Naturally occurring promoter down mutation: nucleotide sequence of the trp promoter/operator/leader region of Shigella dysenteriae 16.

The promoter/operator/leader region of the trp operon of Shigella dysenteriae 16 has single base pair differences from the corresponding region of Escherichia coli at positions -24 and -13. The difference at -13 was shown to be responsible for the 90% reduction in promoter function characteristic of the trp operon of S. dysenteriae. The base pair difference at position -13 also renders the operator partially constitutive. This allows the organism to maintain relatively high repressed levels of the trp enzymes and increases the relative importance of attenuation as a transcription control mechanism. These findings and the earlier observation that the trpE protein of S. dysenteriae is only slightly active explain the low in vivo expression of the trp operon of this organism. Nutritional studies suggest that operons involved in other amino acid biosynthetic pathways in S. dysenteriae 16 may be similarly partially inactivated.