Summary: Phenotypically and genotypically () Leu cells of gave rise to small colonies on medium devoid of leucine. This only occurred on plates with a high density of Leu cells or on medium supplemented with limiting quantities of leucine. Cells from these small colonies retained a growth advantage over their parent on limiting leucine supplements even after growth in a non-selecting (rich) medium. Therefore, the growth variants had acquired a heritable change. The phenotype was recessive and due to a change in a nuclear gene unlinked to the locus. The phenotype provided a growth advantage only during leucine starvation; growth of the variants was indistinguishable from their parent on medium lacking the other essential supplements (histidine and uracil) required for the growth of the strain. [C]Leucine uptake assays demonstrated that the variants were better able than their parents to accumulate leucine from their environments, and this ability extended to other hydrophobic amino acids. These results suggest that in the variants an amino acid uptake system has been derepressed rather than there having been reversion or extragenic suppression of the mutation in leucine biosynthesis. We designate the mutant gene responsible for the phenotype (for leucine uptake). The transport characteristics of the mutants suggested that is a regulatory component of an ammonium-regulated hydrophobic amino acid uptake system.


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