REPRESSIBLE CATION-PHOSPHATE SYMPORTERS IN NEUROSPORA-CRASSA

The filamentous fungus Neurospora crassa possesses two nonhomologous h igh-affinity phosphate permeases, PHO-4 and PHO-5. We have isolated se parate null mutants of these permeases, allowing us to study the remai ning active transporter in vivo in terms of phosphate uptake and sensi tivity to inhibitors, The specificity for the cotransported cation dif fers for PHO-4 and PHO-5, suggesting that these permeases employ diffe rent mechanisms for phosphate translocation. Phosphate uptake by PHO-4 is stimulated 85-fold by the addition of Na+, which supports the idea that PHO-4 is a Na+-phosphate symporter. PHO-5 is unaffected by Na+ c oncentration but is much more sensitive to elevated pH than is PHO-4. Presumably, PHO-5 is a H+-phosphate symporter, Na+-coupled symport is usually associated with animal cells. The finding of such a system in a filamentous fungus is in harmony with the idea that the fungal and a nimal kingdoms are more closely related to each other than either is t o the plant kingdom.