PhoB transcriptional activator binds hierarchically to pho box promoters.

The PhoR-PhoB phosphorelay is a bacterial two-component system that activates the transcription of several genes involved in phosphate uptake and assimilation. The response begins with the autophosphorylation of the sensor kinase PhoR, which activates the response regulator PhoB. Upon binding to the pho box DNA sequence, PhoB recruits the RNA polymerase and thereby activates the transcription of specific genes. To unveil hitherto unknown molecular mechanisms along the activation pathway, we report biochemical data characterizing the PhoB binding to promoters containing multiple pho boxes and describe the crystal structure of two PhoB DNA-binding domains bound in tandem to a 26-mer DNA.