Environmentally Responsive Dna Bending by the Agrobacterium Tumefaciens Transcriptional Activator OccR

Octopine-type Ti plasmids encode a LysR-type protein called OccR that regulates a 14 kb operon containing genes that direct the uptake and catabolism of octopine as well as a second uptake system and the traR gene, which regulates transcription of the Ti plasmid tra regulon. OccR binds to its operator in the presence or absence of octopine, but octopine shortens the DNase I-protected region and decreases the angle of a DNA bend incited by the protein. Experiments adjusting the phase between this bend and a sequence directed bend show that bend direction is not influenced by octopine, and that DNA is bent toward the protein. Operator resection experiments demonstrate that the right half of the operator is essential and sufficient for protein binding, while the left half contains several sites required for ligand-responsive DNA bending. A constitutive OccR protein was unable to cause as high a bend angle in the absence of octopine as the wild type protein.