 mV relative to the surrounding medium. Glucose accelerated uraci l exit while retarding its entry. DNP or sodium azide each lowered the gradient to about -30 mV, simultaneously increasing the rate of uraci l entry. They also lowered cellular ATP content. Manipulation of the e xternal ionic conditions governing Delta<(mu)over bar>(H)+ at the plas ma membrane had no detectable effect on uracil transport in yeast prep arations thoroughly depleted of ATP. It was concluded that uracil exit is probably not driven by the proton gradient but may utilize ATP dir ectly. It is known that thymine is not normally absorbed by yeast. How ever, thymine expulsion was here observed during deamination of the su bstrate 5-methylcytosine in the presence of glucose. In the absence of glucose, or following ATP depletion, thymine uptake from the medium o nly occurred when Delta<(mu)over bar>(H)+ was dissipated, either by DN P or azide. or by manipulation of the external ionic environment. The yeast expelled absorbed thymine when Delta<(mu)over bar>(H)+ was resto red to the physiological range. The properties of the system correspon ded to those of an H+/thymine antiport that is distinct from the mecha nism expelling uracil.