Abstract
The mechanism of uracil uptake and one aspect of its regulation were studied in germinated conidia of Neurospora crassa. Uracil was found to be taken up by a transport mechanism that did not exhibit Michaelis-Menten kinetics. Rather, the kinetic patterns indicated two separate systems or a single transport mechanism with negative cooperativity. Cytosine and thymine inhibited uracil uptake, but uridine did not. The mutant strain uc-5-pyr-1, which failed to transport uracil, was used in reversion studies and to map the uc-5 locus. Spontaneous reversion rates at the uc-5 locus were found to be approximately 2 x 10(-8), indicating that the uc-5 lesion results from a single mutation. Loss of the uracil transport function through a single mutation favors the model of a single transport mechanism with negative cooperativity. Uracil uptake was significantly decreased in the presence of NH 4+, and evidence is presented for repression by NH4+ of a uracil transport system. Growth rates of pyrimidine-requiring and wild-type strains measured in the presence and absence of NH4+, with uracil as the pyrimidine supplement, showed that NH4+ decreased the growth rates of the pyrimidine-requiring strains significantly, while having no effect on wild-type growth rates.
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