Regulation of the operon encoding ribonucleotide reductase in Escherichia coli: evidence for both positive and negative control.

The ribonucleotide reductase genes (nrd) are induced by thymine starvation. Deletion analysis of the sequences upstream of the cloned nrd genes was used to identify several regulatory regions. The start of transcription (nrdP) was mapped 110 bp upstream of nrdA, the first structural gene. A site required for positive regulation of nrd was mapped 135 bp upstream of nrdP in a region with two direct repeat sequences as well as potential secondary structure. Two other sites (one upstream of nrdP, the other downstream) were identified as sequences whose deletion markedly increase expression. These latter sites show sequence homology and probably interact since the effects of their individual deletion are not additive when combined.