Two mutants of were isolated which differ from their respective parental strains in their growth responses to guanine and xanthine. Both mutants had purine phosphoribosyltransferase activities similar to their parental strains. One mutant, CB-3, had a lower guanine uptake rate apparently caused by a genetic lesion in a specific gene (designated ) responsible for facilitating the transport of guanine. This gene mapped at 3.5 min in the sequence The second mutant, GP103, had a purine carrier molecule with altered specificity, as demonstrated by a competition between hypoxanthine and xanthine for uptake.


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