Uptake and efflux of sulfate in Neurospora crassa

Sulfate efflux from an intracellular pool was observed with both wild-type and cys-11 cells of Neurospora and apparently occurs by way of the sulfate transport system. Efflux requires the presence of external sulfate or the related ions, chromate, selenate, or thiosulfate, and probably occurs by an exchange reaction. The sulfur amino acids, cysteine or methionine, do not promote sulfate efflux. The K_m for efflux is much greater than the K_m for sulfate uptake, which permits the accumulation of a considerable intracellular pool before efflux becomes significant. Substantial transmembrane movement of sulfate both influx and exit, was found to occur in azidetreated cells, although the net uptake of sulfate was abolished by this inhibitor. Both sulfate uptake and efflux are inhibited by p-chloromercuribenzoate which suggests that the sulfate permease possesses an essential sulfhydryl group.