A SEQUENCE-INDUCED SUPERHELICAL DNA SEGMENT SERVES AS TRANSCRIPTIONAL ENHANCER

The initiation of transcription at the sigma(54)-dependent promoter glnAp2 of Escherichia coli is activated by the protein NR(I)(NTRC)-phosphate, which binds to two sites located upstream of the promoter that together constitute an enhancer. The cooperative binding facilitates the oligomerization of NR(I)-phosphate endowing it with the ATPase activity required for its ability to serve as transcriptional activator. We show here that these sites can be replaced by sequence-dependent superhelical inserts, lacking any homology to the nucleotide sequence of the enhancers. These superhelical inserts, irrespective of their chirality, are as effective as the paired sites in binding NR,phosphate and in stimulating its oligomerization. We conclude that a specific sequence of nucleotides and the three-dimensional structure of DNA can determine its affinity for the NR(I) activator protein capable of binding to DNA.