The activator/repressor protein DnrO of Streptomyces peucetius binds to DNA without changing its topology.

Regulatory proteins that bind to upstream un-translated region often control transcription of prokaryotic genes. Many of these proteins bend or distort their DNA binding sites, and the induced DNA curvature facilitates protein-protein or protein-DNA contacts essential for transcriptional regulation. DnrO is an essential transcription regulator of Streptomyces peucetius that activates daunorubicin biosynthetic pathway. It binds to a specific sequence adjacent to dnrN promoter to activate transcription. The same binding event represses its own transcription. DNA binding domain of DnrO is within 60 aa from N-terminal end of the 340 aa protein. Helix-turn-helix motif in DnrO is similar to BirA repressor of E. coli. In this study, we show that this dual functional protein does not cause any localized bending of DNA as observed by circular permutation gel shift assay. This observation complements the functional role of DnrO as an activator/repressor, since the change in DNA topology might impede the activation or repression function if this protein. This is in variance with DNA bending property of BirA repressor and many other transcription factors. The possibility of G+C rich sequences in the target DNA not favoring distortion of major groove upon protein binding is discussed.