Summary: The mechanism of resistance to the arginine analogue L-canavanine, and of arginine uptake, were examined in the fission yeast Two mutants with increased resistance to canavanine were analysed genetically: both were double mutants, and in each case one mutation conferred resistance to canavanine, while the other enhanced this resistance. Evidence is presented that strains are defective in one system for arginine uptake, which presumably prevents entry of canavanine into the cell. This system operates in the wild-type whether the nitrogen source supplied is ammonium or glutamate. Double mutants carrying and an arginine requirement are unable to grow on ammonium medium even when supplied with exogenous argine, while growth can occur on glutamate plus arginine. This suggested the existence of a second uptake system for arginine which is absent during growth on ammonium, and direct measurements of the rates of arginine uptake under various conditions confirmed this. Our observations closely parallel those made on the budding yeast The ability to select for or against function of the gene should facilitate certain types of genetical analysis in


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