The reversed SoxS-binding site upstream of the ribA promoter in Escherichia coli

The ribA gene, encoding GTP cyclohydroIase II in Escherichia coli, is a mem ber of the soxRS regulon, which is induced by superoxide-generating agents. By evaluating lacZ expression driven by the ribA promoter carrying differe nt lengths of upstream region in a monolysogen, we found that the superoxid e-responsive element resides between 56 and 94 nucleotides upstream of the transcriptional start site. Purified SoxS protein bound to this region and protected nucleotides between positions -80 and -58 from degradation by DNa se I. This region contains a putative SoxS-binding sequence (soxbox) in rev erse orientation. The SoxS protein Interacted specifically with four guanin e residues within the soxbox sequence, as demonstrated by methylation inter ference analysis. These results clearly indicate that SoxS binds to the rev ersed soxbox sequence in the ribA gene, while in other known genes of the s oxRS regulon it binds to the normally oriented soxbox. Possible modes of in teraction between SoxS and RNA polymerase are discussed.