SUMMARY: Under conditions of limited-adenine supplementation, 8-azaguanine is a potent inhibitor of purine auxotrophic strains of It is relatively ineffective in prototrophic strains. Spontaneous and u.v.-light-induced mutants, resistant to 8-azaguanine, were obtained in a strain carrying the ad-2, ad-3A, and ad-3B markers. The resistant mutants grew in the presence of 200 μg 8-azaguanine and 2.0 μg adenine sulphate/ml medium. Inhibition of the sensitive strain is virtually complete under these conditions. Higher adenine sulphate concentrations prevent inhibition of the sensitive strain by 8-azaguanine. Resistance of the new mutants to 8-azaguanine was demonstrated by growth-tube experiments and by plating efficiencies in the presence and in the absence of the inhibitor. Growth-tube experiments were also used to study the reversal of 8-azaguanine induced inhibition by adenine sulphate. The resistance markers in the five strains studied were assigned to linkage group III. The locus designation -3 was given to the resistance marker in a selected mutant of spontaneous origin. Resistance to 8-azaguanine is stable in the presence and in the absence of the inhibitor. In heterokaryons, 8-azaguanine resistance is recessive.


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