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New Amino Acid Regulatory Locus Having Unusual Properties in Heterozygous Merodiploids

¹ Department of Bacteriology, University of California, Davis, California 95616

ABSTRACT

Spontaneous mutants of Escherichia coli B/r resistant to 5',5',5',-trifluoro-DL-leucine contain defects in a gene which maps to the left of the threonine region. Low-level constitutive expression of the isoleucine-valine and leucine operons is caused by this mutation in haploid strains. This is in contrast to extremely high levels of gene expression in the heterozygous merodiploids (F' wild type/mutant allele). The properties of these mutants define a new locus and suggest that it encodes a subunit protein which is involved in the repression of the structural genes for the branched-chain amino acid pathways.