A MUTATION IN THE RECEIVER DOMAIN OF THE AGROBACTERIUM-TUMEFACIENS TRANSCRIPTIONAL REGULATOR VIRG INCREASES ITS AFFINITY FOR OPERATOR DNA

We fused the wild-type Agrobacterium tumefaciens virG gene and the con stitutive virGN54D allele to the malE gene of Escherichia coli, and st udied the binding of MBP-VirG fusions to the autoregulated virG promot er. MBP-VirGN54D protein bound this promoter with 10-fold higher affin ity than MBP-VirG, and bound to vir box I with eightfold higher affini ty than to vir box III. Disruption of vir box III did not alter the af finity for vir box I, suggesting a lack of cooperativity between these sites. We provide evidence that protein bound at a single vir box may have a higher oligomeric state than non-bound protein, and that a DNA distortion adjacent to vir box I may occur during activation.