Uptake and efflux of adenine and its derivatives in Neurospora crassa.

Conidia of wild-type Neurospora crassa, preincubated for 3 1/2 h in growth medium, showed a typical triphasic pattern of adenine uptake. The three phases consisted of a quick initial uptake, followed by a plateau phase, and then by a resume lowered uptake. A study of the relative influx and efflux of [14C] adenine showed that the plateau phase in fact is a period of transmembrane movement of adenine and adenine metabolites. The efflux during the plateau phase essentially cancelled out all the influx during the same period. The uptake curve derived after taking into account the effluxed portion of radioactivity indicated that the second phase represents a period of lowered uptake activity. The beginning of the lowered uptake activity during the second phase is correlated with the presence of a high intracellular level of ATP derived from exogenous [14C]adenine. At the end of the secod phase, the intracellular level of ATP is much smaller and the rate of adenine uptake increases again. Analysis of the acid-soluble pool after feeding [14C]adenine indicated the presence of other 14C-nucleotides, but no detectable levels of bases and nucleosides were present. However, chromatographic analysis of the medium indicated that efflux results essentially in the accumulation of bases. The significance of this finding in relation to efflux is discussed.