Summary: K12, K26 and K34 are three genetically distinct histidine-requiring mutants of . K12 and K26 are 0·69 units apart in the left arm of linkage group I, and K34 is in linkage group IV 32·6 units from 5531 ().

Response to histidine of these three mutants is markedly affected by the genetic background.

K26 accumulates two imidazole compounds, one of which may be histidinol and the other possibly compound II of Ames, Mitchell & Mitchell (1953). Like other histidine-requiring mutants K12, K26 and K34 are inhibited by certain other amino acids in combination with either arginine or lysine. The degree of inhibition is affected by the genetic background. The effect of the inhibitors is to prevent uptake of histidine from the culture medium. Histidine is accumulated from the culture medium against a concentration gradient and stored in the mycelium, from which it gradually disappears with growth. Inhibitors applied after accumulation have no effect on utilization of this stored histidine. Histidine is similarly accumulated by wild type and uptake is also prevented by a combination of the inhibitors arginine and methionine. The histidine requirement therefore serves to accentuate an effect which already exists in the wild type.


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