Structural analysis of the ileR locus of Escherichia coli K12

The Ile repressor protein negatively controls expression from the ilv and thr promoters of Escherichia coli K12. Its existence was inferred from an analysis of the phenotypes of the ileR mutant avr-16 (Johnson, D. I., and Somerville, R. L. (1983) J. Bacteriol. 155, 49-55). The nucleotide sequence of ileR, the structural gene for Ile repressor, has been determined. A DNA segment of 300 base pairs constitutes the ileR gene. The predicted gene product, a protein of 100 amino acids (molecular weight 11,823) has primary structural features reminiscent of other double-stranded DNA-binding regulatory proteins. S1 nuclease mapping of the 5' terminus of ileR mRNA revealed two discrete species whose startpoints differed by approximately 47 nucleotides. The ileR gene, like other repressors for amino acid biosynthetic systems, is autogenously regulated at the transcriptional level. Within the ileR promoter region lie two 18-base pair segments of DNA bearing significant homology to putative operator targets also found within the thr and ilv promoters. A second open reading frame capable of specifying a protein of 83 amino acids, designated orf83, is transcribed divergently from the ileR gene. There are 202 base pairs separating the first codons of the two genes. S1 nuclease mapping of the 5' terminus of orf83 mRNA revealed two discrete species whose startpoints differed by approximately 27 nucleotides. The upstream promoters for ileR and orf83 overlap in their -35 regions.